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Diabetes-Fish Study Raises Doubts
11/2/2009
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Population study finds no diabetes-prevention effect from fishy diets; authors note that omega-3s dampen risk factors for diabetes and cardiovascular risks linked to diabetes
by Craig Weatherby

A recent epidemiological study by Harvard researchers found a slightly higher rate of type 2 (adult) diabetes among people who reported eating more fish than average.

They examined ties between fish and omega-3 fatty acid intake and the development of diabetes among 152,700 women enrolled in the Nurses’ Health Study and 42,504 men enrolled in the Health Professionals Follow-Up Study.

As they wrote, “…we observed a modest but significant positive relation between fish and omega-3 fatty acid consumption and incidence of T2DM [type 2 diabetes mellitus] after adjustment for established risk factors… and dietary predictors of T2DM...” (Kaushik M et al. 2009).

Naturally, this news resulted in several letters from concerned readers, and we thought it important to publish a response.

We’ve covered research that reveals similarities between the influences that omega-3s from fish have on genetic switches in our cells and those exerted by leading diabetes drugs.

Specifically, omega-3s and certain diabetes drugs both influence genetic “transcription factors” for peroxisome proliferator-activator receptors (PPARs).

For more on that, see “Drug May Help Prevent Diabetes but Omega-3s Offer Comparable Effects” and “Drug Cops Fumble in Diabetes Fiasco.

And the authors of the new study agreed with that assessment:

“Omega-3 fats (also known as n–3 fatty acids), particularly long-chain omega-3 fats from seafood sources, alter the expression of peroxisome proliferator-activator receptor [PPAR] genes, which are involved in signaling nutrition status, and of the production of inflammatory cytokines, which are associated with T2DM [type 2 diabetes mellitus]. These findings suggest that omega-3 fatty acids could lower the risk of T2DM.” (Kaushik M et al. 2009)

In fact, the association this study found—between eating more fish than average and having a slightly higher risk of diabetes—makes little sense in light of what’s known about the cell-level effects of omega-3s.

As the authors themselves noted, “…numerous studies have demonstrated beneficial effects of fish and omega-3 fatty acids on multiple risk factors associated with diabetes, and on heart disease—the major sequelae [subsequent effects] of diabetes”.

They also admitted that people with healthier lifestyles generally eat more fish and undergo more medical tests than average… which could explain the link they observed between fish intake and reporting a diagnosis of diabetes.

In addition, as the Harvard team noted, participants may have increased their consumption of fish after being diagnosed with the cardiovascular signs associated with diabetes, such as high cholesterol and triglycerides or hypertension.

Omega-3s proven to reduce diabetes risk factors and related heart risks
As the authors noted, the epidemiological (population) and clinical studies performed to date paint a mixed picture with regard to omega-3 fatty acids and risk of diabetes:

“Epidemiologic studies of the relation of long-chain [omega-3] fatty acid intake with T2DM have reported conflicting results… intake of long-chain omega-3 fatty acids was associated with better glucose tolerance in some studies but not in others. Some intervention [clinical] studies have found that omega-3 intake resulted in an increase in glycated hemoglobin (HbA1c) and in fasting blood glucose…” (Kaushik M et al. 2009).

(Blood tests for HbA1c are used to monitor blood sugar control in diabetes patients, and to check blood sugar control in people who might be pre-diabetic… increasing HbA1c levels are a sign that blood sugar control is weakening and that diabetes is impending or worsening.)

Despite what the Harvard team wrote about omega-3s and HbA1c levels, the authors of an epidemiological (population) study in 10,009 women and men found “…no evidence of an association between fish consumption and HbA1c [levels] after taking other lifestyle factors into account” (Harding AH et al. 2004).

And as we reported recently, a clinical trial found that participants who took daily fish oil supplements showed lower levels of HbA1c (see “Omega-3s Drop Risk Markers for Diabetes and Heart Disease in Clinical Trial”).

Further, the negative implications of the news study are undermined by the positive outcomes of research we’ve reported in several articles:

Omega-3s Seen to Fight Metabolic Syndrome

Diabetes News: Fish Fats and Tea and Display Preventive Potential

Fish, Omega-3s, and Diabetes

Even if the new study found a real effect, the authors emphasized that omega-3s offer many compensating benefits to people at risk of developing diabetes:

“Given the beneficial effects of fish and omega-3 fatty acids on multiple risk factors associated with diabetes, including triglycerides, HDL cholesterol, blood pressure, and inflammation, and on CHD, the major sequelae of diabetes, the clinical relevance of this relation and its possible mechanisms require further investigation” (Kaushik M et al. 2009).

And we think it needn’t be cause for concern, unless and until the association is repeated in other studies and an explanation is found at the cellular level.

Stay fit, eat colorful plant foods, favor whole grains, and avoid sugars and starches
Studies show that exercise and good diets and cut the risk of diabetes drastically.

In fact, just 30 minutes of walking per day can reduce diabetes risk by half, and the combination of walking or other aerobic exercise and resistance training (isometrics, free weights or resistance machines) works even better.

Finally, focus on eating nutrient dense, low-calorie, high-fiber fruits and vegetables—colorful ones are best—and replace sugary or starchy foods (e.g., white flour) with modest amounts of whole grains.

This multifaceted approach will enhance your overall health while greatly reducing your risk of diabetes.

And it seems extremely unlikely that a diet rich in fish will do you any harm with regard to diabetes risk.


Sources
  • Bassuk SS, Manson JE. Epidemiological evidence for the role of physical activity in reducing risk of type 2 diabetes and cardiovascular disease. J Appl Physiol. 2005 Sep;99(3):1193-204. Review.
  • Castaneda C. Diabetes control with physical activity and exercise. Nutr Clin Care. 2003 May-Sep;6(2):89-96. Review.
  • Colberg SR, Grieco CR. Exercise in the treatment and prevention of diabetes. Curr Sports Med Rep. 2009 Jul-Aug;8(4):169-75. Review.
  • Harding AH, Day NE, Khaw KT, Bingham SA, Luben RN, Welsh A, Wareham NJ. Habitual fish consumption and glycated haemoglobin: the EPIC-Norfolk study. Eur J Clin Nutr. 2004 Feb;58(2):277-84.
  • Kaushik M, Mozaffarian D, Spiegelman D, Manson JE, Willett WC, Hu FB. Long-chain omega-3 fatty acids, fish intake, and the risk of type 2 diabetes mellitus. Am J Clin Nutr. 2009 Jul 22. [Epub ahead of print]
  • Sigal RJ, Kenny GP, Boulé NG, Wells GA, Prud'homme D, Fortier M, Reid RD, Tulloch H, Coyle D, Phillips P, Jennings A, Jaffey J. Effects of aerobic training, resistance training, or both on glycemic control in type 2 diabetes: a randomized trial. Ann Intern Med. 2007 Sep 18;147(6):357-69.

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