Maximal fat oxidation, but not aerobic capacity, is affected by oral contraceptive use in young healthy women

Female Exercise Metabolism: Quality Assessment of Existing Knowledge Base and Key Challenges in Study Design

A better understanding of the practical and methodological challenges inherent in accurately tracking female hormonal status would enhance the clarity of research findings in exercise metabolism. The purpose of this study was to conduct an objective assessment of the quality of existing literature in this area and to provide a general overview of the practical conditions and issues encountered in studies investigating substrate metabolism during exercise in women, both in those using (HC+) or not using (HC-) hormonal contraceptives. Forty-four articles were identified through systematic reviews, meta-analyses, and searches on PubMed/MEDLINE. A quality assessment framework was developed and applied using a double-blind scoring approach. The loss/exclusion of data between baseline and final analyses per study was quantified, and the main challenges were highlighted. A higher mean global score was observed in studies conducted among HC+ women (81%) than those among HC- women (46%). Although the dropout rates were rarely mentioned in HC+ women articles, the mean rate of participant/data loss in HC- groups was 22.1%. The rate of HC- participant loss was positively correlated with the global score (r = .504, p = .02). High-quality research may be easier to achieve in studies involving HC+ women compared with those involving HC- women. Studies on HC- women seem to face more practical challenges, such as tracking the menstrual cycle and targeting specific (sub)phases of the menstrual cycle, along with technical and feasibility limitations. Thus, although the framework for designing such studies exists, the quality assessment of the available literature emphasizes its challenge in terms of research implementation.

Auditing the representation of hormonal contraceptives in studies assessing exercise performance in women

Hormonal contraceptive (HC) users have a different ovarian hormonal profile compared to eumenorrheic women. Due to the prevalence of HC use amongst sportswomen, there has been increased research efforts to understand their impact on exercise performance. The aim was to audit this research. Studies identified were assessed for HC type, athlete calibre, performance outcome, study design, and quality of methodological control regarding ovarian hormonal profiles. Sixty-eight different HCs were reported across 61 studies. Monophasic combined oral contraceptive (OCP) pills represented 60% of HCs, followed by other pills [34%, phasic-combined, progestogen-only, and un-specified], phasic and long acting reversible contraceptives [5%, vaginal ring, patch, implant, injection, intrauterine system] and unspecified HCs (1%). Eleven percent of participants using HCs were classified as highly trained or elite/international with no participants being classed as world class. Whilst the number of studies involving HCs has increased two-fold over the past decade, the number of studies ranked as gold standard has not increased (HC; 2003-57%, 2011-55%, 2022-43%. OCP; 2003-14%, 2011-17%, 2022-12%). Future research assessing HCs and exercise performance should adopt high-quality research designs and include a broader range of HCs in highly trained to world-class populations to increase the reach and impact of research in this area.

Effects of In-Exercise Carbohydrate Supplementation on Prolonged High-Intensity Exercise Performance in Oral Contraceptive Users

PURPOSE

To examine the impact of oral contraceptive (OC) phases on performance, physiological, and subjective responses to prolonged, intensive exercise when carbohydrate (CHO) stores are reduced.

METHODS

Ten well-trained female cyclists using monophasic OC completed 4 identical trials (>150 min) under conditions of in-trial 60-g·h-1 CHO supplementation (CHO+) or placebo (CHO-) during the sugar- (SUG) and active-pill (ACT) phases of their OC cycle. Each trial comprised two 400-kcal time trials (TT) separated by 1 hour of submaximal cycling at first ventilatory threshold.

RESULTS

Change in completion time from TT1 to TT2 was minimized in CHO+ compared with CHO- (4.06 [2.55] vs 6.08 [5.33] min; P = .019, effect size = -0.36). An interaction effect of OC and CHO was observed for time to complete TT (P = .006), mean TT power (P = .002), mean TT heart rate (P = .002), and posttrial emotional balance (P = .020) and negative emotional state (P = .033). In ACT, mean TT power and heart rate were higher in CHO+ when compared with CHO-, resulting in faster TTs in CHO+ and improved posttrial emotional well-being. When CHO was not supplemented, TT power and heart rate were higher in SUG when compared with ACT, resulting in faster TTs in SUG and improved posttrial emotional balance.

