Eating one or more portions of fish per week may decrease the risk of type 2 diabetes, but similar intake of shellfish could increase the risk, according to British study.

Researchers at the Institute of Metabolic Science and the University of Cambridge in Cambridge, U.K., studied 21,984 eligible participants in the European Prospective Investigation of Cancer (EPIC) study, which enrolled men and women aged 40-79 at baseline (1993-1997).

Investigators used a food questionnaire to assess habitual fish and shellfish intake (white fish [such as cod, haddock, sole, and halibut], oily fish [such as mackerel, sardines, herring, tuna, and salmon], fried fish, and shellfish [such as crab, mussels, and prawns]).

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During a median follow-up of 10.2 years, researchers observed 725 new-onset cases of type 2 diabetes. After adjusting for age, gender, family history of diabetes, smoking, alcohol intake, physical activity and other potential confounders, subjects who consumed one or more portions of any kind of fish per week had a 25% decreased risk of type 2 diabetes compared with those who consumed less than one portion per week, investigators reported in Diabetes Care (2009;32:1857-1863).

Consumption of both white and oily fish was inversely associated with diabetes risk, but adjustment for dietary factors and obesity attenuated these associations, the researchers noted. Subjects who consumed one or more portions of shellfish per week had a 36% increased risk.

“The finding that greater shellfish intake may increase the risk of diabetes is surprising and novel,” the authors wrote.

Possible mechanisms that could explain this association may be related to cooking method, such as frying and the type and amount of cooking fat used, and the accompanying condiments with which shellfish frequently is served, such as mayonnaise or garlic butter, according to the researchers. “In addition, shellfish is known to be a rich source of dietary cholesterol, and it has been shown that dietary cholesterol may increase blood cholesterol.”

The association between higher shellfish consumption and increased diabetes risk “highlights the potential importance of seafood preparation and cooking methods,” the investigators observed.

Individuals who developed diabetes were older than those who did not (mean 61.3 vs. 58 years), and they had a higher mean BMI (29.7 vs. 26.2 kg/m2). In addition, they were more likely to be men and less likely to be physically active.