Khursheed N Jeejeebhoy is an important thinker in nutrition
He has written an important Viewpoint in Nature Clinical Practice Gastroenterology & Hepatology (2008) Benefits and risks of a fish dietâ€”should we be eating more or less?) 5, 178-179
There is increasing evidence for the role of omega-3 fatty acids such as eicosapentaenoic acid and docosahexaenoic acid in preventing coronary artery disease and that fish oils have a cardioprotective effect.
Patients who had had a myocardial infarction had a 29% reduction in mortality over 2 years by eating three fish meals a week. By contrast, patients who were randomized to high-fiber and low-fat diets did not have a significant reduction in mortality. The American Heart Association recommends an increase in the dietary intake of omega-3 fatty acids.
A large randomized trial of 18,000 patients with hypercholesterolemia (observed for 5 years) showed that adding 1800 mg/day of eicosapentaenoic acid to statin treatment resulted in a significant reduction in major coronary events compared with the controls who received statin treatment alone.
This excitement has been tempered by a plethora of papers that warn against eating fish because of the risk of mercury poisoning of the central nervous system.
Mercury enters the atmosphere by combustion of waste and coal. The element then enters the oceans from the atmosphere where it is converted to methyl mercury by microorganisms and then taken up by marine life and concentrated in fish. As methyl mercury is not fat soluble, unlike dioxins, it does not reside in the fatty tissues. Methyl mercury is strongly neurotoxic, and can result in mental retardation, seizures and microcephaly in infants.
The concentration of methyl mercury in fish is increased by fish eating other fish for food. Fish that are not predatory, such as sardines, salmon and shrimp, have very low levels of methyl mercury.Predatory fish such as shark, tuna, swordfish and orange roughy have higher levels of methyl mercury. Farmed fish have the lowest levels of methyl mercury. Whilst methyl mercury is very neurotoxic, in fish methyl mercury is bound to cysteine, and this compound has a tenth of the toxicity of pure methyl mercury.6
A study in the Faroe Islands followed the health of infants over a 14-year period. There was a correlation between high prenatal mercury intake by the mother who ate pilot whale meat daily in their diet and neurological developmental deficits in the infant.
In the Seychelle islands where women eat 12 fish meals a week, no effects on infant neurological development were noted despite the fact that the mean methyl mercury concentration in the hair of Seychelle island inhabitants, including infants, was 10â€“20 times that seen in US inhabitants. The concentration of methyl mercury in fish caught around the Seychelles, however, was similar to that found around the USâ€”0.05â€“0.25 ppm. The higher levels of methyl mercury found in the Seychelle islanders were therefore due to the islanders eating more fish rather than eating highly contaminated fish. By contrast, pilot whale meat has 10 times the concentration of methyl mercury that is found in ocean fish (1.6 ppm). The difference between the data from the Faroes and the Seychelles is therefore likely to be largely because individuals in the Faroe Islands had much higher exposures to methyl mercury, because they ate marine mammals and not fish
The degree of methyl mercury contamination in food determines its toxicity. By eating fish with low levels of methyl mercury, such as sardines, salmon and shrimp, and the highly contaminated marine mammals the dietary intake of methyl mercury can be reduced further.
On the basis of the data given in the main body of this article, there is little evidence that 2â€“3 meals of low-mercury-containing fish per week can cause harm. Wild and farmed salmon would provide an ideal option to reduce the risk of both heart disease and methyl mercury poisoning, as these fish are rich in omega-3 fatty acids and very low in methyl mercury.
- Martin Eastwood