CONCLUSION

CHO depletion during ACT negatively influenced TT performance and emotional well-being when compared with SUG. Irrespective of OC pill phase, CHO supplementation should be prioritized to sustain performance and improve postexercise recovery-stress balance.

Impact of aerobic fitness status, menstrual cycle phase, and oral contraceptive use on exercise substrate oxidation and metabolic flexibility in females

The influence of menstrual cycle phase and fitness status on metabolism during high-intensity interval exercise (HIIE) was assessed. Twenty-five females (24.4 (3.6) years) were categorized by normal menstrual cycle (n = 14) vs. oral contraceptive (OC) use (n = 11) and by aerobic fitness, high-fitness females (HFF; n = 13) vs. low-fitness females (LFF; n = 12). HIIE was four sets of four repetitions with a 3 min rest between intervals on a cycle ergometer at a power output halfway between the ventilatory threshold and V̇O2peak and performed during follicular (FOL: days 2-7 or inactive pills) and luteal phases (LUT: day ∼21 or 3rd week of active pills). Substrate oxidation was assessed via indirect calorimetry, blood lactate via finger stick, and recovery of skeletal muscle oxidative metabolism (mV̇O2) via continuous-wave near-infrared spectroscopy. HFF oxidized more fat (g·kg-1) during the full session (FOL: p = 0.050, LUT: p = 0.001), high intervals (FOL: p = 0.048, LUT: p = 0.001), low intervals (FOL: p = 0.032, LUT: p = 0.024), and LUT recovery (p = 0.033). Carbohydrate oxidation area under the curve was greater in HFF during FOL (FOL: p = 0.049, LUT: p = 0.124). Blood lactate was lower in LFF in FOL (p ≤ 0.05) but not in LUT. Metabolic flexibility (Δ fat oxidation g·kg-1·min-1) was greater in HFF than LFF during intervals 2-3 in FOL and 1-4 in LUT (p ≤ 0.05). Fitness status more positively influences exercise metabolic flexibility during HIIE than cycle phase or OC use.

The role of sex in the relationship between fasting adipokines levels, maximal fat oxidation during exercise, and insulin resistance in young adults with excess adiposity

AIM

Previous evidence suggest that a sexual dimorphism in exercise fat oxidation and adipokines levels may explain a lower risk of cardio-metabolic disorders in women. Therefore, we investigated the role of sex in the relationship between adipokines levels, maximal fat oxidation (MFO) during exercise and insulin resistance.

METHODS

Fifty young adults with excess adiposity (31 women; body fat: 38.7 ± 5.3%) were included in this study. The fasting levels of leptin, adiponectin, glucose and insulin were determined from blood samples and the homeostatic model assessment of insulin resistance index (HOMA-IR) subsequently calculated. Body fat percentage and visceral adipose tissue (VAT) were assessed through dual-energy X-ray absorptiometry whereas MFO was estimated during an incremental-load exercise test after an overnight fasting through indirect calorimetry.

RESULTS

Men had lower levels of body fat (d = 1.80), adiponectin (d = 1.35), leptin (d = 0.43) and MFO (d = 1.25) than women. Conversely, men showed higher VAT (d = 0.85) and fasting glucose levels (d = 0.89). No sex differences were observed in HOMA-IR (d = 0.34). Adipokines levels were not associated with MFO in both sexes (r < 0.30), whereas adiponectin levels were inversely related with HOMA-IR in both men (r = -0.58) and women (r = -0.50). Leptin concentration was associated to HOMA-IR only in men (r = 0.41), while no statistically significant relationships were observed between MFO and HOMA-IR in both sexes (r < 0.44).

CONCLUSION

Insulin resistance was similar between sexes regardless of superior levels of adipokines and MFO during exercise in women. Therefore, adiponectin and leptin may regulate glucose homeostasis without altering whole body fat oxidation rate during exercise.

Effect of oral contraceptives on energy balance in women: A review of current knowledge and potential cellular mechanisms

Body weight management is currently of major concern as the obesity epidemic is still a worldwide challenge. As women face more difficulties to lose weight than men, there is an urgent need to better understand the underlying reasons and mechanisms. Recent data have suggested that the use of oral contraceptive (OC) could be involved. The necessity of utilization and development of contraceptive strategies for birth regulation is undeniable and contraceptive pills appear as a quite easy approach. Moreover, OC also represent a strategy for the management of premenstrual symptoms, acne or bulimia nervosa. The exact impact of OC on body weight remains not clearly established. Thus, after exploring the potential underlying mechanisms by which OC could influence the two side of energy balance, we then provide an overview of the available evidence regarding the effects of OC on energy balance (i.e. energy expenditure and energy intake). Finally, we highlight the necessity for future research to clarify the cellular effects of OC and how the individualization of OC prescriptions can improve long-term weight loss management.

