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Increasing the intake of polyunsaturated fats in the diet with supplements of omega-3 fatty acids, for example, is unlikely to affect people’s risk of developing type 2 diabetes. However, this review only looked at the effect of supplements on diabetes, not wider health.
This large systematic review included 83 long-term trials comparing higher and lower intake of omega-3 and omega-6 fatty acids and total polyunsaturated fats in healthy adults and those with existing diabetes. It found no convincing evidence that a higher intake of these fatty acids affected people’s risk of developing type 2 diabetes or improved control of existing diabetes. There was a suggestion that high doses of long-chain omega-3 fatty acids might worsen glucose metabolism.
The finding supports existing recommendations not to offer omega-3 supplements to prevent or treat type 2 diabetes.
Why was this study needed?
Around 4.7 million people in the UK have diabetes, costing the NHS more than £10 billion per year. Around 90% of these people have type 2 diabetes, a condition in which the hormone insulin, which controls the uptake of glucose from the blood, either can’t work properly or is not produced in enough quantities.
People with diabetes are often advised to eat plenty of oily fish and plant oils. These are rich in polyunsaturated fatty acids (PUFA), such as omega-3 and omega-6 fatty acids, but the impact of these substances on diabetes development and glucose metabolism is unclear. Some experimental data has suggested that omega-3 may worsen the control of diabetes.
This study aimed to review the effects of PUFA on diabetes risk and glucose metabolism and to answer these uncertainties. It also looked specifically at omega-6 fatty acids, short-chain omega-3 (α-linolenic acid), long-chain omega-3 (eicosapentaenoic acid [EPA], and docosahexaenoic acid [DHA]) fatty acids.
What did this study do?
This systematic review compared the effects of higher and lower omega-3, omega-6 and total PUFA intake on new diagnoses of diabetes, pre-diabetes, maintenance of normal blood glucose, serum insulin and insulin resistance. Eligible interventions could be dietary advice, supplementation (taken orally as oil, foods or capsules), or diet provided.
The review included 83 randomised controlled trials lasting at least 24 weeks, involving a total of 121,070 adults at any risk of diabetes or with diabetes. Half of the studies were conducted in Europe. They evaluated increasing PUFA on its own, not the effect of a wider change in diet or lifestyle.
There was limited data on the effect of increasing α-linolenic acid, omega-6, and total PUFA, and the authors identified a risk of bias in many of the included trials, though this might be expected to magnify any treatment effect. Also, most trials assessing the impact of boosting long-chain omega-3 consumption involved supplements, making it difficult to draw conclusions about the impact of increasing oily fish consumption.
What did it find?
- Increasing consumption of long-chain omega-3 fatty acids had little or no effect on the likelihood of being diagnosed with type 2 diabetes. This occurred in 3.7% of people over the course of these trials, relative risk 1.00 (95% CI 0.85 to 1.17; 17 trials; 58,643 participants). It also had little or no effect on measures of glucose metabolism, including glycated haemoglobin, insulin resistance (HOMA-IR), plasma glucose or fasting insulin.
- High dose supplementation with long-chain omega-3 (more than 4.4 g/day) may worsen glucose metabolism, although this finding should be interpreted with caution, as subgrouping by dose did not reveal statistically significant differences.
- The impact of increasing α-linolenic acid, omega-6, and total polyunsaturated fatty acids on diabetes diagnosis remains unclear because the evidence was of very low quality.
- There is no evidence that the ratio of omega-3 to omega-6 is important for type 2 diabetes or glucose metabolism.
What does current guidance say on this issue?
The NICE guideline on cardiovascular disease risk assessment and reduction states that omega-3 fatty acids should not be offered to people with type 2 diabetes. However, NICE’s guideline on the management of adults with type 2 diabetes advises including oily fish in the diet.
The American Diabetes Association also recommends consuming a Mediterranean-style diet high in polyunsaturated fats, omega-3, and α-linolenic acid, without taking supplements.
What are the implications?
This study supports recommendations that omega-3 fatty acid supplements should not be offered to people with type 2 diabetes, for their diabetes. Neither should the supplements be offered to prevent diabetes in people at high risk of developing the disease.
People with diabetes or at risk of developing it should still be encouraged to eat a healthy Mediterranean-style diet, including oily fish, as part of a healthy lifestyle.
Citation and Funding
Brown TJ, Brainard J, Song F et al. Omega-3, omega-6, and total dietary polyunsaturated fat for prevention and treatment of type 2 diabetes mellitus: systematic review and meta-analysis of randomised controlled trials. BMJ. 2019;366:14697.
The study was funded by the World Health Organization’s Nutrition Guidance Expert Advisory Group (NUGAG) subgroup on Diet and Health.
Bibliography
Diabetes UK. Tackling the crisis. London: Diabetes UK; 2019.
NICE. Cardiovascular disease: risk assessment and reduction, including lipid modification. CG181. London. National Institute for Health and Care Excellence; 2014 (updated 2016).
NICE. Type 2 diabetes in adults: management. NG28. London: National Institute for Health and Care Excellence; 2015 (updated 2017).
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