People who report regularly eating oily fish had a significantly reduced risk for developing type 2 diabetes in a prospective, observational study of nearly 400,000 UK residents.
The results also show a significant, but weaker, positive link between regular use of fish oil supplements and a drop in the incidence of type 2 diabetes, say Qibin Qi, PhD, and colleagues in a report published in Diabetes Care. Their analysis failed to show a significant link between consumption of non-oily fish and type 2 diabetes onset.
The study is notable for being "the largest so far" to examine the link between fish consumption and type 2 diabetes incidence, and the first to establish a clear, significant association between regularly eating oily fish and a drop in the incidence of diabetes, said Qi, an epidemiologist at Albert Einstein College of Medicine in New York City.
"At present, it is prudent to recommend fresh oily fish as a part of a healthy dietary pattern instead of fish oil supplements for diabetes prevention," say Qi and coauthors.
The study included just over 392,000 adults without type 2 diabetes or cardiovascular disease at baseline enrolled in the UK Biobank. Median follow-up was just over 10 years, during which 7262 participants developed diabetes.
Participants who ate either one, or two or more, servings of oily fish weekly each had a significant 22% lower rate of incident type 2 diabetes than those who ate no oily fish, after adjustment for multiple confounders. Those who reported regularly taking a fish oil supplement had a significant 9% lower incidence of type 2 diabetes than those who didn't.
Evidence Growing to Add Oily Fish to Diet to Prevent Type 2 Diabetes
"Many current dietary guidelines recommend consumption of two servings of fish, preferably oily, per week, primarily based on cardiovascular benefits," Qi said in an interview.
"No prior statements recommended oily fish for prevention of type 2 diabetes," he explained, adding: "Our findings support future recommendations, but the evidence is not strong enough to make a [formal] recommendation now. We need evidence from clinical trials."
Jason Wu, PhD, an epidemiologist at the University of New South Wales in Sydney, Australia, who specializes in this field but was not involved with the current study, said it "is a very well-conducted study, and certainly generates important new evidence supporting the potential benefits of regular consumption of oily fish."
But he agrees that the evidence remains too preliminary for any official recommendations on eating oily fish for preventing the development of type 2 diabetes, including targeting advice to high-risk subgroups such as those with prediabetes or people who are obese.
Before any groups make recommendations, "we need to thoroughly review all the literature in this space to appraise the overall body of evidence," Wu noted in an interview.
Oily Fish: Solid Evidence for Prevention of CVD Events
In contrast, the case for including oily fish in the diet to prevent CVD events seems settled. In 2018, a panel assembled by the American Heart Association to address the issue released a statement that concluded: "Current scientific evidence strongly supports the recommendation that seafood be an integral component of a heart-healthy dietary pattern." It added that "a large body of evidence supports the recommendation to consume nonfried seafood, especially species higher in long-chain n-3 fatty acids, one to two times per week for cardiovascular benefits, including reduced risk of cardiac death, coronary heart disease, and ischemic stroke."
The statement highlighted that "cold-water oily fish such as salmon, anchovies, herring, mackerel (Atlantic and Pacific), tuna (bluefin and albacore), and sardines have the highest levels" of long-chain n-3 fatty acids, notably eicosapentaenoic acid and docosahexaenoic acid, also collectively known as omega-3 fatty acids.
These fish types were among the oily fishes tallied in the UK Biobank data used by Qi and colleagues.
The case for fish oil supplements for preventing CVD events is much rockier, as summarized in a 2019 editorial, with some studies reporting no discernible effect while others indicate efficacy.
A second commentary from December 2020 highlighted how results from the REDUCE-IT trial showed clear benefit for preventing CVD using a highly purified form of fish oil, icosapent ethyl (Vascepa, Amarin). However, findings from two other recent reports, the STRENGTH and OMENI studies, failed to show CVD benefits from more conventional fish oil formulations.
Composite CVD and Diabetes Prevention Effects?
The new findings by Qi and colleagues "highlight the need to specifically test the effect of fish oil supplements on glucose metabolism in people who cannot or choose not to regularly eat oily fish," said Wu, who is also a researcher at the George Institute for Global Health in Newtown, Australia.
"If eventually there is really strong evidence that fish, fish oil, or both have independent effects on both CVD and type 2 diabetes" it would be reasonable to integrate both outcomes into a single, composite, efficacy endpoint for the purpose of future studies, he added.
Qi agrees on both points. "A randomized, controlled trial of fish oil on type 2 diabetes as a primary outcome is needed. Most existing data are based on secondary analyses in the randomized trials for CVD," he explained.
But, he added, "our results suggest a potential beneficial effect from fish oil supplements," which implies that these may be "better than nothing" for people who can't add oily fish to their regular diet.
The means by which fish and fish oil might slow or stop progression to type 2 diabetes remains uncertain.
The mechanisms for preventing both diabetes and CVD events may overlap, Qi noted, such as anti-inflammatory effects and improved insulin sensitivity, both of which have been observed in animal studies.
Evidence is "still lacking from human studies," he explained, but if such mechanisms were at play, Wu said that would "add biologic plausibility" to a possible causal link between oily fish consumption and diabetes prevention.
"But we can't assume that omega-3 fatty acids alone will have the same effect as oily fish, which obviously contains many other components."
The study received no commercial funding. Qi and Wu have reported no relevant financial relationships.
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