FATTY-ACID CARBON-ISOTOPE RATIOS IN HUMANS ON CONTROLLED DIETS

Carbon stable isotope ratios for six serum fatty acids (FA) are report ed for human subjects on controlled fat diets to determine the range o f 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. Di ets were consumed for 8 wk. Serum samples were collected at baseline ( 0), 5, 6, 7, and 8 wk. FA were extracted and methylated. Isotope ratio s were analyzed by high-precision gas chromatography combustion-isotop e ratio mass spectrometry. At baseline, mean delta(13)C for 16:0(b), 1 6:1(a) 18:0(a) 18:1(c), 18:2n-6(d), and 20:4n-6(bc) were -24.1. -21.7. -21.6, -25.6, -29.6, and -25.0 parts per thousand, respectively, with an average standard deviation of 1.9 parts per thousand. Most delta(1 3)C decreased during the diet period and appeared to have stabilized b y week 5 at -25.3, -21.9, -22.3, -26.5, -30.1, and -24.5 parts per tho usand, respectively. Between-subject variability decreased from 1.74 t o 1.20 parts per thousand on the controlled diets. Measurement variabi lity was 0.53 parts per thousand. The within-subject variabi- lity dur ing weeks 5-8 was 0.57 parts per thousand (range of 0.32-0.84 parts pe r thousand), showing a minimum biological fluctuation on controlled di ets. There was no diet group effect on delta(13)C of serum FA. Except for 18:2, the delta(13)C of experimental diets was lower than that of serum FA, consistent with observations in animals. These data show tha t carbon isotope ratios stabilize in response to controlled diets with in 5 wk, reflecting the isotope ratio of their dietary source, and est ablish isotope ratio fluctuations for endogenous compounds for future studies.