Association between Nitrogen Stable Isotope Ratios in Human Hair and Serum Levels of Leptin

Stable isotope ratios have been reported to be potential biomarkers of dietary intake and nutritional status. High serum levels of leptin, a hormone which regulates energy metabolism and food intake, are associated with insulin resistance and metabolic syndrome. However, little is known about the association between stable isotope ratios and the metabolic risk in humans. We investigated whether the carbon and nitrogen stable isotope ratios in hair are associated with serum leptin levels. Hair samples were collected from 399 healthy adults (233 men and 166 women) aged 40 to 70 years of a community-based cohort in Korea and the bulk stable isotope ratios of carbon (PC) and nitrogen (delta N-15) were measured for all hair samples. Serum leptin levels were analyzed by radioimmunoassay. delta N-15 showed positive correlations with serum leptin levels. In multivariate models, increasing delta N-15 were associated with elevated serum leptin levels (defined as the median values), whereas delta C-13 were not significantly associated with serum leptin levels. The odds ratio (95% confidence interval) per 1% increase in delta(NI)-N-15 for an elevated serum leptin level was 1.58 (1.11-2.26). In participants with high body mass index, delta N-15 showed positive associations with serum leptin levels, whereas these associations were not seen in participants with low body mass index. The nitrogen stable isotopic ratio in hair is positively associated with serum leptin levels. The hair delta N-15 could be used as a clinical marker to estimate metabolic risk.