Fish oil in people with type 2 diabetes mellitus
Abstract
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 methods
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.
Authors' 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.
Plain language summary
Adding fish oils to the diet has been proposed as a means of reducing heart disease in people with type 2 diabetes mellitus, although there have been concerns about this having a bad effect on the control of blood sugar levels. We identified eighteen randomized trials all together including over 800 people in which fish oil was compared to a vegetable oil or placebo. None of the studies looked at occurrence of cardiovascular disease (such as heart attack or stroke) or death as an outcome measure. The levels of fats in the blood were reduced in the groups receiving the fish oil, but the clinical relevance of this is uncertain. However, levels of LDL cholesterol (which may promote heart disease) were increased. Control of blood sugar levels was not affected by the treatment. There were no other adverse effects of the interventions either. Further trials of sufficient duration and looking at outcomes relevant to patients are needed to establish if fish oil supplementation in people with type 2 diabetes is justified.
