Aerobic conditioning effects on substrate responses during graded cycling in pregnancy

[Review of nutrition and hydration in relation to physical exercise during pregnancy]

Introduction

Studies on nutritional recommendations for pregnant women who exercise are scarce. The objective of this article is not to focus on the diet of pregnant women as a whole, but to review those aspects of it that may be related to physical exercise. To this end, the nutritional and hydration recommendations contained in the main clinical practice guides on physical exercise during pregnancy are collected, including the first Spanish guides. Likewise, the energy requirements required by pregnant women who perform physical exercise during pregnancy for adequate gestational weight gain are addressed, aspects related to macronutrients in the aforementioned population group, and two specific topics, such as nutritional needs in the adolescent who practices physical exercise during pregnancy and eating disorders in pregnant athletes It is concluded by stating that pregnant women who exercise regularly should eat a varied and balanced diet, such as the Mediterranean diet, avoid long periods of fasting to avoid the appearance of hypoglycemia and maintain adequate fluid intake before, during and after physical exercise. Pregnant adolescents who engage in physical exercise require nutritional supervision to achieve adequate gestational weight gain. The pregnant athlete with an eating disorder has a higher risk of complications during pregnancy and childbirth, gynecological, fetal and neonatal, and, therefore, requires close monitoring by specialists in maternal-fetal medicine.

A narrative review of exercise dose during pregnancy

The current recommendations for prenatal exercise dose align with those from the American College of Sports Medicine; 150 min of moderate intensity every week of pregnancy. However, recent works suggest there may be a dose-dependent beneficial effect for mother and offspring; maternal and offspring health outcomes respond differently to low, medium, and high doses of prenatal exercise. It is, therefore, our aim to summarize the published evidence (years 1950-2023) for five metrics of prenatal exercise training commonly reported, that is, "FITT-V": Frequency (number of sessions), Intensity (metabolic equivalents "METs"), Time (duration of sessions), Type (exercise mode), Volume (exercise MET*mins). The target audience includes clinicians and health care professionals, as well as exercise professionals and physiologists. Data suggest that moderate exercise frequency (3-4 times weekly) appears safe and efficacious for mother and offspring, while there is contradictory evidence for the safety and further benefit of increased frequency beyond 5 sessions per week. Moderate (3-6 METs) and vigorous (>6 METs) intensity prenatal exercise have been shown to promote maternal and offspring health, while little research has been performed on low-intensity (<3 METs) exercise. Exercise sessions lasting less than 1 hr are safe for mother and fetus, while longer-duration exercise should be carefully considered and monitored. Taken together, aerobic, resistance, or a combination of exercise types is well tolerated at medium-to-high volumes and offers a variety of type-specific benefits. Still, research is needed to define (1) the "minimum" effective dose of exercise for mother and offspring health, as well as (2) the maximum tolerable dose from which more benefits may be seen. Additionally, there is a lack of randomized controlled trials addressing exercise doses during the three trimesters of pregnancy. Further, the protocols adopted in research studies should be more standardized and tested for efficacy in different populations of gravid women.

Prenatal Exercise and Cardiovascular Health (PEACH) study: impact of meal timing and pre-exercise blood glucose values on glycemic responses to acute exercise in pregnancy

We retrospectively analyzed data from 28 participants engaging in moderate-intensity aerobic exercise (265 sessions; 25-40 min) between 18-34 weeks gestation (NCT02948439). The mean change in blood glucose (BG) from pre- to post-acute exercise session was -1.0 ± 1.2 mmol/L. Pre-exercise BG significantly predicted the change in BG (p < 0.001), even when controlling for meal timing, exercise duration, and gestational age. Hypoglycemia only occurred in 3% of sessions. Therefore, in healthy pregnancy the change in BG during exercise is small and primarily related to pre-exercise BG values.

Effect of Pregnancy in 42 Elite to World-Class Runners on Training and Performance Outcomes

PURPOSE

The International Olympic Committee expert group on pregnancy has identified a paucity of information regarding training and performance in truly elite athletes. Thus, the purpose of this study was to quantify elite runners' training volume throughout pregnancy and postpartum competition performance outcomes.

