It would be welcome news to millions if fish oil supplements were proven to help prevent diabetes. But new research delivers very disappointing data on the prospect.
Previous research has hinted that fish oil supplements -- which contain omega-3 fatty acids -- might improve blood sugar metabolism and possibly stave off Type 2 diabetes. But this latest research found no evidence that popping a daily fish oil pill could keep diabetes at bay.
"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," said study author Lee Hooper.
Additionally, she said that her group's previous research has shown that these types of supplements also don't protect against heart disease, stroke or early death. Hooper is a reader in research synthesis, nutrition and hydration at Norwich Medical School at the University of East Anglia, in England.
Fish oil supplements can help reduce high levels of triglycerides (a type of blood fat that contributes to heart disease), Hooper noted. She said the supplements are generally considered safe to take, but added that this review found that larger doses -- above four or five grams per day -- of fish oil supplements might actually lead to worse blood sugar metabolism.
So if people do take fish oil supplements, she noted, they should take doses that are less than 4.4 grams a day to avoid any potentially negative effects.
The review included 83 randomized, controlled clinical trials with more than 120,000 participants in total. Most assessed the effects of long-chain omega-3 fatty acid. The effects of alpha-linolenic acid, omega-6 fatty acid and total polyunsaturated fats were also included in some of the studies, but Hooper said there wasn't enough information on these fats to include in the review.
Omega-3 fats are typically found in fatty fish like salmon. They are also available as over-the-counter supplements.
The studies in the review lasted 24 weeks or longer. The study participants were mostly from Europe, but also from North America, South America, Australia and Asia. The studies were published from the 1960s until 2018.
The researchers looked at the effects of omega-3 fats taken as supplements or in foods.
After finding no benefit for diabetes prevention from these fats, the researchers double-checked the data in different ways. In one re-analysis, they looked only at the studies considered to be the highest quality. They also re-checked to see if different dosages or longer studies might have led to different outcomes.
The investigators still found no benefit. Hooper said the results of this review are very definitive: "Taking fish oil supplements does not protect against diabetes."
Hooper said the researchers weren't able to assess the effects of eating oily fish on diabetes, because there wasn't enough published evidence. They also didn't have enough information from the studies to see if omega-3 supplements might be helpful for people who start off with a low level of this nutrient.
Dr. Joel Zonszein, director of the clinical diabetes center at Montefiore Medical Center in New York City, said this study clearly showed that fish oil supplements won't prevent diabetes.
"It's the same story again -- first it was vitamin E, green tea, chromium, cinnamon and then fish oil. Everyone wants a pill to cure or prevent diabetes, but it's not there. And people are spending a lot of money on these supplements," he said.
Zonszein said fish like salmon can be a good option as part of a healthy, balanced diet, but that there's no evidence that fish oil supplements alone can help diabetes.
"Diabetes is a serious disease that can lead to a number of complications. Once you have diabetes, you need to treat it properly," he explained.
The review was published Aug. 21 in the BMJ.
Learn more about omega-3 fatty acid supplements from the U.S. National Center for Complementary and Integrative Health.
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