Fish oil, insulin sensitivity, insulin secretion and glucose tolerance in healthy people: Is there any effect of fish oil supplementation in relation to the type of background diet and habitual dietary intake of n -6 and n -3 fatty acids?

Abstract Aim To evaluate whether a moderate supplementation of long-chain n -3 fatty acids is able to modulate insulin sensitivity, insulin secretion, β-cell function and glucose tolerance in healthy individuals consuming a diet rich in either saturated or monounsaturated fat, also in relation to their habitual dietary intake of n -6 and n -3 fatty acid. Methods and results One hundred and sixty-two healthy individuals were randomly assigned to follow either one of two isoenergetic diets for 3 months, one rich in monounsaturated fats and the other rich in saturated fats. Within each group there was a second randomisation to fish oil ( n -3 fatty acids 3.6 g/day) or placebo. At the beginning and at the end of the treatment periods insulin sensitivity (SI), first phase insulin response (FPIR) and glucose tolerance (KG -value) were evaluated by the intravenous glucose tolerance test (IVGTT). Fish oil did not have any effect on SI, FPIR, KG -value and disposition index in either diet. Even after dividing subjects according to the median value of n -6/ n -3 ratio of serum phospholipids at baseline, there was no change in SI (Δ SI 0.42 ± 0.34 on fish oil vs 0.14 ± 0.23 on placebo for those with n -6/ n -3 4.85) (M ± SE), FPIR (Δ FPIR 135.9 ± 78.9 vs 157.2 ± 157.5 pmol/L; 38.8 ± 181.7 vs 357.1 ± 181.7 pmol/L), KG -value (Δ KG 0.14 ± 0.15 vs 0.12 ± 0.11; −0.32 ± 0.16 vs 0.15 ± 0.15) or disposition index (Δ disposition index 1465.4 ± 830.4 vs 953.8 ± 690.0; −1641.6 ± 1034.3 vs 446.6 ± 905.1). Considering the 75th percentile of n -6/ n -3 ratio (5.82) the results on insulin sensitivity, insulin secretion and disposition index were confirmed, while, in this more extreme situation, n -3 fatty acid supplementation induced a significant deterioration of KG -value ( p = 0.02). Conclusions In healthy individuals a moderate supplementation of fish oil does not affect insulin sensitivity, insulin secretion, β-cell function or glucose tolerance. The same is true even when the habitual dietary intake of n -6 and n -3 fatty acids is taken into account.