METHODS

Forty-two elite (>50% competed at the World Championships/Olympic) middle-/long-distance runners' training before, during, and after pregnancy (quality/quantity/type) data (retrospective questionnaire) and competition data (published online) were collected.

RESULTS

Running volume decreased significantly ( P < 0.01) from the first trimester (63 ± 34 km·wk -1 ) to the third trimester (30 ± 30 km·wk -1 ). Participants returned to activity/exercise at ~6 wk postpartum and to 80% of prepregnancy training volumes by 3 months. Participants who intended to return to equivalent performance levels postpregnancy, there was no statistical decrease in performance in the 1 to 3 yr postpregnancy compared with prepregnancy, and ~56% improved performances postpregnancy.

CONCLUSIONS

This study features the largest cohort of elite runners training and competition outcomes assessed throughout pregnancy, with training volumes being approximately two to four times greater than current guidelines. For the first time, performance was directly assessed (due to the quantifiable nature of elite running), and study participants who intended to return to high-level competition did so at a statistically similar level of performance in the 1- to 3-yr period postpregnancy. Taken together, this article provides much needed insights into current training practices and performance of elite pregnant runners, which should help to inform future training guidelines as well as sport policy and sponsor expectations around return to training timelines and performance.

Assessment of physical fitness during pregnancy: validity and reliability of fitness tests, and relationship with maternal and neonatal health - a systematic review

Objectives

To systematically review studies evaluating one or more components of physical fitness (PF) in pregnant women, to answer two research questions: (1) What tests have been employed to assess PF in pregnant women? and (2) What is the validity and reliability of these tests and their relationship with maternal and neonatal health?

Design

A systematic review.

Data sources

PubMed and Web of Science.

Eligibility criteria

Original English or Spanish full-text articles in a group of healthy pregnant women which at least one component of PF was assessed (field based or laboratory tests).

Results

A total of 149 articles containing a sum of 191 fitness tests were included. Among the 191 fitness tests, 99 (ie, 52%) assessed cardiorespiratory fitness through 75 different protocols, 28 (15%) assessed muscular fitness through 16 different protocols, 14 (7%) assessed flexibility through 13 different protocols, 45 (24%) assessed balance through 40 different protocols, 2 assessed speed with the same protocol and 3 were multidimensional tests using one protocol. A total of 19 articles with 23 tests (13%) assessed either validity (n=4), reliability (n=6) or the relationship of PF with maternal and neonatal health (n=16).

Conclusion

Physical fitness has been assessed through a wide variety of protocols, mostly lacking validity and reliability data, and no consensus exists on the most suitable fitness tests to be performed during pregnancy.

PROSPERO registration number

CRD42018117554.

Glucose responses to acute and chronic exercise during pregnancy: a systematic review and meta-analysis

Objective

To perform a systematic review and meta-analysis to explore the relationship between prenatal exercise and glycaemic control.

Design

Systematic review with random-effects meta-analysis and meta-regression.

Data sources

Online databases were searched up to 6 January 2017.

Study eligibility criteria

Studies of all designs were included (except case studies and reviews) if they were published in English, Spanish or French, and contained information on the population (pregnant women without contraindication to exercise), intervention (subjective or objective measures of frequency, intensity, duration, volume or type of acute or chronic exercise, alone (‘exercise-only’) or in combination with other intervention components (eg, dietary; ‘exercise+cointervention’) at any stage of pregnancy), comparator (no exercise or different frequency, intensity, duration, volume and type of exercise) and outcome (glycaemic control).

Results

A total of 58 studies (n=8699) were included. There was ‘very low’ quality evidence showing that an acute bout of exercise was associated with a decrease in maternal blood glucose from before to during exercise (6 studies, n=123; mean difference (MD) −0.94 mmol/L, 95% CI −1.18 to −0.70, I2=41%) and following exercise (n=333; MD −0.57 mmol/L, 95% CI −0.72 to −0.41, I2=72%). Subgroup analysis showed that there were larger decreases in blood glucose following acute exercise in women with diabetes (n=26; MD −1.42, 95% CI −1.69 to −1.16, I2=8%) compared with those without diabetes (n=285; MD −0.46, 95% CI −0.60 to −0.32, I2=62%). Finally, chronic exercise-only interventions reduced fasting blood glucose compared with no exercise postintervention in women with diabetes (2 studies, n=70; MD −2.76, 95% CI −3.18 to −2.34, I2=52%; ‘low’ quality of evidence), but not in those without diabetes (9 studies, n=2174; MD −0.05, 95% CI −0.16 to 0.05, I2=79%).

