Like many good ideas, the ancient belief that food is medicine is receiving renewed interest in the West. But unlike Hippocrates in ancient Greece nearly 2500 years ago or traditional Chinese medicine, today powerful market forces are behind modern functional foods.
Processed foods are increasingly designed to contain substances manufacturers would like to claim have certain health benefits or reduce the risk of certain diseases. Such claims are not yet allowed in New Zealand on labels of these functional foods, but manufacturers certainly tell you if their products have added calcium, added folate, added omega-3 or other things we are told we need more of.
Omega-3 is one of the fashionable nutritional buzz words.
It's touted as being good for coronary heart disease, circulatory problems, heart irregularities, brain development, children with attention deficit hyperactivity disorder, aggression, depression, bi-polar disorder, dementia, schizophrenia, psoriasis, arthritis and other inflammatory problems, immune function disorders, macular degeneration, eating disorders, inflammatory bowel disease, certain cancers, osteoporosis, asthma and sinus infections, premenstrual tension, diabetes - in fact it would almost seem to be a universal panacea, something that surely must arouse our suspicion.
But how many of these claims are proven?
And how can we get omega-3 naturally rather than by taking expensive supplements or eating fortified processed food?
And, importantly, can too much be bad for you?
Omega-3 fatty acids (of which there are three key types - see break-out story) have been in the limelight since the 1970s when researchers wondered why Eskimos, who had a high-fat diet, seemed to have a low incidence of coronary heart disease, according to Prof Jim Mann of the human nutrition department of the University of Otago.
Eskimos ate mostly fish, seals, whales and other sea food, all high in omega-3s.
"There was this initial enormous enthusiasm which was slightly then tempered by the fact that when people started to look at what these omega-3s did, - the initial interest was all round coronary heart disease - what became clear, [was that] certainly omega-3s had a profound effect on blood clotting.
"If you have an enormously high omega-3 diet, blood clotting is reduced and, of course, blood clotting is a feature in heart disease . . .[Omega-3s] put down one of the bad lipids which is lower trigliceride. That's a good thing to do, but in some people they also seem to increase the bad cholesterol, so it all seemed a bit unclear."
Since then the DART study in South Wales, showed a diet high in omega-3s (from eating oily fish twice a week or taking fish oil capsules) did not necessarily reduce heart attacks, but did reduce sudden death from them.
Another theory had emerged that omega-3s had a favourable effect on the electrical conduction system of the heart, which was borne out in experimental animals, Prof Mann said.
"Many people die from heart attacks because of rhythm abnormalities - ventricular fibrillation, the rhythm goes crazy and you die."
There was good evidence omega-3 fatty acids could help people with heart disease, but in New Zealand there was no funding for using fish oil capsules for this purpose, as there was in some other countries.
One of the reasons, according to Prof Mann, was that food supplements were not regulated, there were few guaranteed preparations, and some might be inadequate.
The other reason was that you could get adequate omega-3s from food if you ate oily fish such as salmon, mackerel, sardines and tuna (tinned or fresh), green leafy vegetables and certain oils, seeds and nuts.
"I tell my patients to have [oily fish] twice a week - it certainly won't do them any harm and there is some evidence that it will do them some good. I certainly do that myself, probably more than twice a week."
Omega-3s were also found in breast milk and were important for brain development in babies. They were now included in most baby formulas, he said.
There was also evidence skin conditions such as psoriasis could be helped by a diet high in omega-3.
"To the best of my knowledge, the heart disease, the skin and children's brain development, that is where it's reasonably well substantiated. The rest - joint issues, PMT, attention deficit - I think that is moving into the realms of not proven, and when you come to something that seems to be a universal panacea one does start to get a bit suspicious," he said.
However, there appears to be anecdotal evidence to support some of the claims.
"I'm not denying the benefits. Somebody may well produce a trial - a good trial which would be published in the Lancet or the British Medical Journal ."
Prof Mann is working on Food Standards Australia New Zealand criteria which will set out the boundaries for manufacturers' claims.
