Biological determinants of spontaneous physical activity

Temporal relationships in the movement behaviour of adolescents: Testing and methodological considerations of the ActivityStat hypothesis

The study aimed to investigate the ActivityStat hypothesis by examining the presence and the timeframe of the temporal relationships among physical activity (PA) levels and stationary time (ST) in adolescents. A secondary analysis was performed on data involving 356 adolescents in Dunedin, New Zealand. Participants wore a waist-worn accelerometer for several consecutive days to assess PA levels [i.e. light PA (LPA), and moderate-to-vigorous PA (MVPA)] and ST. Bayesian continuous-time structural equation modelling (CT-SEM) was used to examine the relationship between the behaviours over time and the timeframe during which these relationships occur. Increases in LPA, MVPA, and ST were positively associated with their later behaviours until 2.5, 1.7, and 2 days later, respectively. A cross-behavioural reciprocal and negative relationship between LPA and ST was demonstrated 0.4 days later. A positive relationship between ST and MVPA was observed until about 0.4 days later. This is the first study to investigate the ActivityStat hypothesis using Bayesian CT-SEM in adolescents, examining the multivariate relationships among different behaviours and the associated timeframes. To conclude, evidence of activity synergy was suggested for the within-behavioural relationships, while behavioural compensation was noted for ST. Thus, the findings provide some support for the ActivityStat hypothesis in adolescents.

Compensatory behavior of physical activity in adolescents - a qualitative analysis of the underlying mechanisms and influencing factors

INTRODUCTION

Compensatory behavior of physical activity (PA) based on the ActivityStat hypothesis in adolescents is scarcely investigated, and existing studies showed inconclusive results. Understanding the compensatory behavior in a holistic way is important as this can help to improve intervention outcomes and thus, increase the PA levels in adolescents. Thus, the aim of the present study is to investigate the occurrence, direction, timeframe, and ratio of habitual activity compensation in adolescents. Furthermore, we want to identify the awareness of compensation and factors that influence compensatory behavior.

METHODS

The present qualitative study used a mixed methods crossover analysis design. Participants (N = 15, 8 boys and 7 girls) were adolescents aged 11-15 years (mean age 13.04 ± 1.28). They provided a habitual weekly schedule with habitual/regular activities and their intensity. Participants then kept an activity diary over one week to capture their actual behavior. After that, data were compared and deviations > ±20% were considered as compensation opportunities. On this basis, deviations were descriptively analyzed for compensatory behavior and were coded as positive and negative compensatory behavior. Further, for each compensation, the ratio of compensation (MET-minutes of the compensating activity/MET-minutes of the activity that was compensated) was calculated. Additionally, interviews were conducted to explore perceptions and influencing factors for (no) compensation.

RESULTS

Overall, 198 compensation opportunities were identified with deviations greater ± 20%. Of these, 109 opportunities were compensated overall (69 within-day, 40 between-day). Negative compensation took place in 57 opportunities and 52 opportunities were compensated positively. Most of the deviations were overcompensated (compensation/deviation > 100%). About half of the adolescents (N = 8) were not aware about their compensatory behavior, and only one boy was aware of all his compensatory behavior. The most mentioned influence for positive compensation were social support by friends and good weather. As influencing factors for negative compensation, tiredness as well as no need for movement were mentioned predominantly. No negative compensation occurred because adolescents wanted to stick to their routines or participated in hedonistic activities.

DISCUSSION

Summarizing the findings, the present study delivered new insights into the field of compensatory behavior in adolescents. Nevertheless, compensatory behavior was not consistently observed regarding the occurrence of compensation, direction, timeframe and ratio. However, social support appears to be an important factor to compensate positively or to avoid a negative compensatory behavior. Further, it seems to be helpful to support individuals in their search for hedonistic activities as well as in the establishment of routines.

Compensation or Displacement of Physical Activity in Children and Adolescents: A Systematic Review of Empirical Studies

Regular physical activity during childhood and adolescence is associated with health benefits. Consequently, numerous health promotion programs for children and adolescents emphasize the enhancement of physical activity. However, the ActivityStat hypothesis states that increases in physical activity in one domain are compensated for by decreasing physical activity in another domain. Currently, little is known about how physical activity varies in children and adolescents within intervals of one day or multiple days. This systematic review provides an overview of studies that analyzed changes in (overall) physical activity, which were assessed with objective measurements, or compensatory mechanisms caused by increases or decreases in physical activity in a specific domain in children and adolescents. A systematic search of electronic databases (PubMed, Scopus, Web of Science, SportDiscus) was performed with a priori defined inclusion criteria. Two independent researchers screened the literature and identified and rated the methodological quality of the studies. A total of 77 peer-reviewed articles were included that analyzed changes in overall physical activity with multiple methodological approaches resulting in compensation or displacement. Of 40,829 participants, 16,265 indicated compensation associated with physical activity. Subgroup analyses separated by study design, participants, measurement instrument, physical activity context, and intervention duration also showed mixed results toward an indication of compensation. Quality assessment of the included studies revealed that they were of high quality (mean = 0.866). This review provides inconclusive results about compensation in relation to physical activity. A trend toward increased compensation in interventional studies and in interventions of longer duration have been observed.

The Central Role of Hypothermia and Hyperactivity in Anorexia Nervosa: A Hypothesis

Typically, the development of anorexia nervosa (AN) is attributed to psycho-social causes. Several researchers have recently challenged this view and suggested that hypothermia and hyperactivity (HyAc) are central to AN. The following hypothesis will attempt to clarify their role in AN. Anorexia nervosa patients (ANs) have significantly lower core temperatures (T_core) compared to healthy controls (HCs). This reduced temperature represents a reset T_core that needs to be maintained. However, ANs cannot maintain this T_core due primarily to a reduced basal metabolic rate (BMR); BMR usually supplies heat to sustain T_core. Therefore, to generate the requisite heat, ANs revert to the behavioral-thermoregulatory strategy of HyAc. The majority of ANs (~89%) are reportedly HyAc. Surprisingly, engagement in HyAc is not motivated by a conscious awareness of low T_core, but rather by the innocuous sensation of "cold- hands" frequently reported by ANs. That is, local hand-thermoreceptors signal the brain to initiate HyAc, which boosts perfusion of the hands and alters the sensation of "cold-discomfort" to one of "comfort." This "rewarding" consequence encourages repetition/habit formation. Simultaneously, hyperactivity increases the availability of heat to assist with the preservation of T_core. Additionally, HyAc induces the synthesis of specific brain neuromodulators that suppress food intake and further promote HyAc; this outcome helps preserve low weight and perpetuates this vicious cycle. Based on this hypothesis and supported by rodent research, external heat availability should reduce the compulsion to be HyAc to thermoregulate. A reduction in HyAc should decrease the production of brain neuromodulators that suppress appetite. If verified, hopefully, this hypothesis will assist with the development of novel treatments to aid in the resolution of this intractable condition.

Do older English adults exhibit day-to-day compensation in sedentary time and in prolonged sedentary bouts? An EPIC-Norfolk cohort analysis

INTRODUCTION

Compensatory behaviours may be one of the reasons for the limited success of sedentary time interventions in older adults, but this possibility remains unexplored. Activity compensation is the idea that if we change activity levels at one time we compensate for them at a later time to maintain a set point. We aimed to assess, among adults aged ≥60 years, whether sedentary time and time spent in prolonged sedentary bouts (≥30 mins) on one day were associated with sedentary time and time spent in prolonged sedentary bouts (≥30 mins) on the following day. We also sought to determine whether these associations varied by sociodemographic and comorbid factors.

METHODS

Sedentary time was assessed for seven days using hip-worn accelerometers (ActiGraph GT1M) for 3459 adults who participated in the EPIC-Norfolk Study between 2004 and 2011. We assessed day-to-day associations in total and prolonged bouts of sedentary time using multi-level regressions. We included interaction terms to determine whether associations varied by age, sex, smoking, body mass index, social class, retirement, education and comorbid factors (stroke, diabetes, myocardial infarction and cancer).