Conclusion

Acute and chronic prenatal exercise reduced maternal circulating blood glucose concentrations, with a larger effect in women with diabetes.

Cerebral blood flow regulation, exercise and pregnancy: why should we care?

Cerebral blood flow (CBF) regulation is an indicator of cerebrovascular health increasingly recognized as being influenced by physical activity. Although regular exercise is recommended during healthy pregnancy, the effects of exercise on CBF regulation during this critical period of important blood flow increase and redistribution remain incompletely understood. Moreover, only a few studies have evaluated the effects of human pregnancy on CBF regulation. The present work summarizes current knowledge on CBF regulation in humans at rest and during aerobic exercise in relation to healthy pregnancy. Important gaps in the literature are highlighted, emphasizing the need to conduct well-designed studies assessing cerebrovascular function before, during and after this crucial life period to evaluate the potential cerebrovascular risks and benefits of exercise during pregnancy.

Fetal and maternal metabolic responses to exercise during pregnancy

Pregnancy is characterized by physiological, endocrine and metabolic adaptations creating a pseudo-diabetogenic state of progressive insulin resistance. These adaptations occur to sustain continuous fetal requirements for nutrients and oxygen. Insulin resistance develops at the level of the skeletal muscle, and maternal exercise, especially activity involving large muscle groups improve glucose tolerance and insulin sensitivity. We discuss the maternal hormonal and metabolic changes associated with a normal pregnancy, the metabolic dysregulation that may occur leading to gestational diabetes mellitus (GDM), and the consequences to mother and fetus. We will then examine the acute and chronic (training) responses to exercise in the non-pregnant state and relate these alterations to maternal exercise in a low-risk pregnancy, how exercise can be used to regulate glucose tolerance in women at risk for or diagnosed with GDM. Lastly, we present key exercise guidelines to help maintain maternal glucose regulation and suggest future research directions.

Physical activity energy expenditure and glucose control in pregnant women with type 1 diabetes: is 30 minutes of daily exercise enough?

OBJECTIVE

To describe activity patterns in pregnant women with type 1 diabetes and evaluate the impact of increased structured physical activity on glucose control.

RESEARCH DESIGN AND METHODS

Physical activity energy expenditure (PAEE) and glucose levels (continuous glucose monitoring) were measured in 10 pregnant women with type 1 diabetes (age 33.2 years, gestation 20 weeks, BMI 27.9 kg/m(2), diabetes duration 16.6 years, HbA1c 6.5% [48 mmol/mol], insulin pump duration 2.4 years) during a day at home (free-living) and during a 24-h visit incorporating controlled diet and structured physical activity with light intensity activity (three 20-min self-paced walks) and moderate intensity activity (two 50-min sessions of brisk treadmill walking). PAEE was evaluated through individually calibrated combined heart rate and movement sensing.

RESULTS

Free-living PAEE was comparable to that under controlled study conditions (3.8 and 5.1 kcal/kg/day, P = 0.241), with women achieving near to the recommended 30 min of moderate physical activity (median 27 min [interquartile range 14-68]). During the free-living period, more time was spent in light activity (10.3 vs. 7.2 h, P = 0.005), with less sedentary time (13.0 vs. 14.9 h, P = 0.047) and less moderate activity (27 vs. 121 min, P = 0.022). The free-living 24-h mean glucose levels by continuous glucose monitoring were significantly higher (7.7 vs. 6.0 mmol/L, P = 0.028). The effect of controlled diet and exercise persisted overnight, with significantly less time spent hyperglycemic (19 vs. 0%, P = 0.028) and less glucose variability (glucose SD 1.3 vs. 0.7 mmol/L, P = 0.022).

CONCLUSIONS

A controlled diet and structured physical activity program may assist women with type 1 diabetes in achieving optimal glucose control during pregnancy.

