Effect of altered dietary n-3 fatty acid intake upon plasma lipid fatty acid composition, conversion of [13C]alpha-linolenic acid to longer-chain fatty acids and partitioning towards beta-oxidation in older men.
Level 2 - randomized trial
Randomized controlled trial with stable-isotope tracer methodology
PubMed 12908891 · doi:10.1079/bjn2003901
What was done
Healthy men (mean age 52 [SD 12] years) completed a 4-week baseline period consuming a control spread, followed by a test meal containing 700 mg [U-13C]alpha-linolenic acid (ALNA) to measure 48-hour conversion to eicosapentaenoic acid (EPA), docosapentaenoic acid (DPA), and docosahexaenoic acid (DHA). Participants were then randomized for 8 weeks to: (1) continued control diet (n = 5), (2) high ALNA intake (10 g/day, n = 4), or (3) high EPA+DHA intake (1.5 g/day, n = 5), before repeating the tracer study to assess changes in plasma lipid composition, conversion capacity, and partitioning toward beta-oxidation.
What was found
At baseline, apparent fractional conversion of labelled ALNA was: EPA 2.80%, DPA 1.20%, and DHA 0.04%. After 8 weeks, the control group showed no changes. The high-ALNA diet increased plasma triacylglycerol-EPA and -DPA concentrations and phosphatidylcholine-EPA concentration, whilst fractional [13C]ALNA conversion remained similar to baseline. The high-(EPA+DHA) diet raised plasma phosphatidylcholine-EPA and -DHA concentrations, decreased [13C]ALNA conversion to EPA 2-fold and to DPA 4-fold, while [13C]ALNA conversion to DHA remained unchanged. Dietary interventions did not alter partitioning of ALNA toward beta-oxidation.
Why it matters
This indicates that human ALNA conversion capacity is down-regulated by increased availability of downstream products (EPA and DHA) but is not up-regulated by increased substrate intake. This feedback mechanism helps explain why increasing dietary ALNA has a limited effect on DHA levels.
Limits
The sample size was very small (total n = 14; 4 to 5 per group), limiting statistical power. The study examined only older men, precluding generalization to women or younger individuals.
Cited by
- supports In humans, the metabolic conversion rate of dietary ALA to EPA is roughly 2% to 5%, with conversion to DHA being even lower.