AbstractThe effects of a diet rich in α‐linolenic acid vs. one rich in oleic acid on the oxidation of uniformly labeled13C‐α‐linolenic acid and its conversion into longer‐chain polyunsaturates (LCP) were investigatedin vivo in healthy human subjects. Volunteers received a diet rich in oleic acid (n=5) or a diet rich in α‐linolenic acid (n=7; 8.3 g/d) for 6 wk before and during the study. After 6 wk, subjects were given 45 mg of13C‐α‐linolenic acid dissolved in olive oil. Blood samples were collected att=0, 5, 11, 24, 96, and 336 h. Breath was sampled and CO2 production was measured each hour for the first 12 h. The mean (±SEM) maximal absolute amount of13C‐eicosapentaenoic acid (EPA) in plasma total lipids was 0.04 ±0.01 mg in the α‐linolenic acid group, which was significantly lower (P=0.01) than the amount of 0.12±0.03 mg13C‐EPA in the oleic acid group. Amounts of13C‐docosapentaenoic acid (DPA) and13C‐docosahexaenoic acid (DHA) tended to be lower as well. The mean proportion of labeled α‐linolenic acid (ALA) recovered as13CO2 in breath after 12 h was 20.4% in the ALA and 15.7% in the oleic acid group, which was not significantly different (P=0.12). The cumulative recovery of13C from13C‐ALA in breath during the first 12 h was negatively correlated with the maximal amounts of plasma13C‐EPA (r=−0.58,P=0.047) and13C‐DPA (r=−0.63,P=0.027), but not of13C‐DHA (r=−0.49,P=0.108). In conclusion, conversion of13C‐ALA into its LCP may be decreased on diets rich in ALA, while oxidation of13C‐ALA is negatively correlated with its conversion into LCP. In a few pilot samples, low13C enrichments of n−3 LCP were observed in a diet rich in EPA/DHA as compared to oleic acid.