Omega-3s and diabetes: a promising but mixed picture
The same Harvard team behind today's good news came to a negative conclusion for omega-3s earlier this year.
In that study, they compared the self-reported diets of 36,328 women in the Women's Health Study to their health status over a 16-year period.
Much to most observers' surprise, that analysis linked fishy diets to higher diabetes risk, while plant-source omega-3s were not linked to risk:
“Our data suggest an increased risk of T2D [type 2 diabetes] with the intake of long-chain omega-3 fatty acids, especially with higher intakes.” (Djoussé L, Gaziano JM, Buring JE et al. 2011)
Specifically, the risk of developing diabetes within 16 years was 44 percent higher among those who reported eating two or more servings of fish daily, and similarly elevated among the women estimated to consume two or more grams of omega-3s per day.
However, as with the Singapore findings we report today, these outcomes strain credulity … a point made by the authors of another survey-based study that linked omega-3s to higher diabetes risk (see “Diabetes-Fish Study Raises Doubts”):
“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.
In truth, no study that relies on diet surveys and uses them to make rough estimates of omega-3 intake can be as reliable as one – like the positive Harvard study – that employs blood tests.
For more on this topic, see the “Omega-3s & Metabolic Health” section of our news archive.
Brostow DP, Odegaard AO, Koh WP, Duval S, Gross MD, Yuan JM, Pereira MA. Omega-3 fatty acids and incident type 2 diabetes: the Singapore Chinese Health Study. Am J Clin Nutr. 2011 May 18. [Epub ahead of print]
Djoussé L, Biggs ML, Lemaitre RN, King IB, Song X, Ix JH, Mukamal KJ, Siscovick DS, Mozaffarian D. Plasma omega-3 fatty acids and incident diabetes in older adults. Am J Clin Nutr. 2011 May 18. [Epub ahead of print] doi: 10.3945/ajcn.111.013334
Djoussé L, Gaziano JM, Buring JE, Lee IM. Dietary omega-3 fatty acids and fish consumption and risk of type 2 diabetes. Am J Clin Nutr. 2011 Jan;93(1):143-50. Epub 2010 Oct 27.
Hodge AM, English DR, O'Dea K, Sinclair AJ, Makrides M, Gibson RA, Giles GG. Plasma phospholipid and dietary fatty acids as predictors of type 2 diabetes: interpreting the role of linoleic acid. Am J Clin Nutr. 2007 Jul;86(1):189-97
Odegaard AO, Koh WP, Butler LM, Duval S, Gross MD, Yu MC, Yuan JM, Pereira MA. Dietary patterns and incident type 2 diabetes in chinese men and women: the singapore chinese health study. Diabetes Care. 2011 Apr;34(4):880-5. Epub 2011 Feb 17.
Vessby B, Aro A, Skarfors E, Berglund L, Salminen I, Lithell H. The risk to develop NIDDM is related to the fatty acid composition of serum cholesterol esters. Diabetes 1994;43:1353-7.
Wang L, Folsom AR, Zheng ZJ, Pankow JS, Eckfeldt JH. Plasma fatty acid composition and incidence of diabetes in middle-aged adults: the Atherosclerosis Risk in Communities (ARIC) Study. Am J Clin Nutr 2003;78:91-8.
Wang L, Folsom AR, Zheng ZJ, Pankow JS, Eckfeldt JH; ARIC Study Investigators. Plasma fatty acid composition and incidence of diabetes in middle-aged adults: the Atherosclerosis Risk in Communities (ARIC) Study. Am J Clin Nutr. 2003 Jul;78(1):91-8.