"It's a perennial nightmare. I don't think there's any suggestion yet that they should be able to make significant claims. Of course, they can say [their product] contains omega-3 if it does," he said.
"I don't think any of the functional foods - foods to which it has been added - have been shown in themselves to be of clinical benefit." The jury was still out on how much omega-3 you needed and what the optimal balance with omega-6 was (see breakout story), he said.
"It's very hard to do these kind of studies. You are saying how much do you need to avoid a deficiency and how much do you need to avoid an overdose. And, of course, it is a fat so the body can't get rid of it, so if you have too much it will accumulate. I think one just doesn't know.
"What is correct I think is that we need some of each [omega-3 and omega-6], and certainly I would not be recommending supplements for routine use. I don't believe there's any indication for use of supplements and there may indeed be side effects." Some studies indicate taking more than 3g of omega-3 a day (the equivalent of three meals of oily fish) may increase the risk of haemorrhagic stroke, a potentially fatal type of stroke in which an artery in the brain leaks or ruptures. However, others claim no significant adverse effects using up to 18g a day, although it can lower blood concentrations of vitamin E.
As Prof Mann said, there is a lot of research left to do. Like everything, it comes back to the basic fact a healthy and varied diet with plenty of vegetables, whole grains, legumes, fruit and fish, with a smaller amount of meat and dairy products, healthy fats such as mono and polyunsaturated ones (but not too much of them), and only the occasional sweet item, along with exercise, is the best recipe for good health.
Essential, but what exactly are they?
Omega-3 and omega-6 are essential fatty acids. Both are polyunsaturated fats and both are needed, in an appropriate balance, for good health.
However, nobody really knows what the right balance is, although many commentators say Western diets have the balance wrong - too much omega-6 and too little omega-3 - compared with the diets we would have evolved on.
There are three key omega-3 fatty acids: ALA (alpha-linolenic acid), which is found in a small number of oils such as flaxseed, canola and soy, and walnuts and green vegetables. Humans can convert some short-chain ALA to longer-chain EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid) which can then be used by the body.
EPA is found in cod-liver oil, fish oils, fish and seafood and in small amounts in lean lamb and beef.
DHA is found in fish oils, especially tuna, and breast milk.
Omega-3 essential fatty acids are found in oily fish such as sardines, salmon, trout, mackerel, pilchards, tuna, herring, warehou, kahawai, mullet, trevally, snapper, eel, squid, octopus, mussels and other shellfish, and muttonbird. White fish such as blue cod or sole have less omega-3. It is best to bake or steam fish rather than fry it.
One of the issues surrounding the increasing intake of omega-3 is the problem of over-fishing.
There is not enough fish in the world and some fish, especially large, older fish, have high levels of toxins such as mercury. Fish farming can harm the environment and some sources say farmed fish has less omega-3 than wild fish. Prof Mann is still awaiting results of an experiment to see if New Zealand-farmed salmon has the same oil composition as wild salmon.
There are also good vegetable sources of omega-3 such as linseeds (there is an increasing number of soy and linseed breads and crackers or you can add them to salads or cereals but it is best to crush them first or they go straight through you), some leafy green vegetables (the Mediterranean herb purslane is the best source), and canola oil and spreads.
Some eggs laid by hens on special diets are high in omega-3.
Most eggs contain an average of 0.1g omega-3 per 100g of egg, but Farmer Brown Nutra eggs contain about 10 times as much, according to Rae Steel, of Mainland Poultry.
The hens are fed a special diet which includes flaxseed meal and oil, legumes, wheat and barley, she said.
Many other natural foods contain some omega-3 and there are an increasing number of processed foods fortified with it, including bread, table spreads, and milk.
Walnuts, soy products such as soy oil, soy milk and tofu contain both omega-3 and omega-6.
Omega-6 essential fatty acids are found in most vegetable oils such as sunflower, corn, peanut and grapeseed oils, soy products, many grains, seeds and nuts.