Omega-3 for T2D? Spend Money on 'Something More Useful'

Nicky Broyd

August 22, 2019

"Spend your money on something more useful," is the conclusion of a UK researcher who looked into evidence on omega-3 fats and type 2 diabetes.

Increasing omega-3 fats in the diet has "little or no effect" on preventing or treating type 2 diabetes, according to the major review of evidence by a team from the Norwich Medical School, University of East Anglia (UEA).

The researchers set out to analyse how omega-3, omega-6, and total polyunsaturated fatty acids (PUFA) affected glucose metabolism and diabetes diagnosis.

The study, said to be the most extensive review so far, was commissioned by the World Health Organisation and its findings are reported online today by the BMJ .

Does Fish Oil Fight Diabetes?

Previous research has been mixed on omega-3 and type 2 diabetes, with data showing both positive and harmful effects.

Many countries recommend increasing intake of polyunsaturated and omega-3 fats as part of a healthy balanced diet. In the UK, this translates into recommendations to eat two portions of fish a week, one of them oily. There are no specific UK recommendations on omega-3 supplementation. Despite this, fish oil supplements are one of the biggest selling supplements in the UK.

The UEA team assessed data from 83 randomised controlled trials lasting at least 24 weeks, involving 121,070 people with and without type 2 diabetes. Some previously unpublished data were included.

Good quality evidence was found for long-chain omega-3 fats (LCn3) from fish oils. Increasing LCn3 by 2g per day over a mean trial duration of 33 months did not significantly affect diabetes diagnosis or glucose metabolism. Neither was there a change with longer trial durations, the review found.

Findings that a dose of more than 4.4g per day could have negative effects on the risk of diabetes and glucose metabolism should be interpreted with caution, they said.

There was less good quality evidence for the effects of alpha-linolenic acid (ALA, another omega-3), omega-6 and total PUFAs on diabetes diagnosis.

However, a meta-analysis again suggested little or no effect on glucose metabolism. Increasing ALA may increase fasting insulin by about 7%, they found.

The researchers found some studies had missing data and others had a risk of bias.

Relieved, Rather Than Surprised

Were the researchers surprised by their findings? "We were actually really relieved because there were suspicions before we started the review that there might be negative effects of the omega-3 on diabetes - both diabetes risk, and diabetes control," Lee Hooper, reader in research synthesis, nutrition & hydration, told Medscape News UK.

"But we didn't find any negative effects, she said. "We were quite relieved about that. Neither did we find any benefit. We have a huge mass of evidence here. It looks as though the effects are entirely neutral, it doesn't go positive, it doesn't go negative – very little effect.

"The exception to that is that when we get to quite high doses of long chain omega-3, over 4.4g a day, it does look as though glucose metabolism may be slightly worse at that point. So while very high doses may be negative, most don't take doses anywhere near that high, so that's probably not an issue for many people."

No Convincing Evidence

The researchers concluded: "There is no convincing evidence to suggest that altering our LCn3, ALA, omega-6 or total PUFA intakes alters glucose metabolism or risk of diabetes."

They also said supplementation "should not be encouraged for diabetes prevention or treatment”.

They did say future evidence from longer, high quality trials lasting at least 12 months would be helpful.

Reacting to the study in a statement, Douglas Twenefour, deputy head of care at Diabetes UK, said: "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."

Doctor, Should I Take Omega-3 Supplements?

This was one of a series of reviews carried out by the UEA team. "We were asked by the World Health Organisation at the health effects of omega-3s as well as omega-6s, and total polyunsaturated fat, across a whole range of health effects," Lee Hooper told us.

"We've already published our work on effects in cardiovascular disease, both heart disease and stroke, also on body weight. And again, it looks as though omega-3s are very neutral on those outcomes, too.

"Adding diabetes into that, and it not being a problem, is great."

So what should doctors say if patients ask about omega-3 supplements?

"What we should be saying is: 'Spend your money on something more useful. Spend it on a lovely sort of physical activity that you really enjoy. Or spend it on some food that you really love. Some lovely healthy fruit or some great fish.

She continued: "Don't spend it on supplements. Go for things that will actually do you some benefit, and you will enjoy'."

BMJ 2019;366:l4697 http://dx.doi.org/10.1136/bmj.l4697

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