This study aims to understand how much leucine, an essential amino acid, and protein building block, healthy pregnant women need during early and late pregnancy. Leucine plays an important role in supporting muscle health and fetal growth. Participants will consume carefully prepared meals and provide breath, urine samples, and one blood sample so researchers can measure how the body uses leucine at different stages of pregnancy. The results will help improve nutrition guidelines for pregnant women to better support maternal health and healthy fetal development.

Pregnancy is a period marked by substantial physiological and metabolic changes that place increased demands on maternal nutrition to support both maternal health and fetal development. Adequate intake of protein and indispensable amino acids is essential during this time, as these nutrients are required for tissue growth, metabolic regulation, and fetal development. Leucine, an indispensable amino acid that must be obtained through the diet, plays a key role in protein synthesis and metabolic signaling, particularly through activation of the mTOR (mechanistic Target of Rapamycin) pathway, which is critical for maternal and fetal tissue growth. Current dietary recommendations for leucine during pregnancy are largely derived from studies conducted in non-pregnant adults and do not account for the dynamic metabolic changes that occur across different stages of gestation. Evidence from our laboratory and others suggests that requirements for several indispensable amino acids differ between early and late pregnancy, highlighting the need for pregnancy-specific data. However, whether leucine requirements vary across gestation remains unclear. The purpose of this study is to determine dietary leucine requirements during early and late pregnancy in healthy pregnant women.

The primary objective is to estimate leucine requirements using the indicator amino acid oxidation (IAAO) technique in healthy singleton pregnant women during early (11-20 weeks) and late (31-40 weeks) gestation. We hypothesize that leucine requirements during pregnancy differ from current generalized recommendations and that requirements are different in late gestation compared to early gestation due to increased fetal growth and maternal tissue accretion.

Participants will undergo a pre-study screening visit to confirm eligibility and collect medical history, demographic information, and baseline anthropometric measurements, including height, weight, and body composition.

Leucine requirements will be assessed using the minimally-invasive IAAO method, which is based on the principle that when dietary leucine intake is insufficient to support protein synthesis, oxidation of a labeled indicator amino acid increases. Participants will consume randomized, graded intakes of dietary leucine across separate study days, provided as isocaloric and isonitrogenous meals over an 8-hour period. Each meal will include a flavored protein-free beverage containing a stable isotope tracer (L-[1-¹³C] phenylalanine) and standardized low-protein foods. During each study day, breath samples will be collected at multiple time points to measure ¹³CO₂ enrichment as an indicator of phenylalanine oxidation, and urine samples will be collected to assess tracer enrichment. A single blood sample will be obtained to measure plasma leucine concentrations and pregnancy-related sex hormones, including progesterone. Hormone levels will be assessed using a chemiluminescent microparticle immunoassay on the Architect i2000SR analyzer (Abbott).

Descriptive statistics will be used to summarize participant characteristics, with unpaired two-sample t-tests used to compare baseline characteristics between early and late gestation groups. Dietary leucine requirements will be estimated using a two-phase linear regression mixed-effects model analyzed using R (version 4.4.1, Windows). Plasma leucine concentrations will be analyzed using linear regression methods in GraphPad Prism. Statistical significance will be set at p ≤ 0.05.

Inclusion Criteria:

• Pregnant women aged 20-40 years. (This age range represents typical reproductive age and minimizes potential confounding effects related to adolescent growth or age-associated metabolic changes.)
• Singleton pregnancy. (Restricting participation to singleton pregnancies reduces variability in nutrient requirements and metabolic demands associated with multiple gestations.)
• Gestational age of either 11-20 weeks (early gestation) or 31-40 weeks (late gestation).

(These time points represent physiologically distinct stages of pregnancy, allowing comparison of leucine requirements across early and late gestation.)

• Participants who take part in early gestation, can participate in late gestation, if they meet all inclusion criteria.
• Good general health with no acute or chronic medical conditions. (Including only healthy participants minimizes confounding factors that could alter amino acid metabolism and ensures participant safety.)
• Fully ambulatory and able to comply with study procedures. (Participants must be able to attend study visits and complete protocol requirements, including dietary intake and sample collection.)
• Pre-pregnancy body mass index (BMI) between 19 and 28 kg/m² (This range excludes underweight and obese individuals, whose altered protein and amino acid metabolism could affect study outcomes.)
• Carrying a viable pregnancy without known complications at enrollment. (Excluding pregnancy complications at baseline reduces risk to participants and supports accurate assessment of physiological amino acid requirements.)

Exclusion Criteria:

-Pregnancy-related complications (e.g., hyperemesis gravidarum, GDM, hypertension, pre-eclampsia, anemia).

(These conditions may alter metabolism and increase risk, potentially confounding study outcomes.)

• History of adverse pregnancy outcomes (spontaneous abortion or preterm birth). (May reflect underlying physiological factors affecting amino acid metabolism.)
• Multiple gestation pregnancies. (Nutritional and metabolic demands differ from singleton pregnancies.)
• Pregnancy loss within six months of the current pregnancy. (Short intervals may affect maternal nutrient status and metabolic adaptation.)
• Chronic metabolic, cardiovascular, neurological, genetic, or immune disorders. (These conditions may independently affect protein and amino acid metabolism.)
• Clinically significant claustrophobia. (May interfere with study procedures requiring ventilated hood measurements.)
• Use of alcohol, tobacco, or illicit substances during pregnancy. (Substance use affects metabolism and poses maternal and fetal health risks.)
• Gestational weight loss greater than 1.5 kg. (May indicate inadequate nutritional status.)
• Pre-pregnancy BMI outside 19-28 kg/m². (Extremes of BMI are associated with altered protein metabolism.)

Cis-women