Psychological predictors of opportunistic snacking in the absence of hunger

Association between ultra-processed food and snacking behavior in Brazil

PURPOSE

Ultra-processed food may play a role in facilitating snacking behavior because of their convenience and low satiety potential. This study aimed to describe the association between consumption of ultra-processed foods and frequency of snacking.

METHODS

We analyzed data from 46,164 participants (≥ 10 years old) in the 2017-2018 Brazilian Household Budget Survey. Dietary data were collected by 24-h dietary recalls over one or two days for each participant. We estimated energy intake, ultra-processed food consumption, and level of snacking. We measured the association between ultra-processed food consumption and level of snacking using multinomial logistic regression, stratified by age group (adolescents, 10-19 years old; adults, 20-64 years old; elders, 65 or older).

RESULTS

We found a statistically significant tendency of increased daily energy intake and consumption of snacks and that ultra-processed food consumption was positively associated with the level of snacking for all age groups. For adolescents, adults, and elders in the highest quintile of ultra-processed food consumption as a share of their entire diet, the relative risk ratio (95% CI) of having more than two snacks per day compared to no snacks was 14.21 (9.09-22.21), 4.44 (3.54-5.57), and 4.21 (2.67-6.64), respectively, when compared to the lowest quintile.

CONCLUSION

Higher consumption of ultra-processed food was associated with snacking behavior, and the strength of this association was stronger among adolescents. Efforts to mitigate ultra-processed food attributes that facilitate snacking should be incorporated into strategies to promote healthier food choices, especially among adolescents.

Validation of the Food Purchase Task (FPT) in a clinical sample of smokers with overweight and obesity

Obesity is a major health problem associated with disease burden and mortality. In this context, analyzing food as a powerful reinforcer from a behavioral economics framework could be relevant for the treatment and prevention of obesity. The purposes of this study were to validate a food purchase task (FPT) in a clinical sample of Spanish smokers with overweight and obesity and to assess the internal structure of the FPT. We also analyzed the clinical utility of single-item breakpoint (i.e., commodity price that suppresses demand). A total of 120 smokers [% females: 54.2; M_age = 52.54; SD = 10.34] with overweight and obesity completed the FPT and weight/eating-related variables. Principal component analysis was used to examine the FPT structure, and a set of correlations were used to examine the relationship between the FPT, eating and weight-related variables. The FPT demonstrated robust convergent validity with other measures of eating. Higher food demand was related to higher food craving (r = .33), more binge eating problems (r = 0.39), more weight gain concerns (r = 0.35), higher frequency of both controlled (r = 0.37) and uncontrolled (r = 0.30) grazing, as well as to an eating style in response to emotions (r = 0.34) and external eating (r = 0.34). Of the demand indices, Intensity and O_max showed the highest magnitudes of effects. The FPT factors, persistence and amplitude, do not improve individual FPT indices; and the single-item breakpoint was not related to any eating or weight variable. The FPT is a valid measure of food reinforcement with potential clinical utility in smokers with obesity/overweight.

Added flavors: potential contributors to body weight gain and obesity?

BACKGROUND

Added flavors are a marker for ultra-processing of food and a strong link exists between the intake of ultra-processed food and the development of obesity. The objective of the present article is to assess animal and human data elucidating the impact of added flavors on the regulation of food intake and body weight gain, as well as to define areas for future research.

MAIN TEXT

Mechanistic studies suggest that added flavors induce overeating and body weight gain by two independent mechanisms: Added flavors promote hedonic eating and override homeostatic control of food intake, as well as disrupt flavor-nutrient learning and impair the ability to predict nutrients in food items. Supporting these potential mechanisms, added flavors increase feed intake and body weight as compared to non-flavored control diets in a broad range of animal studies. They are actively promoted by feed additive manufacturers as useful tools to improve palatability, feed intake, and performance parameters. In humans, added flavors are extensively tested concerning toxicity; however, no data exist concerning their impact on food intake and body weight.

CONCLUSIONS

Added flavors are potential contributors to the obesity epidemic and further studies focusing on their role in humans are urgently required. These studies include obesity interventions specifically targeting food items with added flavors and cohort studies on independent associations between added flavor intake and metabolic, as well as cardiovascular, morbidity, and mortality.