The Effects of Oral Contraceptives on Exercise Performance in Women: A Systematic Review and Meta-analysis

BACKGROUND

Oral contraceptive pills (OCPs) are double agents, which downregulate endogenous concentrations of oestradiol and progesterone whilst simultaneously providing daily supplementation of exogenous oestrogen and progestin during the OCP-taking days. This altered hormonal milieu differs significantly from that of eumenorrheic women and might impact exercise performance, due to changes in ovarian hormone-mediated physiological processes.

OBJECTIVE

To explore the effects of OCPs on exercise performance in women and to provide evidence-based performance recommendations to users.

METHODS

This review complied with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. A between-group analysis was performed, wherein performance of OCP users was compared with naturally menstruating women, and a within-group analysis was conducted, wherein performance during OCP consumption was compared with OCP withdrawal. For the between-group analysis, women were phase matched in two ways: (1) OCP withdrawal versus the early follicular phase of the menstrual cycle and (2) OCP consumption versus all phases of the menstrual cycle except for the early follicular phase. Study quality was assessed using a modified Downs and Black Checklist and a strategy based on the recommendations of the Grading of Recommendations Assessment Development and Evaluation working group. All meta-analyses were conducted within a Bayesian framework to facilitate probabilistic interpretations.

RESULTS

42 studies and 590 participants were included. Most studies (83%) were graded as moderate, low or very low quality, with 17% achieving high quality. For the between-group meta-analysis comparing OCP users with naturally menstruating women, posterior estimates of the pooled effect were used to calculate the probability of at least a small effect (d ≥ 0.2). Across the two between-group comparison methods, the probability of a small effect on performance favouring habitual OCP users was effectually zero (p < 0.001). In contrast, the probability of a small effect on performance favouring naturally menstruating women was moderate under comparison method (1) (d ≥ 0.2; p = 0.40) and small under comparison method (2) (d ≥ 0.2; p = 0.19). Relatively large between-study variance was identified for both between-group comparisons ([Formula: see text]0.5 = 0.16 [95% credible interval (CrI) 0.01-0.44] and [Formula: see text]0.5 = 0.22 [95% CrI 0.06-0.45]). For the within-group analysis comparing OCP consumption with withdrawal, posterior estimates of the pooled effect size identified almost zero probability of a small effect on performance in either direction (d ≥ 0.2; p ≤ 0.001).

CONCLUSIONS

OCP use might result in slightly inferior exercise performance on average when compared to naturally menstruating women, although any group-level effect is most likely to be trivial. Practically, as effects tended to be trivial and variable across studies, the current evidence does not warrant general guidance on OCP use compared with non-use. Therefore, when exercise performance is a priority, an individualised approach might be more appropriate. The analysis also indicated that exercise performance was consistent across the OCP cycle.

Sex-specific alterations in blood-borne factors in physically inactive individuals are detrimental to endothelial cell functions

