Eating more fish is advised by most health professionals - but which fish, why and does such advice have any caveats? Besides being an excellent source of protein, oily fish is recommended for its omega 3 essential fatty acid content; namely eicosapentanoic acid (EPA) and docosahexanoic acid (DHA).

These oils cannot be efficiently made by the human body so must be provided by the diet. Beneficial effects of fish oils include reducing the risk of cardiac mortality, lowering harmful levels of triglycerides fat, more prone to clotting when raised, and delaying the onset of cognitive decline.

Oily fish have such oils dispersed throughout their bodies, rather than confining them to the liver as with white fish such as cod. Oily fish include salmon, trout, sardines, mackerel, anchovies, herring and fresh tuna. Tinned tuna unfortunately has many of the oils removed during the canning process.

The US and UK recommend consumption limits especially for pregnant women. For example the Food Standards Agency (UK) advises 2 portions of oily fish per week, for the potential benefit to the neurological development of the baby, but warns against shark, swordfish and marlin. Tuna should be limited to two fresh steaks or four medium cans per week.

A number of specific studies have been carried out looking at the overall affect on health when considering benefits of fish oils versus the potentially negative effects of the pollutants in some fishes. The evidence is decisively in favor of oily fish consumption with a 36 percent reduction in coronary death, and 17 percent reduction in total mortality in people consuming 1-2 portions per week.

The American Heart Association recommends the general population consume a variety of preferably oily fish at least twice a week. Patients with existing heart disease should aim for 1g of EPA/ DHA per day, preferably from fish. Those with raised triglycerides should take 2-4g fish oil supplement per day under supervision of a doctor.

If you do not or cannot eat fish, then it is advisable to obtain your omega 3s in a supplemental form, but be sure to choose a body oil not a liver oil. Vegetarians wanting to avoid fish oil supplements should include plenty of soybean products, rapeseed (or canola), walnuts, flaxseed and their oils in the diet. These contain alpha-linolenic acid which is converted into omega 3 oils internally, though the conversion rate is modest and controversial.

This nutrition-related column is written by Nina Lenton, a qualified dietitian living in Beijing and working at Bayley and Jackson Medical Center. Contact her at nina.lenton@bjhealthcare.com.

(China Daily 08/16/2007 page14)