Fish/Fish Oil: Diabetes Benefits

 Omega-3 fatty acids help reduce the risk of type 2 diabetes

High concentrations of serum long-chain omega-3 fatty acids may help reduce the risk of type 2 diabetes, according to a University of Eastern Finland study published recently in Diabetes Care. The sources of these fatty acids are fish and fish oils.

Type 2 diabetes is becoming increasingly widespread throughout the world, including Finland. Overweight is the most significant risk factor, which means that diet and other lifestyle factors play important roles in the development of type 2 diabetes. Earlier research has established that weight management, exercise and high serum linoleic acid concentrations, among other things, are associated with reduced risk of diabetes. However, findings on how fish consumption or long-chain omega-3 fatty acids affect the risk of diabetes have been highly contradictory. A protective link has mainly been observed in Asian populations, whereas a similar link has not been observed in European or US studies – and some studies have even linked a high consumption of fish to increased diabetes risk.

Ongoing at the University of Eastern Finland, the Kuopio Ischaemic Heart Disease Risk Factor Study (KIHD) determined the serum omega-3 fatty acid concentrations of 2,212 men between 42 and 60 years of age at the onset of the study, in 1984–1989.

During a follow-up of 19.3 years, 422 men were diagnosed with type 2 diabetes.

Serum long-chain omega-3 fatty acid concentrations were used to divide the subjects into four categories. The risk of men in the highest serum omega-3 fatty acid concentration quarter to develop type 2 diabetes was 33% lower than the risk of men in the lowest quarter.

The study sheds new light on the association between fish consumption and the risk of type 2 diabetes. A well-balanced diet should include at least two fish meals per week, preferably fatty fish. Fish rich in long-chain omega-3 fatty acids include salmon, rainbow trout, vendace, bream, herring, anchovy, sardine and mackerel, whereas for example saithe and Atlantic cod are not so good alternatives. Weight management, increased exercise and a well-rounded diet built around dietary recommendations constitute the cornerstones of diabetes prevention.

Fish oil supplements may help fight against Type 2 diabetes

Widely-used fish oil supplements modestly increase amounts of a hormone that is associated with lower risk of diabetes and heart disease, according to a study accepted for publication in The Endocrine Society's Journal of Clinical Endocrinology & Metabolism (JCEM).

Fish oil supplements, also called omega 3 fatty acid capsules, raise levels of adiponectin in the bloodstream. Adiponectin is an important hormone that has beneficial effects on metabolic processes like glucose regulation and the modulation of inflammation. In long-term human studies, higher levels of adiponectin are associated with lower risks of type 2 diabetes and coronary heart disease.

"While prior animal studies found fish oil increased circulating adiponectin, whether similar effects apply in humans is not established," said the study's lead author, Jason Wu, PhD, of the Harvard School of Public Health. "By reviewing evidence from existing randomized clinical trials, we found that fish oil supplementation caused modest increases in adiponectin in the blood of humans."

The meta-analysis reviewed and analyzed results from 14 randomized, placebo-controlled clinical trials. In total, 682 subjects were treated with fish oil, and 641 were given placebos – most commonly olive and sunflower oils. In those taking fish oil, adiponectin levels increased by 0.37 ug/mL. The results also suggested the effect of fish oil on adiponectin differed substantially across the trials, suggesting that fish oil supplementation may have stronger influence on adiponectin in some populations and weaker effects in others.

This is the first study to pool data from previous trials to suggest that fish oil consumption increases adiponectin in humans. The findings quantify the potential impact of fish oil on adiponectin level, and highlight the need to further investigate populations that may particularly benefit from fish oil supplementation.

"Although higher levels of adiponectin in the bloodstream have been linked to lower risk of diabetes and coronary heart disease, whether fish oil influences glucose metabolism and development of type 2 diabetes remains unclear," said Wu. "However, results from our study suggest that higher intake of fish oil may moderately increase blood level of adiponectin, and these results support potential benefits of fish oil consumption on glucose control and fat cell metabolism."

Despite the uncertainty about the effectiveness of fish oil on cardiovascular and metabolic diseases, about 37 percent of adults and 31 percent of children nationwide use omega-3 supplements, according to the 2007 National Health Interview Survey from the National Institutes of Health's National Center for Complementary and Alternative Medicine (NCCAM).

Why fish oils work swimmingly against diabetes

Researchers at the University of California, San Diego School of Medicine have identified the molecular mechanism that makes omega-3 fatty acids so effective in reducing chronic inflammation and insulin resistance.

The discovery could lead to development of a simple dietary remedy for many of the more than 23 million Americans suffering from diabetes and other conditions.

Writing in the journal Cell, Jerrold Olefsky, MD, and colleagues identified a key receptor on macrophages abundantly found in obese body fat. Obesity and diabetes are closely correlated. The scientists say omega-3 fatty acids activate this macrophage receptor, resulting in broad anti-inflammatory effects and improved systemic insulin sensitivity.

Macrophages are specialized white blood cells that engulf and digest cellular debris and pathogens. Part of this immune system response involves the macrophages secreting cytokines and other proteins that cause inflammation, a method for destroying cells and objects perceived to be harmful. Obese fat tissue contains lots of these macrophages producing lots of cytokines. The result can be chronic inflammation and rising insulin resistance in neighboring cells over-exposed to cytokines. Insulin resistance is the physical condition in which the natural hormone insulin becomes less effective at regulating blood sugar levels in the body, leading to myriad and often severe health problems, most notably type 2 diabetes mellitus.

Olefsky and colleagues looked at cellular receptors known to respond to fatty acids. They eventually narrowed their focus to a G-protein receptor called GPR120, one of a family of signaling molecules involved in numerous cellular functions. The GPR120 receptor is found only on pro-inflammatory macrophages in mature fat cells. When the receptor is turned off, the macrophage produces inflammatory effects. But exposed to omega-3 fatty acids, specifically docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), the GPR120 receptor is activated and generates a strong anti-inflammatory effect.

"It's just an incredibly potent effect," said Olefsky, a professor of medicine and associate dean of scientific affairs for the UC San Diego School of Medicine. "The omega-3 fatty acids switch on the receptor, killing the inflammatory response."

The scientists conducted their research using cell cultures and mice, some of the latter genetically modified to lack the GPR120 receptor. All of the mice were fed a high-fat diet with or without omega-3 fatty acid supplementation. The supplementation treatment inhibited inflammation and enhanced insulin sensitivity in ordinary obese mice, but had no effect in GPR120 knockout mice. A chemical agonist of omega-3 fatty acids produced similar results.

"This is nature at work," said Olefsky. "The receptor evolved to respond to a natural product – omega-3 fatty acids – so that the inflammatory process can be controlled. Our work shows how fish oils safely do this, and suggests a possible way to treating the serious problems of inflammation in obesity and in conditions like diabetes, cancer and cardiovascular disease through simple dietary supplementation."

However, Olefsky said more research is required. For example, it remains unclear how much fish oil constitutes a safe, effective dose. High consumption of fish oil has been linked to increased risk of bleeding and stroke in some people.

Should fish oils prove impractical as a therapeutic agent, Olefsky said the identification of the GPR120 receptor means researchers can work toward developing an alternative drug that mimics the actions of DHA and EPA and provides the same anti-inflammatory effects.