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Science update: Omega-3 and omega-6 fats

Science update: Omega-3 and omega-6 fats

HFG Senior nutritionist Rose Carr looks at interesting research on essential fats.

Omega-3

We hear a lot about omega-3s but they are not all the same. ALA is a short-chain omega-3, whereas the long-chain omega-3s are EPA, DHA and DPA. It is the long-chain omega-3s EPA and DHA from fish oils that have been widely studied and are thought to reduce the risk for heart disease and possibly dementia, diabetes and asthma.

Omega-6

Most of the polyunsaturated fats in our diets are omega-6 fats. These are found in nuts, seeds and seed oils including soy, canola, safflower and sunflower oils. A deficiency of omega-6 fat can cause dry and cracked skin but you are unlikely to be short of omega-6 unless your diet is very low in fat.

We classify fats into three main groups: saturated, monounsaturated and polyunsaturated. Animal foods are higher in saturated fats (seafood is the main exception), and plant foods are higher in the mono- or poly-unsaturated fats (the exceptions are coconut oil, palm kernel oil and palm oil which are high in saturated fat). Our bodies are able to make some fats but we can’t produce the essential polyunsaturated fats omega-3 and omega-6, so we must get these from our diets. You may also have heard of omega-9 fats. These are not described as essential fats as our bodies can produce them.

For some years it was thought the proportion of omega-6 to omega-3 fats in our diets was more important than the absolute amounts we consumed, but the science is moving away from that theory. The ratio theory was plausible: it appeared the two fats competed for access to the same biochemical pathway, and an excess of omega-6 over omega-3 fats might suppress the very important actions of omega-3. We were also told our modern Western diets were exceptionally high in omega-6 fats and we should reduce their intake in favour of omega-3s. Both laboratory and animal experiments seemed to support the theory.

While the absolute amounts of omega-6 and omega-3 fats in our diets will naturally impact their ratio to one another, the focus now is on increasing the long-chain omega-3s in our diets rather than limiting omega-6 fats.

A recent American Heart Association advisory summarised a large body of evidence indicating that omega-6 fats in our diets help reduce inflammatory states and are associated with decreased risk of coronary heart disease. Omega-6 helps to lower plasma LDL (‘bad’) cholesterol levels and also has roles in reducing blood pressure and the adverse factors related to insulin resistance. The authors concluded that to reduce omega-6 intakes from their current levels would be more likely to increase than to decrease the risk for coronary heart disease.

The importance of ALA (short-chain omega-3, from plant foods) in our diets is still subject to debate. While there is a wealth of research showing beneficial effects of fish oils (ie. long-chain omega-3) in our diets, there is relatively little research on the plant sources of ALA in our diets, and their effects.

ALA is found in plant-based foods such as legumes, flaxseed (linseed) products, walnuts, canola oil and smaller amounts in green vegetables. Some ALA can be converted into long-chain omega-3s in the body and although different studies have come up with different conversion rates, most seem to agree the conversion rate is very low. However, a novel British study published in November 2010 in the American Journal of Clinical Nutrition looked at the dietary intakes and blood plasma levels of omega-3s in a group of 14,000 people who were in one of several groups: fish eaters, non-fish meat eaters, vegetarians and vegans. Researchers found that while the differences in omega-3 intakes were substantial, as expected, the differences in blood plasma status were much smaller than expected. While more research is needed, their findings build on previous research which has found conversion rates in people who eat meat but not fish, and in vegetarians, may be higher than thought.

Most of us are not going to worry about the ratio of different fats in our diets. That’s way too hard. What we do know is that we need fat in our diets but most of us still get more saturated fat than we need and increasing our intake of omega-3s would be beneficial.

The simplest way to increase our intake of omega-3s is to eat fish several times a week, especially oily fish. If we don’t eat animal products, it seems increasing plant sources of omega-3s (ALA) such as flaxseed, walnuts and canola oil will enhance our blood levels of long-chain omega-3s.

See How much omega-3 is in that seafoood?




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