Maternal obesity does not influence human milk protein 15N natural isotope abundance

Obesity increases protein metabolism with a potential effect on nitrogen isotope fractionation. The aim of this study was to test the influence of obesity on human milk extracted protein N-15 natural isotope abundance (NIA) at one month post-partum and to compare human milk extracted protein N-15 NIA and bulk infant hair N-15 NIA. This cross-sectional observational study involved 16 obese mothers (body mass index (BMI) >= 30 kg m(-2) before pregnancy) matched with 16 normal-weight mothers (18.5 kg m(-2) <= BMI < 25 kg m(-2)) for age and pregnancy characteristics. Human milk extracted protein and bulk infant hair N-15 NIA were determined by isotope ratio monitoring by mass spectrometry interfaced to an elemental analyser (IRM-EA/MS). No significant difference was found in human milk protein N-15 NIA values between obese and normal-weight mothers (8.93 +/- 0.48 parts per thousand vs. 8.95 +/- 0.27 parts per thousand). However, human milk protein N-15 NIA was significantly lower than bulk infant hair N-15 NIA: 8.94 +/- 0.38 parts per thousand vs. 9.66 +/- 0.69 parts per thousand, respectively. On the basis of these results, it is concluded that human milk protein N-15 NIA measured at one month post-partum is not influenced by maternal obesity. These findings suggest that N-15 NIA may be exploited to study metabolism without considering maternal obesity as a confounder.