RESULTS

Participants (mean age = 70.3, SD = 6.8 years) accumulated 540 sedentary mins/day (SD = 80.1). On any given day, every 60 minutes spent in sedentary time was associated with 9.9 extra sedentary minutes on the following day (95% CI 9.0, 10.2). This association was greater in non-retired compared to retired participants (non-retired 2.57 extra minutes, p = 0.024) and in current compared to former and never-smokers (5.26 extra mins for current vs former; 5.52 extra mins for current vs never, p = 0.023 and 0.017, respectively). On any given day, every 60 minutes spent in prolonged bouts was associated with 7.8 extra minutes in these bouts the following day (95% CI 7.6, 8.4). This association was greater in older individuals (0.18 extra minutes/year of age, 95% CI 0.061, 0.29), and for retired versus non-retired (retired 2.74 extra minutes, 95% CI 0.21, 5.74).

CONCLUSION

Older adults did not display day-to-day compensation. Instead, individuals demonstrate a large stable component of day-to-day time spent sedentary and in prolonged bouts with a small but important capacity for positive variation. Therefore older adults appear to be largely habitual in their sedentary behaviour. Strategies to augment these patterns may be possible, given they may differ by age, smoking, and working status.

Whole-Body ARHGAP21-Deficiency Improves Energetic Homeostasis in Lean and Obese Mice

Inhibition of Rab-GAP TBC1 domain family member 1 (TBC1D1) reduces body weight and increases energy expenditure in mice. Here, we assessed the possible involvement of GTPase activating protein 21 (ARHGAP21), a Rho-GAP protein, in energy homeostasis. Wild-type and whole-body ARHGAP21-haplodeficient mice were fed either chow or high-fat diet for 10 weeks. These mice were analyzed for body weight, food intake, voluntary physical activity, and energy expenditure by indirect calorimetry. Real-time PCR was performed to determine changes in the expression of hypothalamic-anorexic genes. Whole-body ARHGAP21-haplodeficient mice showed lower body weight and food intake associated with increased energy expenditure. These mice also showed higher expression of hypothalamic-anorexic genes such as POMC and CART. Our data suggest that the reduction in body weight of ARHGAP21-haplodeficient mice was related to alterations in the central nervous system. This suggests a new role for ARHGAP21 in energetic metabolism and prompts us to consider GAP protein members as possible targets for the prevention and treatment of obesity and related diseases.

Can body temperature dysregulation explain the co-occurrence between overweight/obesity, sleep impairment, late-night eating, and a sedentary lifestyle?

PURPOSE

Overweight/obesity, sleep disturbance, night eating, and a sedentary lifestyle are common co-occurring problems. There is a tendency for them to co-occur together more often than they occur alone. In some cases, there is clarity as to the time course and evolution of the phenomena. However, specific mechanism(s) that are proposed to explain a single co-occurrence cannot fully explain the more generalized tendency to develop concurrent symptoms and/or disorders after developing one of the phenomena. Nor is there a clinical theory with any utility in explaining the development of co-occurring symptoms, disorders and behaviour and the mechanism(s) by which they occur. Thus, we propose a specific mechanism-dysregulation of core body temperature (CBT) that interferes with sleep onset-to explain the development of the concurrences.

METHODS

A detailed review of the literature related to CBT and the phenomena that can alter CBT or are altered by CBT is provided.

RESULTS

Overweight/obesity, sleep disturbance and certain behaviour (e.g. late-night eating, sedentarism) were linked to elevated CBT, especially an elevated nocturnal CBT. A number of existing therapies including drugs (e.g. antidepressants), behavioural therapies (e.g. sleep restriction therapy) and bright light therapy can also reduce CBT.

CONCLUSIONS

An elevation in nocturnal CBT that interferes with sleep onset can parsimoniously explain the development and perpetuation of common co-occurring symptoms, disorders and behaviour including overweight/obesity, sleep disturbance, late-night eating, and sedentarism. Nonetheless, a significant correlation between CBT and the above symptoms, disorders and behaviour does not necessarily imply causation. Thus, statistical and methodological issues of relevance to this enquiry are discussed including the likely presence of autocorrelation.

LEVEL OF EVIDENCE

Level V, narrative review.

The effects of graded levels of calorie restriction: V. Impact of short term calorie and protein restriction on physical activity in the C57BL/6 mouse

Calorie restriction (CR) delays the onset of age-related disease and extends lifespan in a number of species. When faced with reduced energy supply animals need to lower energy demands, which may be achieved in part by reducing physical activity (PA). We monitored changes in PA using implanted transmitters in male C57BL/6 mice in response to graded levels of CR (10 to 40%) or matched levels of graded protein restriction (PR) for 3 months. Mice were fed at lights out and ad libitum controls were limited to dark-phase feeding (12AL) or 24hr/day. Total daily PA declined in a non-linear manner over the first 30 days of CR or PR, remaining stable thereafter. Total daily PA was not related to the level of CR or PR. Total daily PA over the last 20 days of restriction was related to circulating leptin, insulin, tumor necrosis factor-α (TNF-α) and insulin-like growth factor (IGF)-1 levels, measured after 3 months. Mice under restriction showed a high level of activity in the 2hrs before feeding (food anticipatory activity: FAA). FAA followed a complex pattern, peaking around day 20, falling on ∼day 37 then increasing again. FAA was also positively related to the level of restriction and inversely to leptin, insulin, TNF-α and IGF-1. Non-FAA, in contrast, declined over the period of restriction, generally more so in mice under greater restriction, thereby offsetting to some extent the increase in FAA. Mice under PR displayed no changes in PA over time or in comparison to 12AL, and showed no increase in FAA.

Selection for high aerobic capacity has no protective effect against obesity in laboratory mice

Aerobic capacity (VO_2max measured during intensive physical exercise) both trained and intrinsic (i.e. genetically determined) has recently been deemed a good predictor of cardiometabolic risks. However, the underlying mechanisms linking VO_2max and health risk factors are not entirely clear, as it seems that not VO_2max per se, but rather some correlated traits, like spontaneous physical activity (SPA) are responsible for sustaining the lean phenotype. Here we investigated the link between genetically determined aerobic capacity, SPA and resistance to diet-induced health risks using replicated lines of mice selected for high aerobic capacity during swimming in mid-cold water (25°C) and Randomly Bred control mice. After four months of consumption of the western type HFat and HCarb diets and no forced nor voluntary training, we found no evidence of protective effects of intrinsic high VO_2max. The Selected mice displayed similar levels of blood glucose, cholesterol, triglycerides and body fat as the Random Bred control animals. Most notably we found no correlation between VO_2max and SPA levels. Our results therefore call into question the ubiquity of VO_2max as a predictor of metabolic health and leanness, at least in animal models.

Metabolic risk factors in mice divergently selected for BMR fed high fat and high carb diets

Factors affecting contribution of spontaneous physical activity (SPA; activity associated with everyday tasks) to energy balance of humans are not well understood, as it is not clear whether low activity is related to dietary habits, precedes obesity or is a result of thereof. In particular, human studies on SPA and basal metabolic rates (BMR, accounting for >50% of human energy budget) and their associations with diet composition, metabolic thrift and obesity are equivocal. To clarify these ambiguities we used a unique animal model-mice selected for divergent BMR rates (the H-BMR and L-BMR line type) presenting a 50% between-line type difference in the primary selected trait. Males of each line type were divided into three groups and fed either a high fat, high carb or a control diet. They then spent 4 months in individual cages under conditions emulating human "sedentary lifestyle", with SPA followed every month and measurements of metabolic risk indicators (body fat mass %, blood lipid profile, fasting blood glucose levels and oxidative damage in the livers, kidneys and hearts) taken at the end of study. Mice with genetically determined high BMR assimilated more energy and had higher SPA irrespective of type of diet. H-BMR individuals were characterized by lower dry body fat mass %, better lipid profile and lower fasting blood glucose levels, but higher oxidative damage in the livers and hearts. Genetically determined high BMR may be a protective factor against diet-induced obesity and most of the metabolic syndrome indicators. Elevated spontaneous activity is correlated with high BMR, and constitutes an important factor affecting individual capability to sustain energy balance even under energy dense diets.

