The extensive systematic review of 83 randomised controlled trials assessing effects of polyunsaturated fats on newly diagnosed diabetes and glucose metabolism, found that increasing omega-3 had little or no effect on type 2 diabetes, adding that there may even be a negative outcome at high doses (above 4.4 grams per day).
Researchers concluded that omega 3 supplements should not be encouraged and where supplements are used, doses below 4.4 grams per day should be encouraged.
They also found no evidence that the omega-3/omega-6 ratio is important to type 2 diabetes mellitus or glucose metabolism.
In an interview with the BBC, lead researcher Dr Lee Hooper said there had been concerns omega-3 supplements might harm people with type 2 by making glucose control more difficult. But she explains that those with the condition, or who are at risk of developing it, can also have high levels of triglycerides which omega-3 has been shown to reduce.
She said: "This is really expensive stuff. If somebody's at risk of diabetes, there are much better things to spend money on, like a physical activity - or oily fish."
Researchers searched Cochrane CENTRAL, Medline, and Embase to 27 April 2017 and ClinicalTrials.com and the WHO International Clinical Trials Registry Platform to September 2016, and reassessed all ongoing trials in December 2018.
They checked included trials of relevant systematic reviews and wrote to authors of included studies for additional studies and trial data, creating a database of trials that randomised participants to increased omega-3, omega-6, or total PUFA compared with lower omega-3, omega-6, or total PUFA and assessed effects for at least 24 weeks.
From this database, the researchers chose studies for that had assessed at least one primary review outcome
Statistical analysis included random effects meta-analyses using relative risk and mean difference, and sensitivity analyses.
Funnel plots were examined and subgrouping assessed effects of intervention type, replacement, baseline risk of diabetes and use of antidiabetes drugs, trial duration, and dose. Risk of bias was assessed with the Cochrane tool and quality of evidence with GRADE.
Type 2 diabetes mellitus is a leading cause of death and increases risks of cardiovascular disease, blindness, kidney failure, and lower limb amputation.
More than 400 million adults worldwide live with diabetes. This figure is rising, causing excess mortality, morbidity, and substantial economic cost.
The global annual cost of diabetes is estimated at more than $800bn (£636bn; €709bn) and is increasing.
However this isn’t the first time this has been called into question. A parallel randomised controlled trial in 2015 by researchers from the University of Aberdeen found long-term supplementation with fish oil at doses between 3 g/day and 5 g/day did not affect glycaemic control.
Authors: Brown, T. J., Brainard. J., Song. F., Xia., Abdelhamid. A., Hooper. L.
"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"