Most of the research on this question has come from epidemiological studies that compared people’s self-reported diets to their risk of developing diabetes.
No epidemiological study can prove that a food or nutrient lowers or raises a disease risk ... and people's accounts of their own eating patterns are notoriously unreliable.
Accordingly, as University of North Carolina researchers reported two years ago, the evidence from epidemiological studies has been mixed (Xun P, He K 2011).
And few studies of omega-3s and disease risk account for the enormous, pro-inflammatory excess of omega-6 fats in the subjects' “Western” diets ... which blocks uptake of dietary omega-3s and blunts their benefits.
(To learn more about that, see “America's Sickening Omega Imbalance”, “Omega-6/Omega-3 Imbalance Pushes Heart/Diabetes Perils”, and “Dr. Oz Tells the Omega-3 Story”.)
High hopes persist, thanks to largely positive signs from cell, animal, and small-scale clinical research, which suggest that diets higher in omega-3s – and lower in omega-6s – than average should help.
Omega-3 EPA and DHA are found in almost every human cell, are essential to life, and play critical roles in immune, metabolic, and brain functions.
We can make EPA and DHA from the sole plant-source omega-3s (called ALA), but only in the tiny amounts sufficient to ensure survival.
Omega-3 EPA and DHA exert beneficial effects on key risk factors for diabetes, including inflammation, adiposity (excess abdominal fat), hypertension, and “dyslipidemia” (unhealthful fat and cholesterol profiles) ... see “Omega-3s and Antioxidants Eased Diabetes”, “Fish & Omega-3s May Help Deter Diabetes”.
Long-term, placebo-controlled clinical trials remain the gold standard, but they’re difficult to conduct, control, and fund.
The next best thing are epidemiological studies in which scientists compare people’s omega-3 blood levels to their risk of developing diabetes over a long period of time.
Such blood-test based studies are far more reliable than the more common type, in which researchers must rely on people’s answers to questions about their diets.
Earlier this year, researchers published just such a blood-based study ... and the encouraging results justify urgent funding of controlled clinical trials.
Higher omega-3 blood levels linked to one-third reduction in diabetes risk
Scientists at the University of Eastern Finland recruited 2,212 men aged 42 to 60 years, who were participating in the Kuopio Ischemic Heart Disease Risk Factor Study (KIHD).
All of the participating men were free of diabetes when they joined the 20-year KIHD study between 1984 and 1989.
As in many other studies, the men answered a diet questionnaire … but they also provided blood samples.
This allowed the researchers behind the new analysis to measure the men’s precise omega-3 levels … the best possible indicator of omega-3 intake from fish and/or fish oil.
In addition, the men gave hair samples that were tested for mercury levels, so the researchers were able to compare those levels to the risk of developing diabetes.
Development of diabetes was determined by glucose tolerance tests, blood glucose measurements taken four, 11, and 20 years after the study began, and by looking at the men’s hospital and medical reimbursement records.
At the end of the two-decade-long study, the men whose omega-3 levels ranked in the highest one-quarter – greater than 5.33% of total fatty acids – were 33% less likely to have developed diabetes, compared with the men whose omega-3 levels ranked in the lowest one-quarter.
The authors put it this way: “[Omega-3 blood levels], an objective biomarker for fish intake, was associated with long-term lower risk of type 2 diabetes.” (Virtanen JK et al. 2014)
Given the relative unreliable nature of data from diet questionnaires, it's little surprise that the men’s self-reported fish intakes bore no relationship to their diabetes risk.
As the researchers wrote, “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.” (Virtanen JK et al. 2014)
No link between mercury and diabetes risk
The researchers also looked at the influence of mercury exposure on the risk of diabetes, and whether high mercury levels might reduce any beneficial effects of omega-3-rich diets.
And the results showed no correlation between mercury levels and diabetes risk.
As lead author Jyrki K. Virtanen, PhD told Medscape Medical News, “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.”
That finding fits with the protective effect of the selenium that abounds in virtually all ocean fish. For more information, see “Most Fish Rank as Very Safe on New, Selenium-Based Standard”, “Fish-Mercury Fears Hyped, Despite Pesky Facts”, and “No Heart Risk from Fish-Borne Mercury”.
- Virtanen JK, Mursu J, Voutilainen S, Uusitupa M, Tuomainen TP. Serum omega-3 polyunsaturated fatty acids and risk of incident type 2 diabetes in men: the Kuopio Ischemic Heart Disease Risk Factor study. Diabetes Care. 2014 Jan;37(1):189-96. doi: 10.2337/dc13-1504. Epub 2013 Sep 11.
- Xun P, He K. Fish Consumption and Incidence of Diabetes: meta-analysis of data from 438,000 individuals in 12 independent prospective cohorts with an average 11-year follow-up. Diabetes Care. 2012 Apr;35(4):930-8. doi: 10.2337/dc11-1869.