Placental restriction in multi-fetal pregnancies increases spontaneous ambulatory activity during daylight hours in young adult female sheep

Intrauterine growth restriction (IUGR) has adverse effects on metabolic health and early life, whereas physical activity is protective against later development of metabolic disease. Relationships between birth weight and physical activity in humans, and effects of IUGR on voluntary activity in rodents, are mixed and few studies have measured physical activity in a free-ranging environment. We hypothesized that induced restriction of placental growth and function (PR) in sheep would decrease spontaneous ambulatory activity (SAA) in free-ranging adolescent and young adult progeny from multi-fetal pregnancies. To test this hypothesis, we used Global Positioning System watches to continuously record SAA between 1800 and 1200 h the following day, twice during a 16-day recording period, in progeny of control (CON, n=5 males, 9 females) and PR pregnancies (n=9 males, 10 females) as adolescents (30 weeks) and as young adults (43 weeks). PR reduced size at birth overall, but not in survivors included in SAA studies. In adolescents, SAA did not differ between treatments and females were more active than males overall and during the day (each P<0.001). In adults, daytime SAA was greater in PR than CON females (P=0.020), with a similar trend in males (P=0.053) and was greater in females than males (P=0.016). Adult SAA was negatively correlated with birth weight in females only. Contrary to our hypothesis, restricted placental function and small size at birth did not reduce progeny SAA. The mechanisms for increased daytime SAA in adult female PR and low birth weight sheep require further investigation.

Exploring the Adolescent Fall in Physical Activity: A 10-yr Cohort Study (EarlyBird 41)

INTRODUCTION

Contemporary adolescents are deemed inactive, especially girls, but whether for biological reasons associated with their maturation, changes in their behavior or because of environmental constraints, is uncertain. We examined the trends in physical activity (PA) in relation to both biological and environmental factors in an attempt to establish what drives activity patterns from childhood through adolescence.

METHODS

Physical activity (7-d Actigraph accelerometry) was measured annually from 5 to 15 yr in a single cohort of some 300 UK children. Total PA (TPA; in-school and out-of-school separately and combined as whole day) and intensity-specific PA (sedentary, light, and moderate-and-vigorous [MVPA]) were analyzed. Biological age (years before/after measured peak height velocity) and pubertal stage (self-reported pubic hair development-Tanner staging) were also measured as was socioeconomic status (postcode-derived index of multiple deprivation [IMD]).

RESULTS

Total PA was stable from 5 to 8 yr (trend P = 0.10) but fell progressively from 9 to 15 yr (by approximately 30% in girls and approximately 20% in boys, both P < 0.001; sex interaction, P < 0.01). Half of this fall was attributable to light intensity PA and only a quarter to MVPA. The decline in PA was related similarly to chronological and biological age, whereas pubertal stage explained the more rapid PA decline in girls (puberty-adjusted sex interaction, P = 0.51). Total PA fell to the same extent for in-school and out-of-school settings (both P < 0.001), and for lower and higher IMD areas (both P < 0.001). Total PA tracked moderately to strongly from childhood into adolescence (r = 0.58; P < 0.001).

CONCLUSIONS

The adolescent decline in PA is consistent across different environmental settings, attributable to falls in light-intensity/habitual activity and influenced by puberty, suggesting that the inactivity of adolescence may, in part, be under biological control.

Effect of calorie restriction on spontaneous physical activity and body mass in mice divergently selected for basal metabolic rate (BMR)

Spontaneous physical activity (SPA) represents an important component of daily energy expenditures in animals and humans. Intra-specific variation in SPA may be related to the susceptibility to metabolic disease or obesity. In particular, reduced SPA under conditions of limited food availability may conserve energy and prevent loss of body and fat mass ('thrifty genotype hypothesis'). However, both SPA and its changes during food restriction show wide inter-individual variations. We studied the effect of 30% caloric restriction (CR) on SPA in laboratory mice divergently selected for high (H-BMR) and low (L-BMR) basal metabolic rate. Selection increased SPA in the H-BMR line but did not change it in the L-BMR mice. This effect reflected changes in SPA intensity but not SPA duration. CR increased SPA intensity more strongly in the L-BMR line than in the H-BMR line and significantly modified the temporal variation of SPA. However, the initial between-line differences in SPA were not affected by CR. Loss of body mass during CR did not differ between both lines. Our results show that the H-BMR mice can maintain their genetically determined high SPA under conditions of reduced food intake without sacrificing their body mass. We hypothesize that this pattern may reflect the higher flexibility in the energy budget in the H-BMR line, as we showed previously that mice from this line reduced their BMR during CR. These energy savings may allow for the maintenance of elevated SPA in spite of reduced food intake. We conclude that the effect of CR on SPA is in large part determined by the initial level of BMR, whose variation may account for the lack of universal pattern of behavioural responses to CR.

Physically active rats lose more weight during calorie restriction

Daily physical activity shows substantial inter-individual variation, and low physical activity is associated with obesity and weight gain. Elevated physical activity is also associated with high intrinsic aerobic capacity, which confers considerable metabolic health benefits. Rats artificially selected for high intrinsic aerobic capacity (high-capacity runners, HCR) are more physically active than their low-capacity counterparts (low-capacity runners, LCR). To test the hypothesis that physical activity counters metabolic thriftiness, we measured physical activity and weight loss during three weeks of 50% calorie restriction (CR) in the HCR and LCR rat lines. At baseline, HCR ate more and were more active than LCR; this was seen in male rats, where LCR are considerably heavier than HCR, as well as in a set of female rats where body weight did not differ between the lines, demonstrating that this effect is consistent across sex and not secondary to body weight. We show for the first time that HCR lose more weight than LCR relative to baseline. Physical activity levels declined throughout CR, and this was more pronounced in HCR than in LCR, yet some aspects of activity remained elevated in HCR relative to LCR even during CR. This is consistent with the idea that low physical activity contributes to metabolic thriftiness during food restriction, allowing LCR to defend body mass, particularly lean mass. This has implications for physical activity during diet-induced weight loss, the genetic underpinnings of individual differences in weight loss during a diet, and the potential evolutionary opposition between metabolic thriftiness and aerobic capacity.

Methodological considerations for measuring spontaneous physical activity in rodents

When exploring biological determinants of spontaneous physical activity (SPA), it is critical to consider whether methodological factors differentially affect rodents and the measured SPA. We determined whether acclimation time, sensory stimulation, vendor, or chamber size affected measures in rodents with varying propensity for SPA. We used principal component analysis to determine which SPA components (ambulatory and vertical counts, time in SPA, and distance traveled) best described the variability in SPA measurements. We compared radiotelemetry and infrared photobeams used to measure SPA and exploratory activity. Acclimation time, sensory stimulation, vendor, and chamber size independently influenced SPA, and the effect was moderated by the propensity for SPA. A 24-h acclimation period prior to SPA measurement was sufficient for habituation. Principal component analysis showed that ambulatory and vertical measurements of SPA describe different dimensions of the rodent's SPA behavior. Smaller testing chambers and a sensory attenuation cubicle around the chamber reduced SPA. SPA varies between rodents purchased from different vendors. Radiotelemetry and infrared photobeams differ in their sensitivity to detect phenotypic differences in SPA and exploratory activity. These data highlight methodological considerations in rodent SPA measurement and a need to standardize SPA methodology.

Temporal relationship between insulin sensitivity and the pubertal decline in physical activity in peripubertal Hispanic and African American females

OBJECTIVE

Little attention has been paid to possible intrinsic biological mechanisms for the decline in physical activity that occurs during puberty. This longitudinal observational study examined the association between baseline insulin sensitivity (SI) and declines in physical activity and increases in sedentary behavior in peripubertal minority females over a year.

RESEARCH DESIGN AND METHODS

Participants were Hispanic and African American girls (n = 55; 76% Hispanic; mean age 9.4 years; 36% obese). SI and other insulin indices were measured at baseline using the frequently sampled intravenous glucose tolerance test. Physical activity was measured on a quarterly basis by accelerometry and self-report.