Feasibility of a multiple-component mindfulness intervention for Chinese adolescents living with overweight: A pilot randomized trial

The prevalence of obesity among Chinese adolescents is rising rapidly, and theoretically informed, scalable weight management interventions are needed. We developed and evaluated the feasibility and preliminary effects of an mHealth nutrition education and mindful snacking intervention for weight loss and improved dietary practices among Chinese adolescents with overweight. We examined whether including implementation intention formation (using if-then plans) improved outcomes. With user consultation, we created a 3-week mindful eating intervention delivered as 10 videos to user smartphones. Participants (n = 55) were randomly assigned to mindful eating or mindful eating + planning. Forty-six (83.6%) participants (age = 16.35 ± 0.48 years; body mass index [BMI] = 25.79 ± 2.05 kg/m² ) completed the intervention. Both groups exhibited significant pre- to post-intervention weight loss (M = 1.42 and 1.79 kg, respectively); decreases in snacking frequencies, emotional eating, external eating, and trait craving; and significant increases in mindful eating and eating self-efficacy. No significant intervention group differences were observed. User experience data (n = 16) indicated acceptability and meaningful behavior change. Findings suggest that a smartphone-delivered mindfulness-based intervention for Chinese adolescents living with overweight is feasible and efficacious.

Pregnant Women Consume a Similar Proportion of Highly vs Minimally Processed Foods in the Absence of Hunger, Leading to Large Differences in Energy Intake

BACKGROUND

The eating in the absence of hunger (EAH) experimental paradigm measures intake of highly palatable, highly processed foods when sated. However, no studies have examined EAH in pregnant women.

OBJECTIVE

The objectives were to investigate whether EAH in pregnant women differs by level of food processing and to examine relationships of EAH with hedonic hunger, addictive-like eating, and impulsivity.

DESIGN

EAH was assessed in a counterbalanced crossover feeding substudy in which participants completed two free-access eating occasions following a standardized meal during their second pregnancy trimester. Hedonic hunger (Power of Food Scale), addictive-like eating (modified Yale Food Addiction Scale), and impulsivity (Barratt Impulsiveness Scale-15) were assessed by self-report during early pregnancy.

PARTICIPANTS AND SETTING

Data were collected from March 2015 through September 2016 from a subsample of participants (n = 46) enrolled at ≤12 weeks gestation in an observational, prospective cohort study (the Pregnancy Eating Attributes Study) in North Carolina.

INTERVENTION

Participants were presented with highly processed and minimally processed foods in two separate assessments.

MAIN OUTCOME MEASURES

Energy intake (EAH-kcal) and percent consumed (EAH-%) (calculated as 100 × [amount consumed (g) / amount served (g)]) was measured overall and separately for sweet and savory foods.

STATISTICAL ANALYSES PERFORMED

Linear mixed models estimated the effect of condition on EAH. Hedonic hunger, addictive-like eating, impulsivity and their interaction were examined separately.

RESULTS

EAH-% was similar across conditions (16.3% ± 1.1% highly processed vs 17.9% ± 1.2% minimally processed; P = 0.76), resulting in 338.5 ± 34.2 kcal greater energy intake in the highly processed vs minimally processed condition (P < 0.001). Hedonic hunger was not significantly associated with EAH; reward-related eating was positively associated with EAH-kcal and EAH-% of savory foods, and Barratt Impulsivity was positively associated with EAH-kcal and EAH-% overall, and with EAH-% of sweet foods (P < 0.05). There was little evidence of an interaction of Barratt Impulsivity with hedonic hunger or reward-related eating.

CONCLUSIONS

EAH in pregnant women occurs for both highly processed and minimally processed foods and correlates positively with self-reported addictive-like eating, but not hedonic hunger. Impulsivity did not modify associations of addictive-like eating with EAH in this sample.

