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Abstract

The menstrual cycle is a complex physiological process characterized by distinct hormonal fluctuations that impact energy metabolism, appetite and influence nutritional status. It was previously shown that lysine, an indispensable amino acid, requirements are affected by the phase of the menstrual cycle in young females, yet current dietary reference intakes (DRI) for amino acids in adults are based solely on studies in males. Leucine, one of nine indispensable amino acids, is crucial for protein synthesis, muscle preservation, and overall metabolic health. My aim was to determine dietary leucine requirements directly in females, accounting for the phases of the menstrual cycle. A total of thirteen females (22-32y) participated in the follicular phase (days 3-7 after the onset of the menstrual cycle), and in the luteal phase (days 4-7 before the onset of the next cycle) for a total of 80 study days using the indicator amino acid oxidation method in a repeated measures design. Menstrual phase was determined using app-based cycle tracking combined with ovulation tracking methods. Test leucine intakes were randomly assigned, ranging from 10 to 85 mg∙kg⁻¹∙day⁻¹ provided in eight-hourly, isonitrogenous and isocaloric meals. Breath samples were collected at baseline, and isotopic steady state of orally provided 1-¹³C-Phenylalanine, and samples were analyzed for ¹³C enrichment using an isotope ratio mass spectrometer. Leucine requirement was determined using a two-phasic linear mixed-effects model to identify a breakpoint in ¹³CO₂ production, representing the estimated average requirement (EAR). The results suggest that the EAR for leucine in the follicular phase is 47.4 mg∙kg⁻¹∙day⁻¹ (R²c=0.82; 95% CI [36.7, 58.0 mg∙kg⁻¹∙day⁻¹] and 49.1 mg∙kg⁻¹∙day⁻¹(R²c=0.88; 95% CI [39.5,58.7 mg∙kg⁻¹∙day⁻¹] in the luteal phase. For the first time, dietary leucine requirements have been directly measured in females. These estimated values are 40% higher compared to the current male-derived EAR recommendations of 34 mg∙kg⁻¹∙day⁻¹. Menstrual cycle phase, however, did not influence leucine requirements, as follicular and luteal phase needs are comparable. The findings suggest that sex-specific amino acid intake recommendations are needed and may be essential for promoting women's health and supporting overall reproductive and metabolic well-being.