People with type 2 diabetes "should not be encouraged" to take omega-3 fish oil supplements, researchers from the University of East Anglia say.
Writing in the BMJ* the authors say they carried out the most extensive systematic review of trials to date to assess effects of polyunsaturated fats on newly diagnosed diabetes and glucose metabolism, including previously unpublished data. They reviewed 83 studies and found no evidence of that supplements were either harmful or beneficial.
They found that long chain omega-3 had little or no effect on likelihood of diagnosis of diabetes or measures of glucose metabolism such as HbA1c, plasma glucose or fasting insulin, or homoeostatic model assessment for insulin resistance.
Dr Lee Hooper, who led the research, told the BBC there had been concerns omega-3 supplements might make glucose control more difficult. However, people with diabetes or who are at risk of developing it, can also have high levels of triglycerides - a type of blood fat - which omega-3 has been shown to reduce.
She said: "We found neither harm nor benefit."
She described fish oil supplements as "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," she added.
Douglas Twenefour, deputy head of care at Diabetes UK, said: "Eating a healthy, varied diet is incredibly important, and we know that certain foods - including fruits, vegetables, wholegrains, yoghurt and cheese - can help to lower your risk of type 2 diabetes.
"While omega-3 fatty acids are crucial for our overall health, it's generally better for people with type 2 diabetes to get their intake by eating at least two portions of oily fish a week, than by taking supplements."
But Dr Carrie Ruxton, from the industry-funded Health and Food Supplements Information Service, said: "While I would prefer people to follow the government's advice and eat more fish, this isn't the reality and a daily omega-3 supplement - whether from fish oil or algae - can bridge the gap."
*Brown TJ, Brainard J, Song F, Wang Xia, 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 :l4697