Using pre-prandial blood glucose to assess eating in the absence of hunger in free-living individuals

Our ability to understand and intervene on eating in the absence of hunger (EAH) as it occurs in peoples' natural environments is hindered by biased methods that lack ecological validity. One promising indicator of EAH that does not rely on self-report and is easily assessed in free-living individuals is glucose. Here, we hypothesize that elevated pre-prandial blood glucose concentrations (PPBG), which reflect a source of readily-available, short-term energy, are a biological indicator of EAH. This was a 7-day observational study of N = 41, 18-24 year old men and women with BMI < 25 kg/m² (60%) or BMI ≥ 25 kg/m² (40%). We collected data using ecological momentary assessment from people in their natural environments. We defined EAH by self-report (perceived EAH) and by PPBG thresholds using two methods (standardized, PPBG < 85 mg/dl; personalized, PPBG < individual fasting levels). Multilevel modeling was used to analyze the data. N = 963 eating events were reported. There were significantly (p < .05) fewer perceived EAH events (25%) as compared to standardized (62%) and personalized PPBG-defined EAH events (51%). Consistent with published literature, perceived EAH was more likely to occur at a higher PPBG (p < .01), particularly among participants with a BMI ≥ 25 kg/m² (p_int < .01). Additionally, discordance between perceived EAH and PPBG-defined EAH, indicating a perception of hunger at an eating event when PPBS was elevated, was less likely among participants with a BMI < 25 kg/m² vs. those with a BMI ≥ 25 kg/m² (p_int < .01) as well as at snacks vs. meals (p_int < .01). These findings provide preliminary support for using PPBG as a biological indicator of EAH in free-living individuals.

Psychosociocultural Contributors to Maladaptive Eating Behaviors in African American Youth: Recommendations and Future Directions

Childhood overweight and obesity disproportionately affects African Americans, and these children benefit less from standard pediatric weight management treatment programs compared to other racial/ethnic groups. Maladaptive eating behavior has been identified as a behavioral contributor to obesity and is also associated with the development of nonrestrictive eating disorders over time. Unique psychosociocultural factors have been identified that may promote higher risk for maladaptive eating behaviors in African American children beyond the effects of economic disparity. To best treat this group, it is important for practitioners to have a thorough understanding of these factors. We review several of these considerations and describe ways they may interact to contribute to the subsequent development of maladaptive eating behaviors and increased weight. Recommendations are made regarding how attention to these factors could be incorporated into current pediatric weight management treatments to better serve this population via a patient-centered care approach. Future directions will also be discussed.

When the camera eats first: Exploring how meal-time cell phone photography affects eating behaviours

Advances in cell phone technology have the potential to disrupt eating patterns. In this research, we focused on the camera function of a cell phone, characterizing: (i) the extent to which this function is used during meals; (ii) whether meal-time photographers show signs of pathological eating; and (iii) whether the act of taking food photographs alters the amount and enjoyment of food eaten. In the first study, we used the experience sampling method to track one week of meals from 137 young adults. Although we observed a low base rate of meal-time photography (5.44% of the 1140 meals captured), phone users who engaged in this practice had higher external eating scores than those who did not. That is, these meal-time photographers were more likely to eat in response to external cues (e.g. the sight of palatable food) than to internal cues of hunger. However, when participants were randomly assigned to take either food or non-food photographs within a lab setting (Study 2), we found no evidence that the type of photography influenced either the amount or enjoyment of food eaten. Taken together, our findings suggest a limited role for cell phone photography in an obesogenic environment.

ERP and oscillatory differences in overweight/obese and normal-weight adolescents in response to food stimuli

BACKGROUND

Findings are mixed regarding the association of electroencephalographic (EEG) attentional bias measures and body weight, with few studies measuring food craving or intake and no study reporting oscillatory measures.

METHODS

EEG data were collected while 28 satiated adolescents (14 overweight/obese) viewed pictures of neutral, low-calorie food, and high-calorie food stimuli and rated their desire to eat, before having access to high-calorie snacks.

RESULTS

Unlike normal-weight adolescents, overweight/obese participants showed similar P300 amplitudes for high- and low-calorie food, and strongest event-related alpha band desynchronization for low-calorie stimuli. P300 amplitudes and state craving for low-calorie food furthermore predicted snack intake in this group.

CONCLUSIONS

The current research focus in overweight/obesity might need to be extended to include low-calorie food. While all participants showed an attentional bias for high-calorie food, it was the processing of low-calorie food which distinguished the two weight groups on measures of neural activity and which was associated with snack food intake in the overweight/obese group.

Obesity and Eating Disturbance: the Role of TFEQ Restraint and Disinhibition

PURPOSE OF REVIEW

Literature from the past five years exploring roles of Three-Factor Eating Questionnaire (TFEQ) Restraint and Disinhibition in relation to adult obesity and eating disturbance (ED) was reviewed.

