Becky McCall
January 22, 2014
A higher level of serum long-chain omega-3 polyunsaturated fatty acids (PUFAs) — an objective biomarker of fish intake — is linked to a lower long-term risk for type 2 diabetes, according to the results of a new prospective, population-based cohort study.
Among dietary factors, the long-chain omega-3 PUFAs, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), from fish and other seafood have gained special interest because of their beneficial effects on the risk for cardiovascular diseases and several risk factors for diabetes, including inflammation, adiposity, hypertension, and dyslipidemia, explain Jyrki K. Virtanen, PhD, adjunct professor of nutritional epidemiology at the University of Eastern Finland, Kuopio, and colleagues in their article published in the January issue of Diabetes Care.
But findings from most previous studies have been mixed in relation to whether long-chain omega-3 PUFAs have a beneficial or detrimental effect on type 2 diabetes risk per se, they note. The current study differs from most previous trials in using an objective biomarker as a measure of exposure to serum omega-3 PUFAs, and the fact “that few prior studies have used an objective biomarker may partly explain their diverse findings,” Dr. Virtanen pointed out.
The research also looked at the influence of mercury exposure on incidence of diabetes and whether contamination of this kind modifies any effects of long-chain omega PUFAs. “Despite relatively high exposure to methylmercury, mainly through fish consumption, mercury exposure did not affect the risk of type 2 diabetes in our study population,” he told Medscape Medical News.
Highest Omega-3 PUFA Intake Lowered Diabetes by 33%
Type 2 diabetes is on the rise in Finland, precipitating a need for greater prevention, the researchers explain. Given the prior conflicting data, “We wanted to seek some clarification of the association between serum long-chain omega-3 PUFAs and type 2 diabetes,” said Dr. Virtanen.
In 2012, a meta-analysis of data from 438,000 individuals in 12 independent prospective cohorts with an average 11-year follow-up (Diabetes Care. 2012;35:930-938) concluded that there was no inverse association of fish or fish-oil intake with incidence of diabetes. Variation was seen between Eastern and Western diets, however, with lower risk for type 2 diabetes in Asian and higher risk in US study populations.
Dr. Virtanen explained that it was not well understood why the results from different studies varied, but the geographical differences may be associated with genetic differences or may be due to types of fish consumed or how the fish was prepared.
In their cohort, involving around 2000 men aged 42 to 60 years from the Kuopio Ischaemic Heart Disease Risk Factor Study (KIHD), who were free of diabetes at baseline in 1984–1989, the researchers used serum omega-3 PUFA and hair mercury levels as biomarkers for exposure, as well as dietary intake, assessed with 4-day food recording.
They correlated the findings with the incidence of type 2 diabetes, which was assessed by self-administered questionnaires and glucose tolerance tests, with blood glucose measurements at 4, 11, and 20 years after the baseline, and by record linkage to hospital-discharge registry and the reimbursement register on diabetes medication expenses.
Dr. Virtanen explained that circulating omega-3 PUFAs offer an objective biomarker for exposure in preference to dietary-assessment methods because this avoids reliance on memory about food intake. “Random error from dietary-assessment methods used in some studies can attenuate associations in studies and thus could explain null findings.”
After an average follow-up of 19.3 years, 422 men (19.2%) had developed diabetes, and those in the highest quartile of serum long-chain omega-3 PUFA concentrations (>5.33% total serum fatty acids) had a 33% lower risk for incident type 2 diabetes compared with men in the lowest quartile (P for trend = .01).
In contrast, dietary fish or EPA plus DHA intakes, assessed with 4-day food recording, were not associated with the risk. “This most likely reflects the inability of the 4-day food recording to accurately assess intakes of foods that are usually consumed at most 1 to 2 times per week, such as fish,” the researchers observe.
Mercury Exposure Not Linked to Diabetes
Dr. Virtanen’s group has previously shown that exposure to methylmercury, a major dietary source of which is fish, increased the risk for cardiovascular diseases and also attenuated the benefits of the serum omega-3 PUFAs on risk in the KIHD population.
“There is very little information about the association between the environmental contaminants in fish and risk of type 2 diabetes, with some studies associating mercury exposure with insulin resistance,” he added.
But hair mercury — a marker for long-term exposure to mercury — was not associated with diabetes risk in the current study. “Also, we did not find associations with [alpha-linolenic acid] ALA [an essential fatty acid that is derived from plant sources in the diet] either. This suggests that the findings [regarding a reduction in diabetes risk] were specific to the long-chain omega-3 PUFAs from fish,” Dr. Virtanen remarked.
The existence of very detailed information about type 2 diabetes may have made it easier to diagnose during follow-up, he noted.
Nevertheless, “there is a need for more studies in diverse study populations…to get a better picture of the impact of fish consumption on the risk of type 2 diabetes,” he concluded.
Dr. Virtanen and coauthors have reported no relevant financial relationships.
Diabetes Care. 2014;37:189–196. Abstract