Nitrogen Balance and Protein Quality Ingestion in Pregnant Women: Characterizing a Nutritional Scenario in a Pilot Study in Mexico.

Nitrogen balance (NB) reflects protein metabolism and varies with diet quality. We aimed to estimate the NB using objective standardized methods and evaluate the adequacy and quality of protein intake in Mexican pregnant women. This pilot study was nested in the MAS-Lactancia cohort and included 13 pregnant women in their third trimester. Diet was assessed through the duplicate portion method (DPM) and food diaries on two non-consecutive days. A registered dietitian analyzed photos, recipes, and portion sizes to assess dietary quality using NOVA classification. DPM samples were analyzed by Kjeldahl method to determine nitrogen content. On the same days, 24 h-urine was collected to measure urea and total nitrogen excretion. NB was calculated per day. The Kruskal Wallis test compares the characteristics of the participants according to NB. Mean age (SD) was 27.9 (4.2) years, at 33 (3.4) gestational weeks. Median nitrogen intake was 6.75 g/day (IQR: 6.40-9.74), representing 0.64 g protein/Kg body weight by day (IQR: 0.45-0.92). Median NB was 1.32 g (IQR: -0.15 to 2.63), with 30.7% of women in negative, 23.2% in neutral, and 46.1% in positive balance (+2). Diet quality showed high carbohydrate intake (> 70% of total calories), mainly from ultra-processed foods. Protein came in moderate amounts from foods containing 15%-95% of the edible portion. Negative NB was more common among women with higher BMI and lower education. Over half of pregnant women were in negative or neutral NB, indicating insufficient protein intake from limited sources and low diet quality.