RESULTS

Physical activity declined by 25% and time spent in sedentary behaviors increased by ∼13% over 1 year. Lower baseline SI predicted the decline in physical activity measured by accelerometry, whereas higher baseline acute insulin response to glucose predicted the decline in physical activity measured by self-report. Time spent in sedentary behavior increased by ~13% over 1 year, and this was predicted by lower baseline SI. All models controlled for adiposity, age, pubertal stage, and ethnicity.

CONCLUSIONS

When evaluated using a longitudinal design with strong outcome measures, this study suggests that lower baseline SI predicts a greater decline in physical activity in peripubertal minority females.

The ActivityStat hypothesis: the concept, the evidence and the methodologies

BACKGROUND

The ActivityStat hypothesis suggests that when physical activity is increased or decreased in one domain, there will be a compensatory change in another domain, in order to maintain an overall stable level of physical activity or energy expenditure over time. The ActivityStat debate is gaining momentum in the literature and most of the research to date is based on observational studies.

OBJECTIVE

The objective of this paper is to conceptually clarify the ActivityStat hypothesis and to examine the experimental research aiming to demonstrate or refute compensation using a systematic review process.

METHODS

A systematic review was conducted using electronic database searches with the aim of detecting studies experimentally investigating the ActivityStat hypothesis or compensation in physical activity or energy expenditure. Included studies were critically appraised using a specifically designed tool to address the conceptual considerations of the ActivityStat hypothesis.

RESULTS

Searches identified 28 studies that met the inclusion criteria. Publications spanned 26 years and had multiple methodological approaches, including randomized and non-randomized controlled trials, crossover designs, cluster randomized controlled trials and pre-post trials. Populations of the included studies ranged from children, to adults and the elderly, across a range of weight statuses and used both aerobic, resistance and mixed-exercise interventions. The timeframe of interventions ranged from 1 day to 4 years and outcomes were measured using doubly labelled water, accelerometry, heart rate monitoring, resting metabolic rate, indirect calorimetry, pedometry, subjective recall questionnaire and the activity-related time index. Fifteen of 28 included studies provided evidence of compensation, while 13 did not. Subgroup analyses by population, type and duration of intervention, weight status and study quality also showed mixed findings.

CONCLUSION

There is a substantial body of experimental literature investigating compensation that has largely been overlooked in the ActivityStat debate. However, this evidence is currently inconclusive and lacks a cohesive approach to the question of an ActivityStat. Recommendations for the design of future experimental research investigating the ActivityStat hypothesis are presented.

Mice from lines selectively bred for high voluntary wheel running exhibit lower blood pressure during withdrawal from wheel access

Exercise is known to be rewarding and have positive effects on mental and physical health. Excessive exercise, however, can be the result of an underlying behavioral/physiological addiction. Both humans who exercise regularly and rodent models of exercise addiction sometimes display behavioral withdrawal symptoms, including depression and anxiety, when exercise is denied. However, few studies have examined the physiological state that occurs during this withdrawal period. Alterations in blood pressure (BP) are common physiological indicators of withdrawal in a variety of addictions. In this study, we examined exercise withdrawal in four replicate lines of mice selectively bred for high voluntary wheel running (HR lines). Mice from the HR lines run almost 3-fold greater distances on wheels than those from non-selected control lines, and have altered brain activity as well as increased behavioral despair when wheel access is removed. We tested the hypothesis that male HR mice have an altered cardiovascular response (heart rate, systolic, diastolic, and mean arterial pressure [MAP]) during exercise withdrawal. Measurements using an occlusion tail-cuff system were taken during 8 days of baseline, 6 days of wheel access, and 2 days of withdrawal (wheel access blocked). During withdrawal, HR mice had significantly lower systolic BP, diastolic BP, and MAP than controls, potentially indicating a differential dependence on voluntary wheel running in HR mice. This is the first characterization of a cardiovascular withdrawal response in an animal model of high voluntary exercise.

Testing the activitystat hypothesis: a randomised controlled trial protocol

BACKGROUND

The activitystat hypothesis proposes that when physical activity or energy expenditure is increased or decreased in one domain, there will be a compensatory change in another domain to maintain an overall, stable level of physical activity or energy expenditure. To date, there has been no experimental study primarily designed to test the activitystat hypothesis in adults. The aim of this trial is to determine the effect of two different imposed exercise loads on total daily energy expenditure and physical activity levels.

METHODS

This study will be a randomised, multi-arm, parallel controlled trial. Insufficiently active adults (as determined by the Active Australia survey) aged 18-60 years old will be recruited for this study (n=146). Participants must also satisfy the Sports Medicine Australia Pre-Exercise Screening System and must weigh less than 150 kg. Participants will be randomly assigned to one of three groups using a computer-generated allocation sequence. Participants in the Moderate exercise group will receive an additional 150 minutes of moderate to vigorous physical activity per week for six weeks, and those in the Extensive exercise group will receive an additional 300 minutes of moderate to vigorous physical activity per week for six weeks. Exercise targets will be accumulated through both group and individual exercise sessions monitored by heart rate telemetry. Control participants will not be given any instructions regarding lifestyle. The primary outcome measures are activity energy expenditure (doubly labeled water) and physical activity (accelerometry). Secondary measures will include resting metabolic rate via indirect calorimetry, use of time, maximal oxygen consumption and several anthropometric and physiological measures. Outcome measures will be conducted at baseline (zero weeks), mid- and end-intervention (three and six weeks) with three (12 weeks) and six month (24 week) follow-up. All assessors will be blinded to group allocation.

DISCUSSION

This protocol has been specifically designed to test the activitystat hypothesis while taking into account the key conceptual and methodological considerations of testing a biologically regulated homeostatic feedback loop. Results of this study will be an important addition to the growing literature and debate concerning the possible existence of an activitystat.

TRIAL REGISTRATION

Australian New Zealand Clinical Trials Registry ACTRN12610000248066.

Melanocortin-4 receptor gene polymorphism and the level of physical activity in men (HALS Study)

Melanocortin plays an important role in the energy balance in humans. Actions of melanocortin are exerted through activation of five receptors among which the melanocortin-4 receptor (MC4R) is especially abundant within the central nervous system (CNS). It has been proved that genetic variations of the MC4R gene are associated with the energy intake. Recent data has suggested that MC4R gene polymorphism might influence physical activity/energy expenditure as well. Our aim was to search for associations between MC4R polymorphisms and the level of physical activity. We genotyped MC4R in a population-based cohort of 311 men. The level of physical activity was determined with use of the International Physical Activity Questionnaire. We have found that physical effort expressed as log METs-min/week (corrected for age, BMI and educational status) was 6.61 in men declaring low, 7.56-moderate and 8.96-high level of physical activity. We have not found any associations between the C-2745T MC4R polymorphism and the level of physical activity (P = 0.53). There were no interactions between the level of physical activity and the C-2745T polymorphisms either (P = 0.47). We conclude that the C-2745T genetic polymorphism of the MC4R gene does not influence the level of physical activity in healthy, adult men.

