Effects of Structured Lipid Supplementation for Eight Weeks on Substrate Utilization During Moderate Intensity Exercise in Healthy Untrained Men

<b>Background</b>: Structured lipids, composed of re-esterified triacylglycerols containing eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), and medium-chain fatty acids, may influence metabolism and endurance performance. This trial aimed to evaluate the effects of eight weeks of structured lipid supplementation on substrate utilization, erythrocyte fatty acid content, and endurance performance in healthy untrained men. <b>Methods</b>: In a double-blind, placebo-controlled, randomized design, 36 participants (18 per group) received either structured lipids or placebo supplementation for eight weeks. Pre- and post-supplementation assessments included maximal oxygen uptake, time to exhaustion, substrate oxidation during exercise at 65% VO_2max, and erythrocyte membrane fatty acid content. Non-parametric statistical methods were used to analyze within- and between-group differences. <b>Results</b>: After supplementation, the structured lipids group showed statistically significant within-group changes in substrate utilization, including lower respiratory exchange ratio and higher percentage fat oxidation, total fat oxidation, and mean fat oxidation rate. Statistically significant increases were also observed in erythrocyte EPA + DHA content and time to exhaustion. Compared with the placebo group, the structured lipids group showed statistically significant post-intervention differences in substrate oxidation, erythrocyte EPA + DHA levels, and time to exhaustion. <b>Conclusions</b>: Eight weeks of structured lipid supplementation increased erythrocyte membrane EPA and DHA and enhanced fat oxidation during moderate-intensity exercise in untrained men. Although endurance performance improved, the change was within natural variability and showed substantial interindividual differences. Further rigorously controlled studies are needed to determine whether these metabolic adaptations yield meaningful functional benefits.