Evidence-based risk assessment and recommendations for physical activity clearance: pregnancy1This paper is one of a selection of papers published in this Special Issue, entitled Evidence-based risk assessment and recommendations for physical activity clearance, and has undergone the Journal’s usual peer review process

Recommendations for physical activity during pregnancy have progressed significantly in the last 30 years and continue to evolve as an increasing body of scientific evidence becomes available. In addition, there is an increasing number of women who wish to either maintain physical fitness levels during the prenatal period or initiate exercise for a healthier lifestyle during pregnancy. As such, consistent evaluation of the risks associated with exercise during pregnancy is warranted for maternal and fetal well-being. The primary purpose of this systematic review was to evaluate the scientific information available regarding maternal and fetal responses as it relates to the occurrence of adverse exercise-related events in pregnant women without contraindications to exercise. A systematic and evidence-based approach was used to analyze critically the level of evidence for risks associated with acute and chronic exercise during pregnancy in healthy pregnant women. All articles were screened according to standardized evaluation criteria developed by a panel of experts. A total of 74 investigations that involved 3766 pregnant women were included in the analysis. Of the 74 studies, only 35 studies documented the presence or absence of adverse events. The serious adverse event rate for these studies was 1.4 per 10 000 h of exercise. The adverse event rate increased to 6.8 per 10 000 h of exercise when including the occurrence of more mild events and exercise-related fetal bradycardia and tachycardia. Previously inactive or active women (without contraindications) are at a low risk for adverse fetal or maternal events if they participate in routine physical activity during pregnancy.

Water aerobics in pregnancy: Cardiovascular response, labor and neonatal outcomes

Background

To evaluate the association between water aerobics, maternal cardiovascular capacity during pregnancy, labor and neonatal outcomes.

Methods

A randomized, controlled clinical trial was carried out in which 34 pregnant women were allocated to a water aerobics group and 37 to a control group. All women were submitted to submaximal ergometric tests on a treadmill at 19, 25 and 35 weeks of pregnancy and were followed up until delivery. Oxygen consumption (VO_{2 max}), cardiac output (CO), physical fitness, skin temperature, data on labor and delivery, and neonate outcomes were evaluated. Frequency distributions of the baseline variables of both groups were initially performed and then analysis of the outcomes was carried out. Categorical data were compared using the chi-square test, and numerical using Student's t or Mann-Whitney tests. Wilk's Lambda or Friedman's analysis of repeat measurements were applied for comparison of physical capacity, cardiovascular outcomes and maternal temperature.

Results

VO_{2 max} and physical fitness were higher in both groups in the second trimester, returning to basal levels in the third trimester. In both groups, CO increased as pregnancy progressed and peak exercise temperature was higher than resting temperature, increasing further after five minutes of recovery and remaining at this level until 15 minutes after exercise completion. There was no difference between the two groups regarding duration (457.9 ± SD 249.6 vs 428.9 ± SD 203.2 minutes) or type of delivery. Labor analgesia was requested by significantly fewer women in the water aerobics group (27% vs 65%; RR = 0.42 95%CI 0.23–0.77). Neonatal results were similar in both groups.

Conclusion

The regular practice of moderate water aerobics by sedentary and low risk pregnant women was not detrimental to the health of the mother or the child. There was no influence on maternal cardiovascular capacity, duration of labor or type of delivery; however, there were fewer requests for analgesia during labor in the water aerobics group.

Effects of human pregnancy and aerobic conditioning on alveolar gas exchange during exercise

This study examined the effects of aerobic conditioning during the second and third trimesters of human pregnancy on ventilatory responses to graded cycling. Previously sedentary pregnant women were assigned randomly to an exercise group (n = 14) or a nonexercising control group (n = 14). Data were collected at 15–17 weeks, 25–27 weeks and 34–36 weeks of pregnancy. Testing involved 20 W·min–1 increases in work rate to a heart rate of 170 beats·min–1 and (or) volitional fatigue. Breath-by-breath ventilatory and alveolar gas exchange measurements were compared at rest, a standard submaximal [Formula: see text]O2 and peak exercise. Within both groups, resting [Formula: see text]E, [Formula: see text]A, and VT/TI increased significantly with advancing gestation. Peak work rate, O2 pulse [Formula: see text]O2/HR, [Formula: see text]E, [Formula: see text]A respiratory rate, VT/TI, [Formula: see text]O2, [Formula: see text]CO2, and the ventilatory threshold (Tvent) were increased after physical conditioning. Chronic maternal exercise has no significant effect on pregnancy-induced changes in ventilation and (or) alveolar gas exchange at rest or during standard submaximal exercise. Training-induced increases in Tvent and peak oxygen pulse support the efficacy of prenatal fitness programs to improve maternal work capacity. Key words: human gestation, respiration, chronic exercise.

