Effects of Dietary α-Linolenic Acid on the Conversion and Oxidation of 13C-α-Linolenic Acid

The effects of a diet rich in α-linolenic acid vs. one rich in oleic acid on the oxidation of uniformly labeled 13C-α-linolenic acid and its conversion into longer-chain polyunsaturates (LCP) were investigated in 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 of 13C-α-linolenic acid dissolved in olive oil. Blood samples were collected at t = 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 of 13C-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 mg 13C-EPA in the oleic acid group. Amounts of 13C-docosapentaenoic acid (DPA) and 13C-docosahexaenoic acid (DHA) tended to be lower as well. The mean proportion of labeled α-linolenic acid (ALA) recovered as 13CO2 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 of 13C from 13C-ALA in breath during the first 12 h was negatively correlated with the maximal amounts of plasma 13C-EPA (r = −0.58, P = 0.047) and 13C-DPA (r = −0.63, P = 0.027), but not of 13C-DHA (r = −0.49, P = 0.108). In conclusion, conversion of 13C-ALA into its LCP may be decreased on diets rich in ALA, while oxidation of 13C-ALA is negatively correlated with its conversion into LCP. In a few pilot samples, low 13C enrichments of n-3 LCP were observed in a diet rich in EPA/DHA as compared to oleic acid. Author's Abstract.