medwireNews: A systematic review and meta-analysis published in The BMJ confirms that intake of omega-3 fats does not influence the risk for or progression of type 2 diabetes.
The data were drawn from 83 randomized controlled trials of at least 24 weeks’ duration, involving a total of 121,070 participants, making this “the most extensive systematic review of trials to date” on this topic, according to Lee Hooper (University of East Anglia, Norwich, UK) and study co-authors.
The researchers also contacted the authors of 52 of these trials to obtain further information and data, which they successfully did in 36 cases. But they caution that only 10 of the included trials had a low summary risk for bias.
Long chain omega-3 intake had no effect on the risk for a type 2 diabetes diagnosis (in 17 trials), nor on glycated hemoglobin levels or insulin resistance (also in 17 trials). This did not differ by duration of supplementation, the nutrient replaced, antidiabetes medication use, or baseline glucose metabolism.
Hooper and team therefore believe that omega-3 supplementation should in general “not be encouraged” for people with or at risk for type 2 diabetes, although they note that the predominance of supplementation trials in their analysis “precluded fair assessment of [the] effects of increasing oily fish consumption.”
Furthermore, they found some evidence that doses above 4.4 g/day might increase the risk for a type 2 diabetes diagnosis, and also increase levels of glycated hemoglobin, fasting glucose, and insulin resistance.
Although urging caution in the interpretation of these subgroup analyses, they suggest that where people choose to take supplements, or do so specifically to reduce triglyceride levels, they should stick to doses below 4.4 g/day.
Eleven trials looked at the effects of increasing omega-6 fats, but the evidence was of poor quality and was unclear, with uncertain directions of effect and wide confidence intervals.
The evidence was equally unclear for the effects of replacing omega-3 with omega-6 fats. The researchers say that “[i]f the theory that omega-3 and omega-6 fats have opposing roles is correct, we would expect to see strong effects when omega-3 replaces dietary omega-6.”
However, they note that “doses of omega-6 were often small, so we could be missing important effects of the ratio.”
Twelve trials assessed the effects of α-linoleic acid and indicated that it had little or no effect on type 2 diabetes diagnosis or other outcomes, with the possible exception of it causing an increase in fasting insulin levels (albeit, again, from low-quality evidence).
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