Abstract
Background: Fluctuations in endogenous sex hormones over the menstrual cycle appear to influence dietary intake. Research suggests that energy intake may be greater in the luteal phase compared with the follicular phase of the menstrual cycle, while patterns of macronutrient intake are less clear. However, high-quality evidence is lacking due to poor adherence to recent best-practice guidelines for menstrual cycle research. Furthermore, existing research has almost exclusively been conducted in sedentary populations: the dietary intake and energy availability of female exercisers across the cycle are unclear.
Aims: This pilot study aimed to provide data on: 1) the variability of habitual energy availability, energy intake, and macronutrient intake across the menstrual cycle in female exercisers; 2) phase-related dietary intake in the post-exercise period; and 3) retention rates to aid future research.
Methods: Six regularly menstruating female exercisers (>2.5hrs·week-1 exercise) completed three-day weighed image-assisted diet records and training diaries in the early-follicular phase (days 2-4 of the menstrual cycle) and mid-luteal phase (7-9 days after a positive urinary ovulation test) for two menstrual cycles. On one day during each three-day recording period, participants had their body composition measured via bioelectrical impedance analysis (BIA) and completed a fasted 60-minute cycling time trial, followed by an ad-libitum meal.
Results: Mean energy availability was slightly greater in the mid-luteal phase (35.7 ± 7.1 kcal·kgFFM-1·day-1) compared with the early-follicular phase (32.6 ± 11.5 kcal·kgFFM-1·day-1) of the menstrual cycle. When one participant with a dramatic ~600 kcal·day-1 decrease in EEE in the mid-luteal phase of both cycles was excluded, mean energy availability in the early-follicular and mid-luteal phases still showed the same pattern (31.5 ± 12.5 vs. 34.7 ± 7.5 kcal·kgFFM-1·day-1). Daily macronutrient intakes were similar between the two phases in both the full, and reduced samples, but many participants failed to meet sports nutrition guidelines for carbohydrate and protein in either phase. Energy and macronutrient intakes from the post-exercise ad-libitum meal were similar between phases, apart from fat, which increased from the early-follicular phase (20.7 ± 12.0 g) to the mid-luteal phase
(25.5 ± 12.5 g). The rate of menstrual irregularities was very high, causing 43.8% (n=7) of participants to be excluded.
Conclusion: The trends shown in this pilot study are worthy of further exploration, and future interventional and observational studies investigating energy availability may need to consider the effects of the menstrual cycle. Ideally, studies with large sample sizes and more diverse populations, which adhere to best-practice guidelines for menstrual cycle research, are required. However, without greater collaboration between research centres and increased funding for female-specific research, the feasibility of this is challenging.