Effects of Pregnancy on Maternal Work Tolerance

This review summarizes current information on the tolerance of healthy pregnant women and their fetuses to acute strenuous exertion. Maximal aerobic power, expressed in L•min−1, is not significantly affected in women who maintain an active lifestyle, whereas values expressed in mlùkg−1ùmin−1 decline with advancing gestational age in relation to maternal/fetal weight gain. Efficiency during standard exercise testing and the ventilatory anaerobic threshold (Tvent) also appear to be unaffected by pregnancy, but the ability to utilize carbohydrate and exercise anaerobically during brief strenuous exercise may be reduced. Fetal responses to short strenuous exercise are usually moderate and revert to baseline values within approximately 30 min postexercise. Future studies should examine alveolar gas exchange kinetics at the start of exercise and during recovery from both moderate and strenuous exertion. Existing studies of the responses of pregnant women to prolonged exercise have focused primarily on substrate utilization and glucose homeostasis. Other maternal responses to prolonged exercise that should be examined include acid-base regulation, temperature regulation, fluid and electrolyte balance, and perception of effort. Fetal reactions should also be examined in relation to maternal responses. Until evidence-based, occupation-specific guidelines become available, it is recommended that pregnant women use the Joint SOGC/CSEP Clinical Practice Guideline: Exercise in Pregnancy and the Postpartum Period, published by the Society of Obstetricians and Gynaecologists of Canada and the Canadian Society for Exercise Physiology (2003). Key words: human gestation, maximal exercise, prolonged exercise, fetal well-being

Clinical physiology of exercise in pregnancy: a literature review

OBJECTIVES

To review the existing literature on the physiology of exercise in pregnancy as a basis for clinical practice guidelines for prenatal exercise prescription.

METHODS

MEDLINE search for English language abstracts and articles published between 1966 and 2003 related to physiological adaptations to pregnancy, effects of pregnancy on responses to acute exercise and aerobic conditioning, effects of acute maternal exercise on indexes of fetal well-being, impact of physical conditioning on birth weight and other pregnancy outcomes, and use of exercise to prevent or treat gestational diabetes mellitus and preeclampsia.

RESULTS

Maximal aerobic power (VO(2)max, L/min) is well-preserved in pregnant women who remain physically active, but anaerobic working capacity may be reduced in late gestation. The increase in resting heart rate, reduction in maximal heart rate, and resulting smaller heart rate reserve render heart rate a less precise way of estimating exercise intensity. As rating of perceived exertion (RPE) is not altered by pregnancy, the use of revised pulse rate target zones along with Borg's RPE scale is recommended to prescribe exercise intensity during pregnancy. Responses to prolonged submaximal exercise (>30 min) in late gestation include a moderate reduction in maternal blood glucose concentration, which may transiently reduce fetal glucose availability. The normal response to sustained submaximal exercise is an increase in fetal heart rate (FHR) baseline. Transient reductions in FHR reactivity, fetal breathing movements, and FHR variability may also occur in association with more strenuous exercise. Controlled prospective studies have demonstrated that moderate prenatal exercise during the second and third trimesters is useful to improve aerobic fitness and maternal-fetal physiological reserve without affecting fetal growth.

CONCLUSIONS

The Physical Activity Readiness Medical Examination for Pregnancy is recommended for use by physicians and midwives to provide medical clearance for prenatal exercise. Evidence-based prenatal exercise guidelines are needed to counsel healthy and fit pregnant women on the safety of involvement in more strenuous physical conditioning. Future study is also recommended to determine the usefulness of prenatal exercise in the prevention and treatment of gestational diabetes mellitus and preeclampsia.