The biological control of voluntary exercise, spontaneous physical activity and daily energy expenditure in relation to obesity: human and rodent perspectives

Mammals expend energy in many ways, including basic cellular maintenance and repair, digestion, thermoregulation, locomotion, growth and reproduction. These processes can vary tremendously among species and individuals, potentially leading to large variation in daily energy expenditure (DEE). Locomotor energy costs can be substantial for large-bodied species and those with high-activity lifestyles. For humans in industrialized societies, locomotion necessary for daily activities is often relatively low, so it has been presumed that activity energy expenditure and DEE are lower than in our ancestors. Whether this is true and has contributed to a rise in obesity is controversial. In humans, much attention has centered on spontaneous physical activity (SPA) or non-exercise activity thermogenesis (NEAT), the latter sometimes defined so broadly as to include all energy expended due to activity, exclusive of volitional exercise. Given that most people in Western societies engage in little voluntary exercise, increasing NEAT may be an effective way to maintain DEE and combat overweight and obesity. One way to promote NEAT is to decrease the amount of time spent on sedentary behaviours (e.g. watching television). The effects of voluntary exercise on other components of physical activity are highly variable in humans, partly as a function of age, and have rarely been studied in rodents. However, most rodent studies indicate that food consumption increases in the presence of wheels; therefore, other aspects of physical activity are not reduced enough to compensate for the energetic cost of wheel running. Most rodent studies also show negative effects of wheel access on body fat, especially in males. Sedentary behaviours per se have not been studied in rodents in relation to obesity. Several lines of evidence demonstrate the important role of dopamine, in addition to other neural signaling networks (e.g. the endocannabinoid system), in the control of voluntary exercise. A largely separate literature points to a key role for orexins in SPA and NEAT. Brain reward centers are involved in both types of physical activities and eating behaviours, likely leading to complex interactions. Moreover, voluntary exercise and, possibly, eating can be addictive. A growing body of research considers the relationships between personality traits and physical activity, appetite, obesity and other aspects of physical and mental health. Future studies should explore the neurobiology, endocrinology and genetics of physical activity and sedentary behaviour by examining key brain areas, neurotransmitters and hormones involved in motivation, reward and/or the regulation of energy balance.

Predictors of physical activity energy expenditure in Afro-Caribbean children

BACKGROUND/OBJECTIVES

We hypothesized that maternal size during pregnancy and birth size are determinants of childhood physical activity energy expenditure (PAEE). Also, childhood PAEE is inversely related to adiposity and levels of cardiovascular risk factors.

SUBJECTS/METHODS

The Vulnerable Windows Cohort Study is a longitudinal observational study of 569 Afro-Jamaican mothers recruited from the first trimester and their offspring. Anthropometry, bioelectrical impedance, PAEE (using the Actical monitor) and cardiovascular risk factors (blood pressure, fasting glucose, insulin and lipids) were measured in 124 boys and 160 girls at a mean age of 13.2 years.

RESULTS

Boys had more fat-free mass (FFM) and expended more energy than girls (12.3±3.3 vs 9.6±2.8 kcal/kg/day; P<0.001). Maternal weight was associated with child's PAEE (r=0.29; P<0.001). PAEE was not significantly associated with birth weight. Maternal weight, after adjusting for child's age and sex, was positively associated with the child's FFM, fat mass and %fat (P-values 0.01). Age- and sex-adjusted PAEE was positively associated with FFM, fat mass and % fat (P-values <0.001), but not after adjusting for current weight. Age- and sex-adjusted PAEE was positively associated with triglycerides, insulin and systolic blood pressure (P-values <0.05), but not after adjusting for weight and height. PAEE was associated with fasting glucose after controlling for age, sex, weight and height (r=-0.12; P=0.02).

CONCLUSIONS

Maternal size, but not birth weight, is a determinant of childhood PAEE. PAEE is not strongly associated with childhood body composition, but is inversely related to fasting glucose concentration.

Environmental influences on children's physical activity: quantitative estimates using a twin design

BACKGROUND

Twin studies offer a 'natural experiment' that can estimate the magnitude of environmental and genetic effects on a target phenotype. We hypothesised that fidgetiness and enjoyment of activity would be heritable but that objectively-measured daily activity would show a strong shared environmental effect.

METHODOLOGY/PRINCIPAL FINDINGS

In a sample of 9-12 year-old same-sex twin pairs (234 individuals; 57 MZ, 60 DZ pairs) we assessed three dimensions of physical activity: i) objectively-measured physical activity using accelerometry, ii) 'fidgetiness' using a standard psychometric scale, and iii) enjoyment of physical activity from both parent ratings and children's self-reports. Shared environment effects explained the majority (73%) of the variance in objectively-measured total physical activity (95% confidence intervals (CI): 0.63-0.81) with a smaller unshared environmental effect (27%; CI: 0.19-0.37) and no significant genetic effect. In contrast, fidgetiness was primarily under genetic control, with additive genetic effects explaining 75% (CI: 62-84%) of the variance, as was parent's report of children's enjoyment of low 74% (CI: 61-82%), medium 80% (CI: 71-86%), and high impact activity (85%; CI: 78-90%), and children's expressed activity preferences (60%, CI: 42-72%).

CONCLUSIONS

Consistent with our hypothesis, the shared environment was the dominant influence on children's day-to-day activity levels. This finding gives a strong impetus to research into the specific environmental characteristics influencing children's activity, and supports the value of interventions focused on home or school environments.

Erythropoietin elevates VO2,max but not voluntary wheel running in mice

Voluntary activity is a complex trait, comprising both behavioral (motivation, reward) and anatomical/physiological (ability) elements. In the present study, oxygen transport was investigated as a possible limitation to further increases in running by four replicate lines of mice that have been selectively bred for high voluntary wheel running and have reached an apparent selection limit. To increase oxygen transport capacity, erythrocyte density was elevated by the administration of an erythropoietin (EPO) analogue. Mice were given two EPO injections, two days apart, at one of two dose levels (100 or 300 microg kg(-1)). Hemoglobin concentration ([Hb]), maximal aerobic capacity during forced treadmill exercise (VO2,max) and voluntary wheel running were measured. [Hb] did not differ between high runner (HR) and non-selected control (C) lines without EPO treatment. Both doses of EPO significantly (P<0.0001) increased [Hb] as compared with sham-injected animals, with no difference in [Hb] between the 100 microg kg(-1) and 300 microg kg(-1) dose levels (overall mean of 4.5 g dl(-1) increase). EPO treatment significantly increased VO2,max by approximately 5% in both the HR and C lines, with no dosexline type interaction. However, wheel running (revolutions per day) did not increase with EPO treatment in either the HR or C lines, and in fact significantly decreased at the higher dose in both line types. These results suggest that neither [Hb] per se nor VO2,max is limiting voluntary wheel running in the HR lines. Moreover, we hypothesize that the decrease in wheel running at the higher dose of EPO may reflect direct action on the reward pathway of the brain.

Maturity-related differences in physical activity among 10- to 12-year-old girls

Besides environmental and psycho-social factors explaining the variation in physical activity levels during adolescence, some evidence suggests that biological processes are involved in regulating habitual daily physical activity and energy expenditure. The purpose of this study was to examine the influence of biological maturity status on physical activity. Chronological age, standing height, sitting height, and body mass were measured cross-sectionally in 268 girls, aged 9.5 to 11.5 years. Biological maturity groups (Early, Average, Late) were created according to estimated age at peak-height-velocity (estAPHV). Habitual physical activity was determined with a pedometer (Yamax Digiwalker SW-200) over a 7-day period. Differences in steps/day across maturity groups were examined by ANCOVA, controlling separately for time the pedometer was worn, leg length, and body mass. Mean pedometer steps/day was 10,822 +/- 2,639. As expected, body size varied by maturity status (e.g., early > average > late). Significant maturity group differences were found with early maturing girls showing lower activity levels compared to average or late maturers. These differences remained after controlling for time the pedometer was worn and leg length; however, the differences were no longer significant when controlling for body mass. The results suggest that biological maturity status influences physical activity levels in girls between 10 and 12 years of age but the relationship is not independent of body mass. Further research is needed to establish the complex inter-relationships among adiposity, biological maturation, and energy expenditure during puberty.

