Omega-3 fatty acids and their sources in diet
You can compare fatty acids to a string of beads, made of carbon, with chemical links between them. If all the links are single strands (a single bond), the fatty acid is called a “saturated” fat. If one of the links is a double strand (a double bond), it is called a “monounsaturated” fat, a good example of which is oleic acid. Olive oil, with its large amounts of oleic acid, is an integral part of the so-called “Mediterranean diet”, which reduces blood cholesterol and lessens the risk of suffering a heart attack. If two or more of the links in a fatty acid are double-stranded, it is called a polyunsaturated fatty acid, or PUFA. All of the fatty acids that are essential to human health – the so-called “essential fatty acids” – are PUFAs.
DHA is one of these, with no less than six double strands. The letters DHA stands for “docosahexaenoic acid” – so you can see why even chemists refer to it as DHA! Because the first of the double strands is on the third carbon from the distant (omega) end of the string, it’s called an “omega-3 polyunsaturated fatty acid”. Fat accounts for roughly 55% of the substance of our brains, but this is a very special type of fat, not the kind we gather around our waists, hips and thighs. This so-called “structural fat” is an integral part of the membranes of the nerve cells. There are roughly 100 billion such nerve cells, or “neurons”, in the human brain and DHA is the main fatty acid found in their covering membranes. It is especially concentrated in the vast proliferation of nerve-to-nerve junctions, known as synapses, that ramify throughout the cortex or “thinking part” of our brain. We know that deficiency of DHA causes a reduction in the chemicals that transmit signals in these synapses, causing a demonstrable malfunction in some of the key brain pathways.
DHA is also a major component of the myelin sheath, which is wrapped like insulation around the long processes of nerves. Integrity of the myelin is of the utmost importance for normal brain function. Experiments in animals show that deficiency of omega-3s in the postnatal period leads to delay in forming this insulation, leading to impaired learning and also motor, visual and auditory abnormalities. For example, inflammation and damage to myelin plays an important part in human diseases such as multiple sclerosis.
The highest concentration of DHA is found in the retina – the nerve lining at the back of the eyes – where it plays an important role in vision. Today we suspect that some of the visual disturbance of a hangover may be caused by alcohol’s known effect of leaching out DHA from both the brain and the retina. In men DHA also plays a role in the tail-wagging movement, or motility, of the sperm.
Eicosapentaenoic Acid, or “EPA”, is another omega-3 polyunsaturated fatty acid, with five double strands in a slightly shorter string. Unlike DHA, EPA is not stored in any great amount in the brain or retina but it plays an important part in the health of our hearts and circulation and is important also in reducing inflammation.
We also know that the omega-3s, DHA and EPA, get used up as part of the normal chemistry of our brain and other internal organs so they have to be replaced, mainly from our diet. DHA and EPA are only found in any significant quantity in marine foods, especially the edible flesh of oily fish, such as herrings, salmon and mackerel, and the livers of white fish, such as cod and haddock. Meanwhile another source of DHA, and the only significant vegetarian one, is certain marine algae, which provide the DHA that is now used to fortify bottled milk formulas.
The essential fatty acids include the omega-6 linoleic acid, or LA, which is plentiful in plants, particularly their seeds. It’s a major ingredient in most f the common cooking and salad oils, such as sunflower, soybean, peanut, corn and safflower oil. Note, however, that there is little to none in butter, coconut oil or cocoa butter. LA is readily converted to arachidonic acid (ARA) in the body. Arachidonic acid itself is also found in a normal diet, from meat, eggs and dairy produce. It’s a moot point whether you regard linoleic acid as the essential fatty acid, or both linoleic acid and arachidonic acid, but it hardly matters in practice since 85 to 90 per cent of our dietary intake of omega-6s is LA. As you may have figured out, in ARA and LA the first double strand joint is on the sixth carbon from the distant, or omega, end of the string, hence the term omega-6. Arachidonic acid also plays an essential role in brain development and normal day-to-day function. But given the many sources of both these omega-6s in the adult diet, deficiency of ARA is unlikely in normal circumstances, though linoleic acid is a normal component of bottle or formula feeds for infants.
Alpha-linolenic acid, or ALA, is another omega-3 fatty acid of plant origin, being found for example in broadleaf green vegetables, flaxseeds, walnuts, soy nuts, canola oil (another name for rapeseed) and soybean oil. It acts as a source of energy for the body. It can also be used as a building block for DHA and EPA. But this conversion process is slow and inefficient, with only 5-10% of ALA being converted to EPA and even less being converted to DHA. This means that the only reliable way to get enough of these essential ingredients is to consume DHA and EPA in our diets. Ideally we should aim to eat a meal of oily fish at least twice a week, and this is what I recommend.
General symptoms of essential fatty acid deficiency include fatigue, skin and hair problems, and weakening of the immunity. In the longer term deficiency of omega-3s appears to make us more liable to heart attacks, strokes, cognitive decline and more rapid development of conditions associated with ageing. It also appears to make us more liable to auto-immune diseases such as arthritis. So there are some pretty sound reasons for taking the recommended dose of omega-3 in your diet. There is also early evidence that lack of omega-3s in diet may make women more liable to breast cancer – but this latter is just a suggestion and needs a good deal more research before we can be sure.