Phenylalanine is an essential amino acid that must be obtained through food. Current adult phenylalanine recommendation is based on male-conducted studies and set the same for females. These recommendations may not be appropriate for females, as they do not take into consideration the hormonal changes over the menstrual cycle. The menstrual cycle is a rhythmic hormonal process split into two phases, with hormones regulating metabolism differently. The aim of this study is to find phenylalanine requirement for healthy, menstruating females. 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 females as well as in children.

The objective of this study is to assess phenylalanine requirements in healthy, menstruating females, between the ages 20-35, during the luteal and follicular phases using the minimally invasive indicator amino acid oxidation method.

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 phenylalanine requirement in females to help set nutrition guidelines for females that heavily consider the metabolic and hormonal effects of the differences between the menstrual cycle phases. The existing values, derived from guidelines designed for males, overlook the crucial impact of the menstrual cycle on energy and dietary intake. Relying on amino acid requirements derived from studies in males for females significantly heightens the risk of potential harm. It is crucial to identify the distinct amino acid requirements for females and grasp how these needs are affected by various phases of the menstrual cycle. A minimum of 15 healthy females (20-35 y) will be recruited and studied at two menstrual phases over a range of test phenylalanine intakes (2.5, 5, 7.5 10, 12.5 17.5, and 22.5 mg/kg/day). Subjects 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 participants 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.