Endurance capacity of mice selectively bred for high voluntary wheel running

Mice from four lines bred for high voluntary wheel activity run approximately 3-fold more revolutions per day and have elevated maximal oxygen consumption during forced treadmill exercise, as compared with four unselected control (C) lines. We hypothesized that these high runner (HR) lines would have greater treadmill endurance-running capacity. Ninety-six mice from generation 49 were familiarized with running on a motorized treadmill for 3 days. On days 4 and 5, mice were given an incremental speed test (starting at 20 m min(-1), increased 1.5 m min(-1) every 2 min) and endurance was measured as the total time or distance run to exhaustion. Blood samples were taken to measure glucose and lactate concentrations at rest during the photophase, during peak nightly wheel running, and immediately following the second endurance test. Individual differences in endurance time were highly repeatable between days (r=0.79), and mice tended to run longer on the second day (paired t-test, P<0.0001). Blood glucose following the treadmill test was low for all animals ( approximately 53 mg dl(-1)) and lactate was high ( approximately 6.5 mmol l(-1)), suggesting that exhaustion occurred. The HR lines had significantly higher endurance than the C lines (1-tailed P<0.05), whether or not body mass was used as a covariate in the analysis. The relationship between line means for wheel running and treadmill endurance differed between the sexes, reinforcing previous studies that indicate sex-specific responses to selective breeding. HR mice appear to have a higher endurance capacity than reported in the literature for inbred strains of mice or transgenics intended to enhance endurance.

Behavioural mechanisms affecting energy regulation in mice prone or resistant to diet- induced obesity

We investigated inbred SWR/J and AKR/J mice, two established models for different susceptibility to diet-induced obesity (DIO), to scrutinize the contribution of physical activity and energy assimilation to the etiology of developing obesity. Body mass gain and body composition of mice fed a high-energy (HE) or a low caloric control diet were monitored. In parallel, assimilated energy, locomotor activity and thermoregulatory behaviour were measured. Activity was continuously registered by radio telemetry and, in addition, Open Field (OF) behaviour was used as a quick screening tool for spontaneous activity before and after the feeding trial. Energy assimilation was increased in both strains on HE (AKR/J: +60.7% and SWR/J: +42.8%) but only in AKR/J, body mass (+8.1%) and fat mass (+40.7%) were significantly elevated. As a trend, total home cage activity was increased and was more scattered in SWR/J. Interestingly, HE stimulated OF activity only in SWR/J in the second trial at the end of the feeding experiment. The spatial pattern of OF activity also differed between strains with obese mice avoiding the core area. Under housing conditions, nest building behaviour was more pronounced in AKR/J. To further evaluate OF behaviour as a marker for spontaneous activity an obese mouse line was investigated. Mice lacking the leptin receptor (db/db) showed already before the onset of obesity lowest activity levels in OF. Adjustment of energy intake, higher activity levels and energy consuming thermoregulatory behaviour are mechanisms employed by SWR/J mice to dissipate excess energy as a defence against the onset of obesity. Therefore our results deciphering mechanisms of DIO-sensitivity in mice contribute to the understanding of inter-individual differences in body weight development in an adipogenic environment.

Central overexpression of leptin antagonist reduces wheel running and underscores importance of endogenous leptin receptor activity in energy homeostasis

We used recombinant adeno-associated virus (rAAV)-mediated gene delivery to overexpress a mutant of rat leptin yielding a protein that acts as a neutral leptin receptor antagonist. The long-term consequences of this overexpression on body weight homeostasis and physical activity, as assessed by voluntary wheel running (WR), were determined in F344 x Brown Norway (BN) rats. Leptin antagonist overexpression was confirmed by examination of mRNA levels in the hypothalamus. Food consumption and body weight gain were exacerbated in the antagonist group during both chow and high-fat feeding periods over the 192-day experiment. In a second experiment, a lower dose of antagonist vector was used that resulted in no change in food consumption but still increased body weight. The degree of antagonist overexpression was sufficient to partially block signal transducer and activator of transcription 3 (STAT3) phosphorylation due to administration of an acute submaximal dose of leptin. Rats were provided free access to running wheels for 4 days during both the chow and high-fat feeding periods. With both antagonist doses and during both chow and high-fat feeding, WR was substantially less with antagonist overexpression. In contrast, when leptin was overexpressed in the hypothalamus, WR activity was increased by greater than twofold. At death, adiposity and serum leptin levels were greater in the antagonist group. These data indicate that submaximal central leptin receptor blockade promotes obesity and diminishes WR activity. These findings underscore the critical role of unrestrained leptin receptor activity in long-term energy homeostasis and suggest that even minor disruption of leptin receptor function can promote obesity.

Day-to-day variability in voluntary wheel running among genetically differentiated lines of mice that vary in activity level

This study examined the day-to-day variability in voluntary wheel-running behavior among three genetically distinct lines of young male and female mice. Daily wheel revolutions were recorded at an age of 6-8 weeks in 10 males and 10 females from each of 3 lines: selectively bred line for high wheel running (Line 8), selectively bred for high wheel-running activity and fixed for a Mendelian recessive allele that reduces hind-limb muscle mass by 50% (Line 3), non-selected control (Line 2). There were significant mean differences in revolutions/day among weeks (P = 0.003), but the effect size was small (10%). Significant main effects for wheel running were also revealed for sex and line (P < 0.001). The grand mean +/- SD for the coefficient of variation (CV) of intra-individual wheel running was 23.0 +/- 10.8%. Although a significant main effect for the CV was found for week, the effect size was low (7%) (age 6 weeks, 23.4 +/- 10.9%; age 7 weeks, 25.1 +/- 13.2%; age 8 weeks, 20.1 +/- 7.8%). The overall mean CV was similar between females (21.4 +/- 9.8%) and males (24.4 +/- 12.0%) and among lines (Line 2, 23.4 +/- 9.8%; Line 3, 20.4 +/- 7.6%; and Line 8, 25.0 +/- 14.4%). These findings are consistent with our previous work in young humans and lend further support for the hypothesis that biological mechanisms influence daily levels of physical activity.

Association between short sleeping hours and physical activity in boys playing ice hockey

OBJECTIVES

To determine physical activity in healthy boys and how physical activity relates to training and daily awake hours.

STUDY DESIGN

In 66 boys (5 to 15 years) affiliated with an ice-hockey club, we measured total daily energy expenditure (TDEE, doubly-labeled water) and basal metabolic rate (ventilated-hood method). Physical activity energy expenditure for the whole day (DAEE), during training, and during spontaneous physical activity was measured by accelerometry and activity protocols. Univariate (UA) and multivariate (MA) correlation analysis was applied.

RESULTS

Physical activity level, DAEE, and TDEE for prepubertal (2.0 and 2.2 Mcal/d) and pubertal (bone age >or=13 years; 1.8 and 2.8 Mcal/d) boys were matched to literature data from normal boys of equal age. In prepubertal boys DAEE correlated positively with awake hours (r(UA) = 0.55, r(MA) = 0.39, P < .01). In pubertal boys this correlation was not significant, the slopes between the 2 groups being significantly different (P = .025). In prepubertal boys spontaneous physical activity expenditure correlated significantly positively with training activity expenditure (r(UA) = 0.72, r(MA) = 0.52, P < .001).

CONCLUSION

Contrary to findings in adults, where short sleepers had lower physical activity and intensive training was negatively compensated reducing spontaneous physical activity, in physically active prepubertal boys, total daily and spontaneous physical activity relate positively to awake hours and training; suggesting child-specific control of physical activity.

Spontaneous physical activity: relationship between fidgeting and body weight control

PURPOSE OF REVIEW

To discuss the potential importance of spontaneous physical activity in regulating body weight and outline possible reasons for the large interindividual variance in spontaneous physical activity.

RECENT FINDINGS

Spontaneous physical activity is highly variable among people, with some having high levels and some low, and can contribute significantly to interindividual differences in total daily energy expenditure. Cross-sectionally, spontaneous physical activity is inversely related to body weight; however, more importantly, spontaneous physical activity is inversely associated with weight gain in prospective studies, and experimental weight perturbations do not appear to change spontaneous physical activity behavior. Spontaneous physical activity is a familial trait and is biologically influenced, although the environment exerts a significant impact.

SUMMARY

Although spontaneous physical activity is a biologically driven behavior, interventions to increase nonexercise activity within the workplace and school hold promise in increasing daily energy expenditure for the average sedentary American. However, many large-scale efforts will need to take place within our sedentary-promoting environment to encourage more daily spontaneous physical activity-related activity.

