It is virtually impossible to avoid the incessant advertising which proclaims the virtues of various food products which are high in PUFAs(polyunsaturated fatty acids). Invariably the message is that these are “healthy” because they will lower blood cholesterol(TC). In Blog 8 I have explained that the rationale used to support the case for reducing TC is fundamentally flawed. For people over aged 60, which is when most deaths occur, the tendency is for life expectancy to increase at higher levels of TC.
So if there are no benefits from lowering TC then the justification for consuming lots of products which are high in polyunsaturates oils is effectively blown out of the water. But it gets worse because there are genuine concerns about consuming excessive amounts of these products.
First of all, It is important to understand that the body has a requirement for long chain fats which are referred to as essential fatty acids(EFA) but there are actually 2 distinct families, namely the omega-3s and the omega-6s.
These are:
- Omega-3s based on alpha linoleic acid(ALA) which has 18 carbon atoms with 3 double bonds beginning in the 3-position(C18 3n-3). In the body the ALA may be converted to eicosapentaenoic acid (EPA C20 4n-3) and docosahexaenoic acid (DHA C22 5n-3). These are primarily found in fish oils
- Omega-6s based on linoleic acid(LA) which also has 18 carbons atoms but 2 double bonds beginning in the 6-position(C18 6n-6). LA can be converted into arachadonic acid (AA C20 3n-6). It is this group, found in spreads and vegetables oils, which is promoted as good for health because it lowers TC
The reason these are essential for human nutrition is because they cannot be synthesised in the body and must therefore be provided in the diet. Because the body is unable to convert omega 6s into omega 3s and vice versa the balance between the 2 groups is critical.
Up to about 1900 the diet contained approximately equal amounts of each group. However since as industrialisation has progressed the intake of the omega 3s has fallen while that of the omega 6s has increased with the result that the ratio of omega 6:omega 3 is now 15-20 in countries such as the USA and the UK.
There is convincing evidence that these high ratios are damaging to health.
- During the 1980s research in Greenland with Eskimos, who consumed a diet with a high seafood content rich in EPA and DHA, had low rates of coronary heart disease, asthma, type 1 diabetes mellitus, and multiple sclerosis. Since that observation, the beneficial health effects of omega 3 fatty acids have been extended to include benefits related to cancer, inflammatory bowel disease, rheumatoid arthritis, and psoriasis.
- The longer chain fatty acids are converted into eicosanoids. If there a relatively high proportion of omega 6s in the diet there will be an excessive amount of eicosanoids derived from AA which will contribute to the formation of thrombus and atheromas; to allergic and inflammatory disorders, particularly in susceptible people; and to proliferation of cells.Diets which are high in LA increase the susceptibility of the LDL cholesterol to oxidation which in turn increase the severity of coronary atherosclerosis.
- The higher the ratio of omega-6/omega-3 fatty acids in platelet phospholipids, the higher the death rate from cardiovascular disease.
- Excessive amounts of omega-6 PUFA and a very high omega-6/omega-3 ratio promote the pathogenesis of many diseases, including cardiovascular disease, cancer, and inflammatory and autoimmune diseases, whereas increased levels of omega-3 PUFA (a lower omega-6/omega-3 ratio),exert suppressive effects(1,2).
In addition there are many studies which indicate that excessive levels of LA per se may pose a hazard to health. It is known that a high content of LA in LDL cholesterol increases its’ sensitivity to oxidation. Oxidised LDL cholesterol is involved in the development of atherosclerotic plaque, which leads to heart disease(3).
A study in Sweden monitored 61471 women for an average period of 4.2 years between 1987 and 1993 during which time there were 674 cases of breast cancer recorded. It was found that increasing amounts of LA in their diets was associated with an increased likelihood of developing breast cancer. By contrast olive oil which has a high content of oleic acid(C 18) was found to be protective(4).
This information strongly indicates that the anyone who consumed products high in polyunsaturates in the belief that they will be good for their health should seriously reconsider their position. The reality is that may do more harm than good. Personally I prefer to choose butter, especially from grass-fed cows. I will explain all shortly.
REFERENCES
- A P Simopoulos(2002) Journal of the American College of Nutrition 21 (6) pp 495-505
- A P Simopoulos(2008) Experimental Biology and Medicine 233 (6) pp 674-688
- P C Calder (2010) British Journal of Nutrition 104 pp1575-1576
- A Wolk et al(1998) Archives of Internal Medicine 158 (1) pp 41-45