RECENT FINDINGS

Restraint has a mixed impact on weight regulation, diet quality, and vulnerability to ED, where it is related detrimentally to weight regulation, diet, and psychopathology, yet can serve as a protective factor. The impact of Disinhibition is potently related to increased obesity, poorer diet, hedonically driven food choices, and a higher susceptibility to ED. Restraint and Disinhibition have distinct influences on obesity and ED and should be targeted differently in interventions. Further work is required to elucidate the mechanisms underlying TFEQ eating behavior traits.

Factors behind healthy snack consumption at school among high-school students: a qualitative study

BACKGROUND

The prevalence of consuming fast foods and non-nutritious snacks is progressively increasing among adolescents. This study aimed to explore factors behind snack consumption at school among Iranian high-school students.

METHODS

This descriptive qualitative study was conducted in 2017 in four boys' and four girls' high-schools located in Isfahan, Khorramabad, and Tehran, Iran. Data were collected through 42 in-depth semi-structured interviews and four focus groups with male and female students, their parents, and their school teachers and administrators. The conventional content analysis approach was used for data analysis. Trustworthiness was applied to the study through prolonged engagement, maximum variation sampling, and member checking techniques.

RESULTS

Factors behind students' snack consumption came into two main groups, namely influential behaviors, and influential emotions and perceptions. Influential behaviors included the behaviors of students, their family members, peers, school administrators, and snack sellers. Moreover, influential emotions and perceptions included positive and negative feelings towards healthy snacks, fear over the consequences of unhealthy snacks, and perceived positive outcomes of healthy snacks.

CONCLUSIONS

Students' snack consumption at school is affected not only by their own behaviors, emotions, and perceptions, but also by significant others' behaviors and environmental factors. School administrators need to make environmental modifications to turn school environment into a pleasant place for healthy snack consumption and make healthy snack consumption a pleasurable experience for students.

Breaking habits with mindful snacking? An email-based intervention targeting unwanted snacking habits in an Australian sample

OBJECTIVE

To investigate the potential for an email-based, mindful eating exercise to improve unwanted snacking habits.

METHOD

Australian participants (N = 78, 86% female) with unwanted snacking habits engaged in a mindful eating email-based intervention, over a practice period of two weeks. All measures were completed using an online survey. Habit strength was measured using the Self-Report Habit Index (SHRI). Mindful eating was measured using the Mindful Eating Questionnaire (MEQ) and self-compassion was measured using the Self-Compassion Scale (SCS). Participants were emailed a mindful eating exercise and asked to make action plans to practice it. Reminders to perform the task were sent via email after one week. After two weeks, the MEQ, SCS and SHRI were readministered.

RESULTS

Habit strength significantly increased over the two-week period and self-compassion significantly decreased. The amount of practice reported was not associated with these changes, but self-reported effort expended during practice sessions was partially associated.

DISCUSSION

The intervention resulted in increased habit strength; potential mechanisms underlying these changes are discussed. Decreased self-compassion may be linked to both the electronic delivery of the intervention and the nature of the mindful eating task.

Problematic Eating Behaviors Are More Prevalent in African American Women Who Are Overweight or Obese Than African American Women Who Are Lean or Normal Weight

Problematic eating behaviors such as overeating and loss of control over consumption can lead to obesity. Problematic eating behaviors among women of differing body mass indexes were explored through focus group methodology, the Palatable Eating Motives Scale (PEMS), and a taste test in a sample of low-income African American women (n = 45). Women who were overweight or obese (W-O/O) reported more problematic eating behaviors including eating in the absence of hunger, frequent overeating, and increased food thoughts than women who were lean or normal weight (W-L/N). The W-O/O appear to possess more problematic eating behaviors than W-L/N.

Top-down guidance of attention to food cues is enhanced in individuals with overweight/obesity and predicts change in weight at one-year follow up

Background

Changing eating behaviour may be challenging for individuals with obesity and this may be related to attentional bias towards food. Previous paradigms used to assess attentional bias to food stimuli have not distinguished between bottom-up processes related to assessment of rewarding stimuli versus top-down processes related to effects of mind-set on attention. We investigated whether attentional bias for food cues varies between individuals with overweight/obesity and healthy weight individuals, due to differential top-down control of attention. We also determined whether top-down biases predict food consumption in the lab and weight change in our sample over one-year.