Genes, exercise, growth, and the sedentary, obese child

It is still not possible to provide an evidence-based answer to the question of whether regular exercise is essential for normal growth. It is also unclear whether very low levels of exercise result in growth deficits. Regular exposure to exercise is characterized by heterogeneity in responsiveness, with most individuals experiencing improvements in fitness traits but a significant proportion showing only very minor gains. Whether a sedentary mode of life during the growing years results in a permanent deficit in cardiorespiratory fitness or a diminished ability to respond favorably to regular exercise later in life remains to be investigated. Although several genes have been associated with fitness levels or response to regular exercise, the quality of the evidence is weak mainly because studies are statistically underpowered. The special case of the obese, sedentary child is discussed, and the importance of the "energy gap" in the excess weight gain during growth is highlighted. Obese, sedentary children have high blood pressure, dyslipidemia, elevated glycemia and type 2 diabetes, hepatic steatosis, respiratory problems, orthopedic complications, and other health disorders more frequently than normal weight, physically active children. The role of genetic differences in the inclination to be sedentary or physically active is reviewed. An understanding of the true role of genetic differences and regular exercise on the growth of children will require more elaborate paradigms incorporating not only DNA sequence variants and exercise exposure but also information on nutrition, programming, and epigenetic events during fetal life and early postnatal years.

Patterns of habitual activity across weekdays and weekend days in 9-11-year-old children

OBJECTIVE

To characterize the pattern of activity in boys and girls across weekdays and weekend days.

METHODS

Physical activity was recorded every 2 s by uniaxial accelerometry in 84 children, aged 9-11 years, for up to four weekdays and two weekend days. Activity bouts (>or= 4 s and >or= 5 min) greater than light (>or= LIGHT), moderate (>or= MOD) and vigorous (>or= VIG) intensity were recorded. The study took place in the South-West of England in 2007.

RESULTS

The mean duration of activity bouts decreased as intensity increased from 11.0+/-1.3 s for >or= LIGHT activity to 6.1+/-1.0 s for >or= VIG activity. The frequency, duration and intensity of bouts were greater in boys than girls, and the frequency and duration of bouts were greater on weekdays than weekend days. Girls accumulated more activity sporadically than boys, whereas boys accumulated more activity in >or= 5-min bouts.

CONCLUSION

Sex differences and weekday/weekend differences in activity were largely due to the intensity of the most frequent bouts of activity and frequency of the most intense bouts. Information regarding the pattern of children's habitual activity can be used to inform activity interventions and assess the aspects of the activity pattern that are related to health.

Determinantes biológicos e sócio-culturais associados à prática de atividade física de adolescentes

A atividade física é um comportamento importante na promoção de saúde e na prevenção de doenças. Para que se desenvolvam programas eficazes no incentivo à prática de atividade física em adolescentes, torna-se necessário que se identifiquem os fatores que a determinam. Este estudo pretende rever alguns dos aspectos do estado atual do conhecimento acerca da influência de determinantes demográfico-biológicos (idade, sexo, estatuto sócio-econômico) e sócio-culturais (família, pares e professor de educação física) na atividade física de adolescentes. Nesta revisão apenas foram incluídos estudos efetuados com amostras superiores a 100 adolescentes com idades entre os 10 e os 18 anos, que tenham adotado delineamentos de pesquisa transversal e que tenham utilizado questionários. Os principais resultados e conclusões foram que: a idade parece estar negativamente associada à atividade física; o sexo masculino tende a estar mais envolvido nessas atividades; o estatuto sócio-econômico elevado parece ser um fator protetor do risco de inatividade física; a participação da família e dos pares em atividades físicas parece estar positivamente associada às atividades por parte dos adolescentes; o professor de educação física parece não representar um fator propiciador da atividade física.

Evolutionary origins of obesity

Although it appeared relatively suddenly, the current obesity epidemic - largely manifest in industrialized societies but now spreading to the rest of the world - is the result of interaction between human biology and human culture over the long period of human evolution. As mammals and primates, humans have the capacity to store body fat when opportunities to consume excess energy arise. But during the millions of years of human evolution such opportunities were rare and transient. More commonly ancestral hominins and modern humans were confronted with food scarcity and had to engage in high levels of physical activity. In tandem with encephalization, humans evolved elaborate and complex genetic and physiological systems to protect against starvation and defend stored body fat. They also devised technological aids for increasing energy consumption and reducing physical effort. In the last century, industrialization provided access to great quantities of mass-produced, high-calorie foods and many labour-saving and transportation devices, virtually abolishing starvation and heavy manual work. In the modern obesogenic environment, individuals possessing the appropriate combination of ancestral energy-conserving genes are at greater risk for overweight and obesity and associated chronic diseases.

Early life determinants of physical activity in 11 to 12 year olds: cohort study

OBJECTIVE

To examine factors in early life (up to age 5 years) that are associated with objectively measured physical activity in 11-12 year olds.

DESIGN

Prospective cohort study.

SETTING

Avon longitudinal study of parents and children, United Kingdom.

PARTICIPANTS

Children aged 11-12 years from the Avon longitudinal study of parents and children.

MAIN OUTCOME MEASURE

Physical activity levels in counts per minute (cpm) and minutes of moderate to vigorous physical activity for seven days measured with a uniaxial actigraph accelerometer.

RESULTS

Valid actigraph data, defined as at least three days of physical activity for at least 10 hours a day, were collected from 5451 children. Several factors were associated with physical activity at ages 11-12 years. Regression coefficients are compared with the baseline of "none" for categorical variables: maternal brisk walking during pregnancy (regression coefficient 5.0, 95% confidence interval -8.5 to 18.5; cpm for <1 h/wk and > or =2 h/wk of physical activity 17.7, 5.3 to 30.1), maternal swimming during pregnancy (21.5, 10.9 to 32.1 and cpm for <1 h/wk and > or =2 h/wk of physical activity 24.2, 7.8 to 40.7), parents' physical activity when the child was aged 21 months (28.5, 15.2 to 41.8 and cpm of physical activity for either parent active and both parents active 33.5, 17.8 to 49.3), and parity assessed during pregnancy (2.9, -7.6 to 13.4 and cpm of physical activity for 1 and > or =2 parity 21.2, 7.1 to 35.3).

CONCLUSIONS

Few factors in early life predicted later physical activity in 11-12 year olds. Parents' physical activity during pregnancy and early in the child's life showed a modest association with physical activity of the child at age 11-12 years, suggesting that active parents tend to raise active children. Helping parents to increase their physical activity therefore may promote children's activity.

Do children take the same number of steps every day?

The purpose of this study was to examine the day-to-day variability in pedometer-assessed physical activity (steps/day). A total of 1,443 children aged 6-12 years from the United States (195 boys, 254 girls), Sweden (257 boys, 252 girls), and Australia (229 boys, 256 girls) wore a pedometer for 4 consecutive weekdays. Repeated measures analysis of variance was used to examine daily differences in steps/day and the coefficient of variation (CV) was calculated for each individual to describe the day-to-day variability. Overall, mean steps/day were higher among boys (14,698 +/- 3,373 steps/day) than girls (12,086 +/- 2,929 steps/day). Significant differences were found between the 4 monitoring days for the entire sample; however, the absolute mean differences were small (55-958 steps) with an overall effect size of 0.01. This trend was apparent regardless of age, gender, and country. Individual CVs ranged from approximately 2 to 88% and the overall mean CV approximated 22%. An age-related increase in the mean CV was observed between 6- and 12-year-old children. The age x gender x country interaction was not significant (P > 0.05). These findings have implications toward the proper design, analysis, and interpretation of studies regarding physical activity among children. Beyond this aspect, our results lend insight into potential age-related biological mechanisms that may also influence daily levels and patterns of physical activity.

Estradiol and tamoxifen reverse ovariectomy-induced physical inactivity in mice

Decreased physical activity and increased body mass are associated with estrogen deficiency.

PURPOSE

To determine whether estrogen or the estrogen analog, tamoxifen, could reverse those detrimental effects after surgical ovariectomy in mice.

