Effect of dietary fish oil on body fat mass and basal fat oxidation in healthy adults

Summary

This small but mechanistically important crossover trial asked a focused question: does substituting fish oil for visible dietary fat — without changing total calories or other diet composition — actually shift body fat mass and substrate oxidation? Six healthy young volunteers (five men, mean age 23, normal BMI) ate a controlled diet for three weeks, then 10–12 weeks later ate the same diet with 6 grams per day of visible fat replaced by 6 grams of fish oil for another three weeks. The fish-oil arm produced a small but statistically significant body-fat-mass reduction relative to control (-0.88 vs -0.3 kg). Basal respiratory quotient dropped (0.815 to 0.834), indicating a shift toward fat as the primary fuel at rest. Basal lipid oxidation rose roughly 22 percent (1.06 vs 0.87 mg/kg/min). Resting metabolic rate adjusted for lean body mass was unchanged — meaning the body wasn't burning more calories overall, just shifting the substrate mix toward fat oxidation. The paper is one of the cleanest demonstrations that fish-oil intake can shift substrate metabolism in healthy adults independent of overall calorie change.