Omega-3 fatty acids

Omega-3 fatty acids are attracting a great deal of interest. And this is not for nothing. As well as the fact that they reduce the risk of cardiovascular disease there are many research reports that describe additional positive health effects of these fatty acids. Although not all the effects have actually been proven, the general picture is a positive one.

What are omega-3 fatty acids?

Fats are built up from a glycerol molecule and three fatty acids that together form a triglyceride. An omega-3 fatty acid is a polyunsaturated fatty acid with the first double bond positioned on the third carbon atom, counted from the methyl end. Another common name for an omega-3 fatty acid is therefore an n-3 fatty acid.

An important omega-3 fatty acid is alpha-linolenic acid (ALA). ALA has a chain length of 18 carbon atoms with three unsaturated bonds (C18:3, n-3; see the figure). Other important omega-3 fatty acids are eicosapentanoic acid (EPA; C20-5, n-3) and docosahexanoic acid (DHA; C22-6 n-3).

Essential fatty acids

Essential fatty acids are important for optimum health, but the body cannot make them itself. It is therefore important that we take in sufficient of these fatty acids in our food. This group of essential fatty acids includes the omega-3 fatty acid ALA and the omega-6 fatty acid linoleic acid. Linoleic acid (LA; C18-2 n-6) is converted in the body to the longer arachidonic acid (AA; C20:4 n-6). ALA is converted to EPA, and this can be converted into DHA. However, the body can only carry out this last conversion to a very limited extent.

Conversion in the body

Enzymes called desaturases and elongases are used for the conversion of LA and ALA to the long chain fatty acids. Because both the omega-3 and the omega-6 fatty acids use the same enzymes there is a competition. When ALA is converted it appears that what is important is not the ratio between LA and ALA in the food but the absolute amount of fatty acids consumed. A high intake of LA restricts the conversion of ALA to EPA.

Health effects of omega-3 fatty acids

The Netherlands Health Council has concluded that EPA and DHA reduce the risk of cardiovascular disease. EPA and DHA appear to have favourable effects on risk factors such as arrhythmia, the functioning of the blood vessel wall (endothelium function), inflammation diseases and blood coagulation. ALA also has a protective effect against coronary heart disease. Because the results are not yet completely consistent research into the effect of alpha-linolenic acid on the risk of cardiovascular diseases remains necessary.

Mechanism

There are various scientific theories about the mechanism of the working of omega-3 fatty acids. One possible mechanism is that the fatty acids have an effect on heart rhythm abnormalities. As well as this, the omega-3 fatty acids become built into the cell membranes and the myelin sheath around the tips of the brain cells so that they can influence the conductance of stimuli and the transfer of information. More research is necessary to clarify the precise mechanism of the working of omega-3 fatty acids.

Sources of omega-3 fatty acids

ALA is present in vegetable oils such as linseed oil and soya oil. It is also present in products that are made from these oils such as sauces and margarines. ALA also occurs naturally in lentils, soya beans, green leaf vegetables such as spinach and in nuts (see the table). The long chain omega-3 fatty acids EPA and DHA occur in fatty fish such as herring, salmon and mackerel.

Advice

The recommendation made by the Netherlands Health Council for an adult is 450 mg per day of the omega-3-fatty acids EPA and DHA. In order to achieve the amount made in this recommendation two portions of fish, one of which should be a fatty fish, need to be eaten each week. The Health Council recommends that it is preferable to eat one portion of fish per week as a hot meal and one portion with a bread meal. In practice this means replacing meat products by fish products at one lunch and at one hot meal. The amount required depends on the type of fish. Fatty fish, such as herring, mackerel, sardines and salmon contain relatively high contents of EPA and DHA (see the table), so less of these fish is required to achieve the recommended intake than of leaner fish such as cod or pollack. The use of foodstuffs that have been enriched with fish oil fatty acids can be an acceptable alternative for people who do not eat fish. It is emphasised that the use of substances prepared from fish oil as a source of fish oil fatty acids is naturally not the most preferred method.

Most people in the Netherlands do not achieve the recommendations for eating fish. The Food consumption survey (Netherlands Nutrition Centre, 1998) showed that the intake of fish by adults and children is low; for children this is even less than 28 grams per week. Additional research carried out during the Food Consumption survey 2003 on young adults showed that the intake of EPA and DHA in this group is low: 80 (women)-100 (men) mg per day.

An adequate level of intake for adults for ALA is 1 % of the energy requirement. This is equivalent to a daily intake of 2-3 grams (see the sample calculation). (The recommendation for LA is 2 % of the energy requirement per day). The intake of ALA is also lower than that recommended: 0.59 (women)-0.63 (men) percent of the energy requirement.

Sample calculation

The average man in the Netherlands has an energy requirement of 2500 kcal per day and a women 2000 kcal per day. Recommended intake of ALA = 1 percent of the energy requirement per day.

Recommended intake of ALA in kcal/day for an average energy intake is:
men: (2500* 1% =) 25 kcal
women: (2000*1% =) 20 kcal.

1 gram of fat supplies 9 kcal
Recommended intake of ALA in grams/day for an average energy intake is:
men: (25/9=) 2.8 grams
women: (20/9=) 2.2 grams