Determination of Glycine Requirement in Pregnancy

Purpose:

To determine the dietary glycine requirement in healthy pregnant women during mid (20 - 30 weeks) and late (31 - 40 weeks) gestation.

Hypothesis:

We hypothesize that there is a dietary requirement for glycine during pregnancy due to an increased metabolic demand. With increased glycine intake we hypothesize to observe an increase in protein synthesis, as measured by the indicator amino acid oxidation (IAAO) method.

Justification:

Inadequate protein intake during pregnancy may cause intrauterine growth restriction, which is associated with increased risk of chronic diseases later in life. Currently, the dietary intake recommendations for amino acids are greatly based on nitrogen balance studies of non-pregnant adults. There is little scientific information regarding dietary glycine needs during different stages of pregnancy, despite empirical evidence indicating gestational differences in protein metabolism.

Objectives:

The main objective of the current study is to determine the glycine requirement using the minimally invasive indicator amino acid oxidation (IAAO) method in healthy pregnant women. Our second objective is to compare the glycine requirement during mid (20-30 weeks gestation) and late (31 - 40 weeks gestation) stages of pregnancy.

Research methods:

We will recruit a minimum of 18 pregnant women who may take part in 2 study days in both gestational stages, for a total of 72 study days. Potential participants will meet us for a preliminary assessment where we will evaluate their eligibility to participate in the study. The preliminary study will take approximately 1 hr during which we will examine body composition (bioelectrical impedance, skin-fold measurements, height and weight), resting energy expenditure (indirect calorimetry), blood glucose (glucometer), and medical history (questionnaire). Fasted blood glucose above 6.7 mmol/L is indicative of gestational diabetes. Participants with fasted blood glucose concentrations above 6.7 mmol/L will be referred to follow-up with their primary health care provider (see letter to physician).

We will test the participants urine using a dipstick, to assess whether proteinurea is present. If there is an indication of proteinurea we will refer the participant to their primary physician (See letter to physician - Urine results).

We will collect a 3 day diet record to determine usual food intake and food preferences. Based on the 3 day food record we will prescribe a standardized diet two days prior to the study day to ensure protein intake of 1.5 g/kg/d and energy intake of 1.7 x resting energy.

During each study day, eligible participants will randomly receive 1 of 36 test glycine intakes during both mid and late pregnancy, ranging from 10 to 100 mg/kg/d. Each subject may participate in 2 study days between 20-30 weeks gestation and 2 study days between 31 - 40 weeks gestation, for a total of 4 study days. The test glycine intakes will contain a crystalline L-amino acid mixture based on the composition of egg protein, except for glycine. The test glycine intake will be administered in an experimental diet, along with enough non-protein energy to meet 1.7 x resting energy expenditure of each participant. The diet will be provided as 8 meals, each representing 1/12th of the participant's daily intake requirement. These meals will be provided over the course of an 8 hour study day. A stable isotope tracer will be added to the 5th - 8th meal. We will measure the rate of oxidation of this tracer in expired breath (F13CO2), and flux of this tracer by its enrichment in urine.