Background: People with type 2 diabetes mellitus are at increased risk from cardiovascular disease. Dietary fish oils are known to reduce triglyceride levels, but their impact on cholesterol levels, glycemic control and vascular outcomes are not well known.
Objectives: To determine the effects of fish oil supplementation on cardiovascular outcomes, cholesterol levels and glycemic control in people with type 2 diabetes mellitus.
Search strategy: We carried out a comprehensive search of the Cochrane Controlled Trials Register, Medline, Embase, Lilacs, bibliographies of relevant papers and contacted experts for identifying additional trials. Date of last search: September 2000.
Selection criteria: All randomized placebo-controlled trials in which fish oil supplementation was the only intervention in people with type 2 diabetes were included. Authors were contacted for missing information.
Data collection and analysis: Three investigators performed data extraction and quality scoring independently with discrepancies resolved by consensus.
Main results: Eighteen trials including 823 participants followed for a mean of 12 weeks were included. Doses of fish oil used ranged from 3 to 18 g/day. No trials with vascular event or mortality endpoints were identified. The outcomes studied were glycemic control and lipid levels. Meta-analysis of pooled data demonstrated a statistically significant effect of fish oil in lowering triglycerides by 0.56 mmol/l (95% CI -0.71 to -0.40 mmol/l) and raising LDL cholesterol by 0.21 mmol/l (95% CI 0.02 to 0.41 mmol/l). No statistically significant effect was observed for fasting glucose, HbA1c, total or HDL cholesterol. The triglyceride lowering effect and the elevation in LDL cholesterol were most marked in those trials that recruited people with hypertriglyceridemia and used higher doses of fish oil. No adverse effects of the intervention were reported.
Reviewer's conclusions: Fish oil supplementation in type 2 diabetes lowers triglycerides, may raise LDL cholesterol (especially in hypertriglyceridemic patients on higher doses of fish oil) and has no statistically significant effect on glycemic control. Trials with vascular event or mortality defined endpoints are needed.