Fatty acid carbon isotope ratios in humans on controlled diets

Carbon stable isotope ratios for six serum fatty acids (FA) are reported for human subjects on controlled fat diets to determine the range of natural isotope abundance and to demonstrate the leveling effect of a well-controlled diet. Twenty-nine subjects were randomly assigned to one of three controlled diets containing high, medium, or low fat. Diets were consumed for 8 wk. Serum samples were collected at baseline (0), 5, 6, 7, and 8 wk. FA were extracted and methylated. Isotope ratios were analyzed by high-precision gas chromatography combustion-isotope ratio mass spectrometry. At baseline, mean delta 13C for 16:0b, 16:1a, 18:0a, 18:1c,18:2n-6d and 20:4n-6bc were -24.1, -21.7, -21.6, -25.6, -29.6, and -25.0/1000, respectively, with an average standard deviation of 1.9/1000. Most delta 13C decreased during the diet period and appeared to have stabilized by week 5 at -25.3, -21.9, -22.3, -26.5, -30.1, and -24.5/1000, respectively. Between-subjected variability decreased from 1.74 to 1.20/1000 on the controlled diets. Measurement variability was 0.53/1000. The within-subject variability during weeks 5-8 was 0.57/1000 (range of 0.32-0.84/1000), showing a minimum biological fluctuation on controlled diets. There was no diet group effect on delta 13C of serum FA. Except for 18:2, the delta 13C of experimental diets was lower than that of serum FA, consistent with observations in animals. These data show that carbon isotope ratios stabilize in response to controlled diets within 5 wk, reflecting the isotope ratio of their dietary source, and establish isotope ratio fluctuations for endogenous compounds for future studies.