Total body water by deuterium dilution

Total body water was measured by D₂O⁽¹³⁾ at 17 and 34 weeks of gestation. Participants reported in the evening before the study and at 20.00 h received a standard dinner and then rested for 5 h. About 01.00 h, participants provided a basal urine sample. They then drank 75 mg/kg body weight of deuterated water (Europa Scientific, Crewe, UK) from a sterile plastic container with the aid of a straw. The dose was weighed to three decimal places on an Afcoset (FX-400) electronic balance (Mumbai, India). This was followed by 3 g/kg body weight of plain water using the same straw and the container to ensure that the complete dose was ingested. Urine samples were collected at the fifth and sixth hours after drinking the deuterated water. Body weight measured after the last urine sample was used for all calculations.

Urine samples were frozen (at −70°C) until transportation to St John's Hospital, Bangalore, India for further analysis. Deuterium enrichment was analysed by zinc reduction followed by dual-inlet mass spectroscopy (Europa Scientific, Crewe, UK)⁽¹⁴⁾. Each sample was analysed in duplicate, and the mean value was used for calculation. Repeated analysis for natural background samples gave a coefficient of variation (CV, delta v. Standard Mean Ocean Water, SMOW) equal to 0⋅02 %. With high enrichment samples using IAEA standard no. 302 (enrichment equal to 500 v. SMOW), CV was equal to 0⋅22 %. The sample dose of deuterated water was also analysed to ascertain its enrichment and included in the calculation of the deuterium dilution space. It is agreed that deuterium over estimates total body water by 4 % related to the exchange of hydrogen with other molecules,⁽¹⁵⁾ so the space was divided by 1⋅04 to derive total body water.

Based on the study by Butte that showed that in sixty-three women living in the United States, birth weight was correlated positively with gestational gain in TBW measured by deuterium dilution (r 0⋅37, P = 0⋅006), we calculated that forty-nine women were required from both the rural and urban environment to achieve an α value of 0⋅05 and power of 80 %.