23 Aug Omega-3 Supplements Did Not Protect Agains Diabetes
MedicalResearch.com Interview with:
Lee Hooper PhD, RD
Reader in Research Synthesis, Nutrition & Hydration
Norwich Medical School
University of East Anglia
MedicalResearch.com: What is the background for this study?
Response: The World Health Organization asked us to carry out a set of studies (systematic reviews of randomised controlled trials) assessing health effects of omega-3 and omega-6, which are polyunsaturated fats. This is because the WHO are planning to update their dietary guidance on fats in the near future.
Worries about effects of long chain omega-3 on control of diabetes have long existed, and some experimental studies have suggested that omega-3 supplementation and diets high in PUFA and omega-3 raise fasting glucose. Pollutants such as methylmercury and polychlorinated biphenyl levels exceeding recommended thresholds are rarer now, but have been reported in seafoods and fish oil supplements; elevated mercury levels interrupt insulin signalling, raising fasting glucose, in mouse models. Body concentrations of organic pollutants are correlated with prevalence of diabetes in the US, but other cross sectional studies have suggested either no association with or benefits of eating fish on glycaemic control. Systematic reviews of observational studies have suggested both positive and negative associations with glucose metabolism, but strong evidence shows that omega-3 supplements reduce raised triglycerides and have little or no effect on body weight. Theories suggest that omega-3 and omega-6 fats compete in some metabolic pathways so that the omega-3/omega-6 ratio is more important than absolute intakes of either.
MedicalResearch.com: What are the main findings?
Response: This was a systematic review. We searched for and assessed all trials that assessed the effects of increasing long-chain omega-3, alpha-linolenic acid, omega-6 or total PUFAs. The fats could be taken as supplements or via enriched or naturally rich foods. The trials randomly allocated participants to either eating more long-chain omega-3 (or ALA, omega-6 or total PUFA) or their usual amount, and to continue this for at least 6 months. Then they compared the health of the two groups, eating more or less of the polyunsaturated fat.
We combined the results of all of these trials to see how taking more or less long-chain omega-3 affected new diabetes diagnoses and measures of glucose metabolism (how well the body processes sugars). We asked whether, across all the trials, did the people randomised to more long-chain omega-3 have more or fewer diagnoses of diabetes than the people who ate their usual diet? How did the glycosylated haemoglobin, insulin and blood glucose levels (measures of glucose metabolism) change in the two groups of people?
We found that despite over 58,000 participants being randomised into long-term trials, and 4 per cent of those participants developing diabetes, the people who were randomised to consume more long-chain omega-3 fats (fish oils) had the same risk of diabetes diagnosis as the group who did not take more fish oil (the control group). This was despite good long-chain omega-3 doses (mean dose was 2 grams per day of long-chain omega-3 fats in the intervention groups, mean trial duration was 33 months). This means that the lack of effect was NOT due to low doses or the trials being too short to see effects.
Similarly, there was a consistent lack of effect of fish oils (long-chain omega-3 fats) on any measures of glucose metabolism, which are related to diabetes risk. However, there was some (weak) evidence that when people take high doses of fish oils (long-chain omega-3) they may experience worsening glucose metabolism.
There was insufficient information from trials of ALA, omega-6 or total polyunsaturated fats to assess either protective or harmful effects – so we don’t know whether these fats are neutral, protective or harmful as regards diabetes risk.
MedicalResearch.com: What should readers take away from your report?
Response: This is definitive – taking fish oil supplements does not protect against diabetes, but neither is it harmful except possibly at high doses.
Some people with diabetes may be prescribed fish oil supplements or long-chain omega-3 fats to reduce levels of triglycerides in their blood. Long-chain omega-3 is effective at reducing blood triglycerides. However, the doses should be lower than 4.4 grams per day of long-chain omega-3 fats to prevent possible harmful effects on glucose metabolism.
MedicalResearch.com: What recommendations do you have for future research as a result of this work?
Response: We would also have liked to find out whether taking more omega-3 might be useful in those people with especially low omega-3 intakes in their day to day life. This is because giving more omega-3 is more likely to be useful in adults with low intakes. But unfortunately most trials didn’t report omega-3 intake levels of participants at the start of the trial, so we still don’t know.
Future trials need to measure and assess baseline omega-3 intakes, and assess effects of eating more oily fish – not just supplements.
MedicalResearch.com: Is there anything else you would like to add?
Response: Our previous research has shown that long-chain omega 3 supplements, including fish oils, do not protect against conditions such as heart disease, stroke or death. This review shows that they do not prevent or treat diabetes either.
This large systematic review included information from many thousands of people over long periods. Despite all this information, we don’t see protective effects, and the most trustworthy studies consistently showed little or no effect of long-chain omega 3 fats on diabetes.
Omega-3 supplements should not be encouraged for diabetes prevention or treatment. If people do choose to take supplementary fish oil capsules to treat or prevent diabetes, or to reduce levels of triglycerides in their blood, then they should use doses of less than 4.4 grams per day to avoid possible negative outcomes.
We tried very hard to assess effects of eating oily fish (rather than taking supplements), but there are few trials – most trials give supplements. So at present the effects of eating more oily fish on diabetes risk is unclear.
Our set of systematic reviews has so far assessed effects of fish oil supplements (and also the other fats) on diabetes, risk of death from any cause, heart disease and other cardiovascular diseases like stroke, body weight, serum lipids (cholesterol and triglycerides) and musculoskeletal outcomes. There appears to be little or no effect of fish oil supplements on any of these outcomes except for blood triglycerides.(1, 2)
This research was carried out with funding from the World Health Organization. No further funding was received.
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 doi: https://doi.org/10.1136/bmj.l4697 (Published 21 August 2019)Cite this as: BMJ 2019;366:l4697
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