Omega-3 fatty acids - the jury is still out

In recent years -3 fatty acids have been hailed as having many health benefits. The list has grown: from preventing heart attacks, through control of inflammatory illnesses such as rheumatoid arthritis and asthma, to reducing the risk of certain mental illnesses, such as depression and schizophrenia. There has also been some evidence to suggest that -3 supplements improve IQ, behaviour and concentration in children. What are -3 fatty acids, what is their role in the body, and what does the available research tell us? 

The omega-fatty acids

-3 fatty acids are polyunsaturated fatty acids (PUFAs). They are characterised by having a double bond in the -position, ie at the third carbon atom from the methyl end of the hydrocarbon chain. The important -3 fatty acids are -linolenic acid (1), eicosapentaenoic acid (EPA, 2) and docosahexaenoic acid (DHA, 3). They are essential for maintaining the membranes of cells, but are not synthesised in the body and therefore have to be obtained from the diet. The same is true for the related -6 PUFAs, the 'essential' ones of which are linoleic acid (4) and arachidonic acid (5).  

In the late 1950s scientists discovered that these essential -3 and -6 fatty acids were precursors to hormone-like molecules, the prostaglandins and leukotrienes. In cells -linolenic acid converts to arachidonic acid, which converts easily to prostaglandin E₂ (6). Other prostaglandins vary in the degree of reduction of the ring oxygens and the presence of double bonds in the chain. The prostaglandins have many physiological roles in the body. For example, some are responsible for the inflammatory response and pain that occurs when tissues are damaged. Others, notably thromboxane, also synthesised from arachidonic acid, stimulate blood clotting when a blood vessel is damaged, and can act as markers of heart disease. Leukotrienes, which are also synthesised from arachidonic acid, promote constriction of bronchi which is associated with asthma.  

More recently chemists have discovered that DHA is found abundantly in the membranes of brain cells and the retina, and is thought to be critical to the functioning of the brain and nervous system. Other research suggests that it is important to maintain a balance -6:-3 in cells in the ratio of 2:1-4:1. This is because fatty acids compete with each other for enzymes in the cells, which in turn influences the amount of prostaglandins, leukotrienes and thromboxanes produced. Since -3 fatty acids help to reduce inflammation and most -6 fatty acids tend to promote inflammation, scientists reason that an inappropriate balance could contribute to the development of disease, while the right balance should maintain health.  

Sources. The main sources of -linolenic acid are certain seed oils, eg linseed oil, rapeseed oil, soya oil, walnut oil and peanut oil. EPA and DHA are obtained from oily fish such as salmon, mackerel, sardines and tuna. There are also small amounts of -3 fatty acids in meat and eggs. The major sources of -6 fatty acids are corn, sunflower and soyabean oils. The Government recommends eating one-two portions of oil-rich fish a week. This would provide ca 2-3 g of -3 fatty acids.  

Healthy evidence?

In the 1970s interest in the dietary benefits of the -fatty acids was sparked when researchers discovered that Greenland Eskimos had a remarkably low incidence of heart disease. These people eat a lot of oily fish and so their diet is high in polyunsaturated fats, specifically -3 fatty acids. Since then a lot of research has focused on the potential health benefits of -3 fatty acids.  

In the late 1980s, a group of male patients who had suffered heart attacks were asked to eat 200-400 g of oily fish per week (which provided an additional 500-800 mg/day of -3 fatty acids) over a period of two years against a control group who did not. The trial reported a reduction of almost 30 per cent in deaths among those men who had eaten the fish, though the total number of heart attacks was not reduced. A follow-up study 10 years later found there were no long-term survival benefits. 

Another trial in Italy focused on patients who had survived a heart attack. They were given fish oil extracts containing EPA and DHA over three and a half years. Results pointed to a 20 per cent reduction in deaths against a control group who did not take the supplements. However, another trial in Cardiff on patients with 'stable angina' (chest pain caused by reduced blood flow to the heart muscle) showed no benefit from taking EPA and DHA supplements and in fact there was some evidence of an increased risk of death among this group of patients.  

More recently, the British Medical Journal (BMJ., 2006, 332, 752) published a review of published -3 fatty acid trials up to February 2002 - a total of 15,159 titles and abstracts, covering some 36,913 patients. Analysis didn't confirm a statistically lower risk of death from heart disease in patients taking additional -3 oils. But the researchers concluded that the health benefits may not be as clear as previously suggested and more research needs to be done. 

Other potential benefits?

Tom Sanders, professor of nutrition and dietetics at King's College London has recently been involved in a study looking at the optimal -6/-3 ratio in the UK diet. It's not clear, for example, whether -linolenic acid, the -3 fatty acid found in plant foods, has a comparable effect to the long-chain -3 fatty acids, EPA and DHA, found in fish oils. Any effect -linolenic acid has will depend on how much of it is converted to EPA and DHA in the body. 

The study looked at 258 people in the 45-70 age group who were given spreads and oils in their diet for six months which contained different ratios of  -6/-3 fatty acids. Increasing the amount of oily fish increased the amount of DHA and EPA in blood and cell membranes. Increasing the amount of -linolenic acid had no effect. However, one effect of increased levels of EPA/DHA was a decreased stiffness in the arteries. According to Sanders' research small amounts of long chain -3 fatty acids (approximately 1 g per day) do have significant effects on the risk of heart disease and supports the current advice to increase the amount of oily fish in our diet. Professor Sanders told InfoChem, '-3 has been thought of as the elixir of life. If it does reduce the risk of sudden cardiac death this is extremely important because a high proportion of first heart attacks are fatal and happen without warning'. 

Other scientists are looking at the possibility that -3 fatty acids may help in reducing the risk of depression and schizophrenia and help prevent Alzheimer's disease. This is being tested in trials, with the first results expected in 2008. There has also been a lot of media coverage over the past few years about the potential of -3s for raising children's IQ. A recent study of more that 100 children with developmental coordination disorder (DCD) from 12 Durham schools showed that the children receiving a fish oil supplement made significantly better progress in both reading and spelling than children who received a placebo. The researchers reason that DHA may help to ease the flow of electrical signal from one brain cell to the next by making the cell membrane more elastic.

Overall, scientists agree that more work needs to be done into the health benefits of -3 fatty acids before we rush to buy supplements or indeed throw them out. As always though, a balanced diet which includes these essential -fatty acids is important. 

John Johnston