Methods

Forty-three participants with overweight/obesity and 49 healthy weight participants between the ages of 18 and 58 participated. Participants completed two attention tasks in a counterbalanced order: (i) a priming task assessing bottom-up control of attention and (ii) a working memory task assessing top-down control of attention. Eating behaviour was assessed by a taste test. At one-year follow-up participants returned to the laboratory to assess changes in their body mass index (BMI).

Results

The healthy weight and overweight/obese groups did not differ in demographics and baseline measures (appetite, food liking, taste test food intake). Participants with overweight/obesity showed greater top-down attentional bias towards food cues than did healthy weight participants but had no difference in bottom-up attentional bias. Top down attentional bias towards food cues predicted weight change over one-year but did not predict food intake in the taste test.

Conclusions

The present findings illustrate that the relationship between attentional bias for food, food intake, and body weight is complex. Top-down effects of mind-set on attention, rather than bottom-up control of attention to food may contribute to patterns of eating that result in development and/or maintenance of overweight/obesity. Interventions targeted at top down biases could be effective in facilitating prevention of weight gain.

The benefits of defining "snacks"

Whether eating a "snack" is considered a beneficial or detrimental behavior is largely based on how "snack" is defined. The term "snack food" tends to connote energy-dense, nutrient-poor foods high in nutrients to limit (sugar, sodium, and/or saturated fat) like cakes, cookies, chips and other salty snacks, and sugar-sweetened beverages. Eating a "snack food" is often conflated with eating a "snack," however, leading to an overall perception of snacks as a dietary negative. Yet the term "snack" can also refer simply to an eating occasion outside of breakfast, lunch, or dinner. With this definition, the evidence to support health benefits or detriments to eating a "snack" remains unclear, in part because relatively few well-designed studies that specifically focus on the impact of eating frequency on health have been conducted. Despite these inconsistencies and research gaps, in much of the nutrition literature, "snacking" is still referred to as detrimental to health. As discussed in this review, however, there are multiple factors that influence the health impacts of snacking, including the definition of "snack" itself, the motivation to snack, body mass index of snack eaters, and the food selected as a snack. Without a definition of "snack" and a body of research using methodologically rigorous protocols, determining the health impact of eating a "snack" will continue to elude the nutrition research community and prevent the development of evidence-based policies about snacking that support public health.

Objective quantification of the food proximity effect on grapes, chocolate and cracker consumption in a Swedish high school. A temporal analysis

Close food proximity leads to increased short-term energy intake, potentially contributing to the long-term development of obesity. However, its precise effects on eating behaviour are still unclear, especially with food available for extended periods of time. This study involved two similar high school student groups (15–17 years old), which had ad libitum access to grapes, chocolates and crackers during an hour-long experimental session. In the distal condition the foods were placed 6 meters away from the students (n = 24), in contrast to the proximal condition (n = 17) were the food was placed near the students. The identification of the type and the quantification of the amount of each food selected, for each individual serving, was facilitated through use of food scales and video recording. In the proximal condition individuals served themselves grapes and crackers more often and consumed more chocolate than in the distal condition. In total, participants in the proximal condition ingested significantly more energy (726 kcal vs. 504 kcal; p = 0.029), without reporting higher fullness. Food proximity also affected the temporal distribution of servings, with the first five minutes of the sessions corresponding to 53.1% and 45.6% of the total energy intake for the distal and proximal conditions, respectively. After the first five minutes, the servings in the distal condition were strongly clustered in time, with many students getting food together. In the proximal condition however, students displayed an unstructured pattern of servings over time. In conclusion, this study strengthens past evidence regarding the important role of food proximity on individual energy intake and, for the first time, it associates continuous food proximity to the emergence of unstructured eating over time. These conclusions, expanded upon by future studies, could support the creation of meaningful intervention strategies based on spatially and temporally controlled food availability.

