Nitrogen balance and delta N-15: why you're not what you eat during pregnancy

Carbon (C-13/C-12) and nitrogen (N-15/N-14) stable isotope ratios were longitudinally measured in human hair that reflected the period from pre-conception to delivery in 10 pregnant women. There was no significant change in the delta(13)C results, but all subjects showed a decrease in delta(15)N values (-0.3 to -1.1parts per thousand) during gestation. The mechanisms causing this decrease in hair delta(15)N have not been fully elucidated. However, since the delta(15)N values of dietary nitrogen and urea nitrogen are significantly lower compared to maternal tissues, it is hypothesized that the increased utilization of dietary and urea nitrogen for tissue synthesis during pregnancy resulted in a reduction of the steady state diet to a body trophic level effect by approximately 0.5-1parts per thousand. An inverse correlation (R-2 = 0.67) between hair delta(15)N and weight gain was also found, suggesting that positive nitrogen balance results in a reduction of delta(15)N values independent of diet. These results indicate that delta(15)N measurements have the ability to monitor not only dietary inputs, but also the nitrogen balance of an organism. A potential application of this technique is the detection of fertility patterns in modern and ancient species that have tissues that linearly record stable isotope ratios through time. Copyright (C) 2004 John Wiley Sons, Ltd.