METHODS

Ten-week-old C57BL/6 mice were sham operated (sham, N = 6) or ovariectomized (OVX, N = 9). After 4 wk of voluntary wheel running, placebo (OVX-P) or 17beta-estradiol (OVX-E2) pellets were implanted and the mice ran an additional 4 wk. A second study followed in which mice received placebo, 17beta-estradiol, or tamoxifen (OVX-Tam) simultaneously with ovariectomies. Distances run per 24 h and body masses were analyzed by two-way ANOVA with repeated measures.

RESULTS

During the initial 4 wk, OVX mice ran approximately 80% less and had approximately 20% greater body masses compared with sham mice (P < 0.001). Estradiol replacement quickly reversed the inactivity as OVX-E2 mice increased their running from 1.9 +/- 0.3 km x 24 h(-1) to 6.9 +/- 0.7 km within a week of replacement, which was equivalent to shams (8.1 +/- 0.7 km), whereas OVX-P mice ran only 0.5 +/- 0.2 km (P < 0.01). OVX-E2 mice tended to maintain body mass after estradiol replacement, whereas the OVX-P mice continued to increase mass. OVX mice that received tamoxifen had high running activity, approximately 9 km x 24 h(-1), and maintained body mass.

CONCLUSION

The removal of ovarian hormones caused mice to become inactive and gain body mass. Hormone therapy in the form of 17beta-estradiol or tamoxifen rapidly stimulated voluntary wheel running and reversed body mass gains, indicating that estrogen receptor binding was involved in regulating physical activity.

Utility of pedometer step recommendations for predicting overweight in children

OBJECTIVE

The purpose of this study was to examine the utility of pedometer-based physical activity recommendations in predicting childhood adiposity.

DESIGN

Subjects (n=608) (9.6 years) were from two Midwestern USA communities. Physical activity was assessed by a pedometer. The percentage of subjects meeting physical activity recommendations was determined using published recommendations. Overweight and obesity were determined based on reference values for the body mass index (BMI) developed by the International Obesity Task Force. An elevated waist circumference (WC) was determined based on age- and sex-specific reference values >75th percentile.

RESULTS

Children who did not meet the pedometer recommendations were about two times more likely to be overweight/obese and have an elevated WC compared with those meeting recommendations. The BMI and WC were significantly different across pedometer step count groups in males and females.

CONCLUSION

Subjects not meeting the recommendations for steps per day were more likely to be classified with the overweight phenotype than those meeting the recommendation. A dose-response relationship between pedometer steps per day and adiposity is also apparent.

Genetic influences on exercise participation in 37,051 twin pairs from seven countries

BACKGROUND

A sedentary lifestyle remains a major threat to health in contemporary societies. To get more insight in the relative contribution of genetic and environmental influences on individual differences in exercise participation, twin samples from seven countries participating in the GenomEUtwin project were used.

METHODOLOGY

Self-reported data on leisure time exercise behavior from Australia, Denmark, Finland, Norway, The Netherlands, Sweden and United Kingdom were used to create a comparable index of exercise participation in each country (60 minutes weekly at a minimum intensity of four metabolic equivalents).

PRINCIPAL FINDINGS

Modest geographical variation in exercise participation was revealed in 85,198 subjects, aged 19-40 years. Modeling of monozygotic and dizygotic twin resemblance showed that genetic effects play an important role in explaining individual differences in exercise participation in each country. Shared environmental effects played no role except for Norwegian males. Heritability of exercise participation in males and females was similar and ranged from 48% to 71% (excluding Norwegian males).

CONCLUSIONS

Genetic variation is important in individual exercise behavior and may involve genes influencing the acute mood effects of exercise, high exercise ability, high weight loss ability, and personality. This collaborative study suggests that attempts to find genes influencing exercise participation can pool exercise data across multiple countries and different instruments.

Within- and between-individual variability in estimated energy expenditure and habitual physical activity among young adults

OBJECTIVE

To examine the within- and between-individual variability in the estimated total energy expenditure (TEE) and habitual physical activity (PA) in free-living young adults. We were specifically interested in the day-to-day variation.

DESIGN

Cross-sectional.

SETTING

University.

SUBJECTS

A total of 277 (125 males, 152 females) participants aged 18-24 years.

INTERVENTIONS

None.

MAIN OUTCOME MEASURES

Subjects recorded their daily PA with the Bouchard 3-day diary. Descriptive statistics for TEE, moderate-to-vigorous physical activity (MVPA), inactivity (IA), and activity energy expenditure (AEE) were calculated and presented as a frequency distribution. Within- and between-individual variability were tested by RMANOVA and also presented as the coefficient of variation (CV).

RESULTS

Mean values for TEE, MVPA, AEE, and IA were comparable between males and females; however, considerable between-individual variability existed. Only MVPA showed a significant difference between sexes (P<0.05), due to slightly greater vigorous PA in males compared to females (8.0 and 6.8 kcal/kg/day, respectively; P=0.12). Results from the RMANOVA revealed no significant day-to-day variability in any TEE or PA variable and the CVs were not significantly different between males and females. TEE and IA displayed mean CVs of approximately 12% while mean CVs for AEE and MVPA approximated 32 and 84%, respectively.

CONCLUSIONS

Results from this study suggest (1) a limited day-to-day variability in TEE and (2) a large degree of between-individual variation for TEE and PA exist. This study highlights the importance of understanding the variability associated with TEE, which is important for PA measurement and proper PA and nutritional recommendations.

SPONSORSHIP

None.

Individual differences in physical activity are closely associated with changes in body weight in adult female rhesus monkeys (Macaca mulatta)

The increased prevalence of overweight adults has serious health consequences. Epidemiological studies suggest an association between low activity and being overweight; however, few studies have objectively measured activity during a period of weight gain, so it is unknown whether low activity is a cause or consequence of being overweight. To determine whether individual differences in adult weight gain are linked to an individual's activity level, we measured activity, via accelerometry, over a prolonged period (9 mo) in 18 adult female rhesus monkeys. Weight, food intake, metabolic rate, and activity were first monitored over a 3-mo period. During this period, there was mild but significant weight gain (5.5 +/- 0.88%; t =-6.3, df = 17, P < 0.0001), whereas caloric intake and activity remained stable. Metabolic rate increased, as expected, with weight gain. Activity level correlated with weight gain (r = -0.52, P = 0.04), and the most active monkeys gained less weight than the least active monkeys (t = -2.74, df = 8, P = 0.03). Moreover, there was an eightfold difference in activity between the most and least active monkeys, and initial activity of each monkey was highly correlated with their activity after 9 mo (r = 0.85, P < 0.0001). In contrast, food intake did not correlate with weight gain, and there was no difference in weight gain between monkeys with the highest vs. lowest caloric intake, total metabolic rate, or basal metabolic rate. We conclude that physical activity is a particularly important factor contributing to weight change in adulthood and that there are large, but stable, differences in physical activity among individuals.

NEAT--non-exercise activity thermogenesis--egocentric & geocentric environmental factors vs. biological regulation

Non-exercise activity thermogenesis (NEAT) is the energy expenditure of all physical activities other than volitional sporting-like exercise. NEAT includes all those activities that render us vibrant, unique and independent beings such as going to work, playing guitar, toe-tapping and dancing. The factors that account for the 2000 kcal day(-1) variability of NEAT can be categorized as environmental or biological. The environmental determinants of NEAT can be view using one of two models. In the egocentric model we consider a single person as the focus, e.g. 'my job'. In the geocentric model we consider the 'environment' as the focus, e.g. well-lit and safe walk ways. These models provide us with a theoretical framework to understand NEAT and how best to intervene to promote NEAT. As well as environmental effectors of NEAT, there are also biological regulatory mechanisms that enable us to account for three-quarters of the biological variance in susceptibility and resistance to fat gain with human over-feeding. NEAT is likely to be regulated through a central mechanism that integrates NEAT with energy intake and energy stores so that NEAT is activated with over-feeding and suppressed with under-feeding. In conclusion, NEAT is likely to serve as a crucial thermoregulatory switch between energy storage and dissipation that is biologically regulated and influenced, and perhaps over-ridden, by environment. Deciphering the role of NEAT may lead to a better understanding of the pathogenesis, prevention and treatment of obesity.