The existing guidelines for dietary amino acid intake for women are set the same as those established for men. These recommendations might not be appropriate for women, as they do not take into consideration important differences between male and female physiology, like the menstrual cycle. This study aims to determine the leucine requirements during the phases of the menstrual cycle. Leucine is an essential amino acid (cannot be made naturally in the body) and is necessary for protein synthesis, and other important cellular functions. Consuming an adequate amount daily is crucial for maintaining overall health. An innovative, non-invasive technique utilizing pre-determined diets, safe stable isotopes, and a simple breath collection method will be used. Previous work done by Elango Lab investigators has applied this method to investigate other amino acids in pregnant, non-pregnant, and lactating women.

The objective of the study is to determine leucine requirements using the indicator amino acid oxidation method during the follicular phase and the luteal phase of the menstrual cycle in healthy women (between the ages of 20-35 y).In comprehending protein needs and achieving protein targets, it is imperative to establish individual amino acid requirements. This clinical study aims to determine the dietary requirements of leucine for women, helping build a strong foundation for setting dietary guidelines for women. The existing values, derived from guidelines designed for men, overlook the crucial impact of the menstrual cycle on energy and dietary intake. Relying on amino acid requirements derived from studies in men for women significantly heightens the risk of potential harm. It is crucial to identify the distinct amino acid requirements for women and grasp how these needs are affected by various phases of the menstrual cycle. A minimum of 15 healthy women (20-35 y) will be recruited and studied at two menstrual phases over a range of test leucine intakes (10, 20, 30, 40, 55, 70, and 85 mg/kg/day). Women will be invited to participate in up to 7 study days/phase, and randomized to a different intake at each study day. Participants' eligibility will be assessed during a pre-study day. The goal is to study a minimum of 7 women at each intake, for a total of 49 study days in each stage.

Inclusion Criteria:

• People with menstrual cycles
• Between the ages of 20 to 35y
• Regular menstrual cycle (21-35 day cycle) for the past 12 months
• Body mass index (BMI) of between 18 and 28 kg/m2
• Free from pre-existing health conditions

Exclusion Criteria:

• People who are currently using hormonal birth control or any contraceptive that would affect sex hormones
• People on hormone therapy
• People with an abnormal menstrual cycle history
• People with non-traditional dietary practices
• Recent weight loss/weight gain
• People with a history of endocrine disorders
• People who are pregnant
• People who are breastfeeding
• People who have given birth in the last 18 months
• People who depend on medication that affects normal metabolism. Any medication that alters normal body metabolism would skew the results and compromise the validity of the data set.
• People with a metabolic, neurological, genetic, or immune disorder likely to affect nutritional requirements or overall body metabolism.