Not all omega-3s created equal

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At the same time, the better known omega-3 fatty acids from fish oil, EPA and DHA, had negative impacts on LDL (‘bad’) cholesterol levels, but did lead to beneficial effects to levels of a protein that affects blood clotting.

Effect on specific risk markers

The results add weight to a diet rich in both plant and marine sources of omega-3s to reduce the levels of cardiovascular disease markers in the elderly, a group at high risk of heart problems.

“This suggests that these n-3 fatty acids derived from different sources might have their own specific effects on cardiovascular risk markers,” wrote the scientists in the European Journal of Clinical Nutrition (Vol. 60, pp. 978-984).

Alpha-linolenic acid is a plant-derived polyunsaturated fatty acid (PUFA) with 18 carbon atoms in the chain. It can be converted to the longer chain fatty acids, EPA and DHA (the PUFAs found in oily fish), although tracer studies have reported that this conversion is limited in humans.

Omega-3s from fish oil have been studied extensively for their potential to decrease the risk of cardiovascular disease.

While a lot of attention has focused on EPA and DHA, only a limited number of studies have looked at the effects of ALA and EPA/DHA on cardiovascular risk factors in an elderly population, said the researchers behind the new study.

The research study

The researchers, led by Professor Roland Mensink from the University of Maastricht, recruited 37 subjects (23 women) with high cholesterol levels and an average age of 64,5, and, after a three week run-in diet rich in oleic acid, randomly assigned them to one of three experimental diets: control diet (oleic acid-rich); ALA-rich diet (6,8g per day); EPA/DHA-rich diet (1,05g EPA, 0,55g DHA per day).

After a further six weeks, the researchers found that levels of LDL cholesterol had increased in the EPA/DHA diet by 0,39 millimoles per litre, compared to the ALA-rich diet.

Levels of the apolipoprotein B (ApoB) also increased for the EPA/DHA group by 14mg/dL and 12mg/dL compared to oleic acid and ALA-rich diet, respectively.

ApoB is the main apolipoprotein of LDL cholesterol and is responsible for the transport of cholesterol to tissues. In high concentrations it has been linked to plaque formation in the blood vessels, although the mechanism behind this is not clear.

The EPA/DHA diet did, however, lead to beneficial improvements in the levels of a substance called tissue factor pathway inhibitor (TFPI), a protein that is involved in formation of blood clots that may lead to thrombosis, heart attack or stroke.

“It has been suggested… that the elderly might benefit more from an improvement of factors influencing blood clotting and fibrinolysis than any of the other factors involved in the atherogenic process,” wrote the researchers.

More research needed

The researchers do not discuss the reasons as to why each fatty acid may produce differing effects and further mechanistic studies, as well as larger and longer interventions in similar populations, are needed.

In spite of these limitations, and a scarcity of other studies for comparison, the researchers concluded: “Our findings indicate that n-3 fatty acids from both plant and marine sources do not affect the lipid profile equally favourable in elderly subjects as oleic acid.” - (Decision News Media, August 2006)

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