Mechanisms underlying the protective effects of both habitual endurance exercise and the female sex on vascular function are incompletely understood. Blood-borne circulating factors, such as circulating microRNAs (ci-miRs), may partially explain these effects. Blood samples were obtained from young, healthy men and women who either habitually performed endurance exercise (endurance trained) or were relatively inactive (sedentary). Women were tested during the early follicular phase of the menstrual cycle or the placebo pill phase of oral contraceptive to control for estrogen. Cultured human umbilical vein endothelial cells (HUVECs) were exposed to participants' serum in migration, proliferation, and reactive oxygen species (ROS) assays. Real-time quantitative polymerase chain reaction was used to quantify an initial array of 84 cardiovascular disease (CVD)-related ci-miRs, followed by validation of 10 ci-miRs. All participants were devoid of traditional CVD risk factors, and circulating estradiol concentration was not different between groups. Serum of endurance-trained women induced greater HUVEC migration compared with serum of sedentary women. HUVEC ROS production was greater in response to serum of sedentary men compared with serum of endurance-trained men and sedentary women. There were sex effects on the levels of nine ci-miRs, with greater levels in men, while ci-miRs-140-5p and 145-5p were also higher in sedentary compared with endurance-trained men and/or women. In a sex-specific manner, habitual endurance exercise was associated with beneficial effects of serum on HUVECs. Thus, alterations in circulating factors may contribute to the protective effects of habitual endurance exercise on vascular health. Additionally, sex had a greater impact than habitual activity level on the levels of vascular-related ci-miRs.NEW & NOTEWORTHY Serum from sedentary women caused impaired endothelial migration, whereas serum from sedentary men elicited increased endothelial reactive oxygen species production as compared with serum from their endurance-trained counterparts. Select CVD-related circulating microRNAs (ci-miRs) were higher in men than women, while ci-miRs-140-5p and 145-5p were also higher in sedentary versus trained men and/or women. Our data suggest that alterations in circulating factors may contribute to the protective effects of habitual exercise and sex on vascular health.

Gender-specific considerations in physical activity, thermogenesis and fat oxidation: implications for obesity management

With current 'one-size-fits-all' obesity prevention and management strategies proving largely ineffective, the focus has shifted towards a more tailored, individualized approach. However, investigation of the mechanisms underlying inter-individual variability in metabolic profile and response to intervention often yield conflicting results. Indeed, it is perhaps surprising that despite at least a century of recognition that sex hormones influence metabolism, firm conclusions regarding the effects of the menstrual cycle, hormonal contraception and menopause on many aspects of energy expenditure and substrate utilization remain to be drawn. In this review, we examine current evidence relating to gender-specific considerations in the promotion of physical activity, thermogenesis and fat oxidation for body-weight regulation, including the relationship between sex hormone status and non-exercise activity thermogenesis - an energy expenditure compartment that is often overlooked in favour of traditional exercise/sport physical activities yet presents a viable target in the search for effective weight management.

Contextualising Maximal Fat Oxidation During Exercise: Determinants and Normative Values

Using a short-duration step protocol and continuous indirect calorimetry, whole-body rates of fat and carbohydrate oxidation can be estimated across a range of exercise workloads, along with the individual maximal rate of fat oxidation (MFO) and the exercise intensity at which MFO occurs (Fatmax). These variables appear to have implications both in sport and health contexts. After discussion of the key determinants of MFO and Fatmax that must be considered during laboratory measurement, the present review sought to synthesize existing data in order to contextualize individually measured fat oxidation values. Data collected in homogenous cohorts on cycle ergometers after an overnight fast was synthesized to produce normative values in given subject populations. These normative values might be used to contextualize individual measurements and define research cohorts according their capacity for fat oxidation during exercise. Pertinent directions for future research were identified.

Sex difference in substrate oxidation during low-intensity isometric exercise in young adults

Low-intensity physical activity is increasingly promoted as an alternative to sedentary behavior. However, much research to date has focused on moderate- to vigorous-intensity physical activity, and in particular dynamic work, with the effect of low-intensity isometric exercise (<4 METs) on substrate utilization yet to be explored. Here we investigate the effects of such exercise on respiratory quotient (RQ) and determine the extent of intra- and inter-individual variability in response. Energy expenditure, RQ, and substrate oxidation were measured by ventilated-hood indirect calorimetry at rest and in response to standardized, intermittent, low-level isometric leg-press exercises at 5 loads (+5, +10, +15, +20, +25 kg) in 26 healthy, young adults. Nine participants repeated the experiment on 3 separate days to assess within-subject, between-day variability. There was no significant difference in energy cost and heart rate responses to low-intensity isometric exercise (<2 METs) between men and women. However, a sex difference was apparent in terms of substrate oxidation - with men increasing both fat and carbohydrate oxidation, and women only increasing fat oxidation while maintaining carbohydrate oxidation at baseline, resting levels. This sex difference was repeatable and persisted when substrate oxidation was adjusted for differences in body weight or body composition. Individual variability in RQ was relatively low, with both intra- and inter-individual coefficients of variation in the range of 3%-6% in both sexes. These results suggest that women preferentially increase fat oxidation during low-level isometric exercise. Whether such physical activity could be incorporated into treatment/prevention strategies aimed at optimizing fat oxidation in women warrants further investigation.