It’s healthy to eat fish, but here's why you should choose it carefully

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Choose your fish wisely to ward off diabetes – and protect your heart.

If you eat fish in an effort to guard against heart disease, new study findings suggest you might also be increasing your risk of diabetes.

While fish does contain omega-3 fatty acids found to have multiple heart benefits, some types also deliver a hefty dose of mercury, a contaminant thought to impair blood-sugar control.

The new study, soon to be published in the journal Diabetes Care, revealed that people with high mercury exposure were 65 per cent more likely to develop diabetes than their peers who had much lower levels of the heavy metal in the body. Our main sources of mercury are fish and seafood consumption and exposure from dental amalgam fillings.

Mercury occurs naturally in the environment but it's also an industrial pollutant. Once mercury is released into the water, fish absorb it. Larger, longer-living predatory fish such as grouper, swordfish, shark, marlin, Chilean sea bass and many types of tuna end up with the most mercury. Cooking fish has little impact on its mercury content.

The concern with regularly eating high-mercury fish is that the metal can build up in your body and lead to health problems such as birth defects in newborns, learning disabilities in children and, now, diabetes in adults.

For the study, researchers followed 3,875 Americans, aged 20 to 32, for 18 years. After accounting for factors linked to diabetes risk – such as BMI (body mass index), waist circumference, family history, physical activity – having a high mercury level in the body was associated with substantially greater odds of developing diabetes. (Mercury levels were measured from toenail clippings, a robust indicator of long-term mercury exposure.)

The data did hint that higher intakes of omega-3 fats and magnesium, a mineral found in fish as well as legumes, nuts, seeds, wheat bran and Swiss chard and spinach, dampened some of the harm from mercury.

The researchers also linked high mercury exposure to elevated fasting blood sugar and insulin levels. Earlier studies have shown that mercury exposure – at a level similar to that found in some types of seafood – can damage cells in the pancreas that make insulin, the hormone that clears sugar from the bloodstream.

Should you stop eating fish to ward off a future diagnosis of diabetes? No, definitely not. Eating fish rich in omega-3 fatty acids like salmon, trout and sardines may help reduce the risk of heart attack, stroke, Alzheimer's disease and macular degeneration (an eye disease that can lead to blindness).

In fact, a study published earlier this month tied higher blood levels of omega-3 fatty acids to a lower risk of dying from any cause, especially coronary heart disease and arrhythmia (irregular heart beat).

But you do need to choose your fish wisely. Based on these new findings, women and young children aren't the only ones who need to limit their intake of mercury from fish. It seems we all should.

It's far too early to set mercury-consumption limits for the prevention of diabetes. However, Health Canada advises women of childbearing age and young children to limit intake of high-mercury fish to less than 2.5 ounces per month.

Fish high in mercury include tuna (albacore, bigeye, blackfin, bluefin), king mackerel, shark, swordfish, marlin, orange roughy, tilefish and escolar. Canned albacore (white) tuna should also be limited since this species accumulates moderate amounts of mercury. Canned light tuna, usually skipjack tuna, is lower in mercury.

There are other things to consider when it comes to choosing fish. Besides mercury, industrial contaminants called PCBs (polychlorinated biphenyls) have also been demonstrated to show adverse health effects. As well, it's important to many people – myself included – to buy so-called eco-friendly fish that's caught or raised in ways that consider sustainability of the species and the ocean.

Back to the matter of diabetes. Limiting your mercury exposure may very well turn out to be an important measure to reduce diabetes risk, but let's not forget scientifically proven strategies to prevent the condition: losing excess weight, preventing abdominal weight gain, getting regular exercise and eating a healthy diet that minimizes (or avoids) refined starches and added sugars.

So yes, a health-promoting diet can include fish. Use the following chart to help you choose fish low in contaminants, high in omega-3 fats and easier on the environment.

Smart fish choices

The following fish choices are low in contaminants (versus moderate or high) and eco-friendly. An asterisk denotes fish high in omega-3s.

Black sea bass

Clams

Crab (Dungeness, red king, U.S. blue king, snow, blue, Jonah)

Herring, Atlantic*

Mackerel, Atlantic (Canada and U.S.)*

Mussels (blue, Mediterranean, New Zealand)

Pollock, Atlantic

Salmon, canned*

Salmon, wild (Alaska, California, Oregon)*

Sardines*

Scallops (bay, sea)

Sole/flounder

Steelhead trout*

Tilapia (U.S., Asia, Latin America)

Source: Environmental Defense Fund (EDF) Seafood Selector (seafood.edf.org).