Fish consumption in relation to type 2 diabetes and cardiovascular complications

Type 2 diabetes is a major public health threat globally, associated with severe medical complications such as cardiovascular diseases and damage to kidneys, eyes, and nerves, as well as with premature death. Dietary factors are of importance in both primary prevention and disease management. Fish consumption is of interest given the observed benefits on several cardiometabolic risk factors. Such benefits have largely been attributed to the content of long-chain omega-3 fatty acids, but may also be related to a wider range of nutrients. On the negative side, there is also the potential of adverse effects by contaminants present in some types of fish.

This thesis includes four papers in which we aimed to: 1) summarize results from prospective studies on the associations of fish consumption and intake of long-chain omega-3 fatty acids with risk of type 2 diabetes; 2) evaluate the validity of food frequency questionnaire (FFQ)- based estimates of these exposures; 3) examine the associations of total and specific types of fish with risk of type 2 diabetes, taking environmental contaminants into account; and 4) examine the associations of total and specific types of fish with risk of myocardial infarction (MI), stroke, and mortality among people with diagnosed type 2 diabetes.

Our meta-analysis showed that the accumulated observational evidence on fish consumption and long-chain omega-3 fatty acid intake in relation to type 2 diabetes risk is heterogeneous across regions, with an increased risk reported in studies conducted in the US, no association reported in European studies, and lower risk reported in Asian/Australian studies.

The three latter papers are based on data from two large Swedish population-based cohorts of nearly 90,000 women and men, whose dietary habits were assessed via FFQs in 1997. We showed that the estimates of long-chain omega-3 fatty acid intake correlated with adipose tissue content of these fatty acids in a subsample of 239 women, supporting their validity. Participants were followed for the outcomes of interest through December 2012, by linkage to nationwide registers. In men, total fish consumption was not associated with type 2 diabetes in the primary analysis. Taking dietary exposure to polychlorinated biphenyls (PCBs) and methylmercury (MeHg) into account, there was however a suggestion of an inverse association. High consumption of shellfish and fried fish was associated with higher risk of type 2 diabetes. Among women and men with type 2 diabetes before baseline, fish consumption was inversely associated with MI, while there was no support for an association with stroke. The results for total and coronary heart disease (CHD)-related mortality were inconclusive with some suggestion of lower risk associated with moderate fish consumption.

Overall, the results presented in this thesis do not challenge the current recommendations on regular fish consumption. General advice may however be too imprecise, and more specific recommendations on fish species and preparation methods may be warranted.

List of scientific papers

I. Wallin A, Di Giuseppe D, Orsini N, Patel PS, Forouhi NG, Wolk A. Fish consumption, dietary long-chain n-3 fatty acids, and risk of type 2 diabetes: systematic review and meta-analysis of prospective studies. Diabetes Care 2012;35:918-29.
https://doi.org/10.2337/dc11-1631

II. Wallin A, Di Giuseppe D, Burgaz A, Håkansson N, Cederholm T, Michaëlsson K, Wolk A. Validity of food frequency questionnaire-based estimates of long-term long-chain n-3 polyunsaturated fatty acid intake. Eur J Nutr 2014;53:549-55.
https://doi.org/10.1007/s00394-013-0563-1

III. Wallin A, Di Giuseppe D, Orsini N, Åkesson A, Forouhi NG, Wolk A. Fish consumption and frying of fish in relation to type 2 diabetes incidence: a prospective cohort study of Swedish men. Eur J Nutr. 2015 Dec 21.
https://doi.org/10.1007/s00394-015-1132-6

IV. Wallin A, Orsini N, Forouhi NG, Wolk A. Fish consumption in relation to myocardial infarction, stroke and mortality among women and men with type 2 diabetes: a prospective cohort study. [Manuscript]