Contextual factors associated with eating in the absence of hunger among adults with obesity

Eating in the absence of hunger (EAH) is under-explored in adults with obesity. In this study, 50 adults with obesity recorded eating episodes and theoretically-relevant environmental, perceptual, and emotional correlates in the natural environment for 2weeks via ecological momentary assessment (EMA). Generalized linear models and mixed models were used to characterize correlates and consequences of EAH vs. non-EAH episodes/tendencies (within-subjects and between-subjects effects, respectively), time of day, and time of day×EAH interactions. Approximately 21% of EMA-recorded eating episodes involved EAH, and 70% of participants reported at least 1 EAH episode. At the within-person level, participants' EAH episodes were associated with greater self-labeled overeating than their non-EAH episodes. At the between-person level, participants who tended to engage in more EAH reported less self-labeled overeating than those who engaged in less EAH. Across EAH and non-EAH episodes, eating in the evening was associated with overeating, expecting eating to be more rewarding, greater alcoholic beverage consumption, eating alone, eating because others are eating, and eating while watching television. Significant EAH×time of day interactions were also observed but the pattern of findings was not consistent. Findings suggest that EAH may be a relevant target for reducing food intake in individuals with obesity given its high prevalence and association with perceptions of overeating, although results should be extended using objective measures of food intake. Associations between evening eating episodes and perceptual and environmental factors should be further explored.

The neurobiological and behavioral overlaps of nicotine and food addiction

Both cigarette smoking and obesity are significant public health concerns and are associated with increased risk of early mortality. It is well established that the mesolimbic dopamine pathway is an important component of the reward system within the brain and is implicated in the development of addiction. Indeed, nicotine and highly palatable foods are capable of altering dopamine release within this system, engendering addictive like responses in susceptible individuals. Although additional research is warranted, findings from animal and human literature have elucidated many of neuroadaptions that occur from exposure to nicotine and highly palatable foods, leading to a greater understanding of the underlying mechanisms contributing to these aberrant behaviors. In this review we present the findings taken from preclinical and clinical literature of the known effects of exposure to nicotine and highly palatable foods on the reward related circuitry within the brain. Further, we compare the neurobiological and behavioral overlaps between nicotine, highly palatable foods and obesity. Lastly, we examine the stigma associated with smoking, obesity and food addiction, and the consequences stigma has on the overall health and wellbeing of an individual.

What Is a Snack, Why Do We Snack, and How Can We Choose Better Snacks? A Review of the Definitions of Snacking, Motivations to Snack, Contributions to Dietary Intake, and Recommendations for Improvement

Around the world, adults consume energy outside of traditional meals such as breakfast, lunch, and dinner. However, because there is no consistent definition of a "snack," it is unclear whether those extra eating occasions represent additional meals or snacks. The manner in which an eating occasion is labeled (e.g., as a meal or a snack) may influence other food choices an individual makes on the same day and satiety after consumption. Therefore, a clear distinction between "meals" and "snacks" is important. This review aims to assess the definition of extra eating occasions, to understand why eating is initiated at these occasions, and to determine what food choices are common at these eating occasions in order to identify areas for dietary intervention and improvement. Part I of this review discusses how snacking is defined and the social, environmental, and individual influences on the desire to snack and choice of snack. The section concludes with a brief discussion of the associations of snacking with cardiometabolic health markers, especially lipid profiles and weight. Part II addresses popular snack choices, overall snacking frequencies, and the demographic characteristics of frequent snackers in several different countries. This review concludes with a recommendation for nutrition policymakers to encourage specific health-promoting snacks that address nutrient insufficiencies and excesses.

A systematic review of the relationship between eating, weight and inhibitory control using the stop signal task

Altered inhibitory control (response inhibition, reward-based inhibition, cognitive inhibition, reversal learning) has been implicated in eating disorders (EDs) and obesity. It is unclear, however, how different types of inhibitory control contribute to eating and weight-control behaviours. This review evaluates the relationship between one aspect of inhibitory control (a reactive component of motor response inhibition measured by the stop signal task) and eating/weight in clinical and non-clinical populations. Sixty-two studies from 58 journal articles were included. Restrained eaters had diminished reactive inhibitory control compared to unrestrained eaters, and showed greatest benefit to their eating behaviour from manipulations of inhibitory control. Obese individuals may show less reactive inhibitory control but only in the context of food-specific inhibition or after executive resources are depleted. Of the limited studies in EDs, the majority found no impairment in reactive inhibitory control, although findings are inconsistent. Thus, altered reactive inhibitory control is related to some maladaptive eating behaviours, and hence may provide a therapeutic target for behavioural manipulations and/or neuromodulation. However, other types of inhibitory control may also contribute. Methodological and theoretical considerations are discussed.