Urging on Omegas: Omega-3s, from [alpha]-linolenic acid to DHA, are related fatty acids that differ in structure, source, and disease prevention abilities

Prepared Foods, May, 2003 by Claudia Dziuk O'Donnell

The following article is derived from Chapter 8, written by Andrew Sinclair and colleagues, Lipids for Functional Foods and Nutraceuticals, 2003, Frank Gunstone, ed., with full permission from Peter J. Barnes, Publisher at The Oily Press. See article end for more information.

The food and dietary supplement industries have promoted an increased consumption of omega fats. In developed countries such as the U.S. and Canada, intakes of long-chain n-3 PUFA (polyunsaturated fatty acids) are below recommended levels for optimal health. The UK Department of Health sets this at 1.5g per week and the British Nutrition Foundation at 1.25g per day.

So, what are these fats, where do they come from and what are their benefits?

Sourcing Fatty Acids

The main omega-3 (also referred to as [omega]-3 or n-3) fatty acids in food are [alpha]-linolenic acid (ALA or 18:3n-3), docosahexaenoic acid (DHA or 22:6n-3), eicosapentaenoic acid (EPA or 20:5n-3) and docosapentaenoic acid (DPA or 22:5n-3).

ALA, an essential fatty acid, is present in high levels in certain plant oils. Our bodies metabolize ALA into DHA, EPA and DPA, which are longer-chained omega-3 poly-unsaturated fatty acids (PUFAs). DHA and EPA can also be consumed in the diet. They are present in fish and other marine organisms. (See chart.) Similarly, an omega-6 fatty acid, linoleic acid (LA or 18:2n-6) can be metabolized into the longer chain arachidonic acid (AA or 20:4n-6).

In the past, it has been debated as to whether ALA and LA are essential in the diet, or if it is more important to consume directly the longer chain AA, EPA and DHA into which they are metabolized. (1) It has been shown that LA itself is required for normal skin functioning such as the prevention of water loss and epidermal integrity. (2) Specific functions of ALA have not been confirmed other than its benefits to animal fur.

However, more recently, it has been learned that ALA is metabolized by several other pathways, which may mean it has other important roles than just serving as a precursor for the longer chain PUFAs. ALA is deposed in skin and fur. (3) Some studies show its importance for healthy skin.

ALA or DHA

Studies show that, on a gram-for-gram basis, dietary DHA is a more effective source of DHA in the body than consuming ALA. In one study with baboons, dietary DHA was found to be seven times more effective than dietary ALA as a substrate for neural DHA. (4) Also, diets high in ALA do not necessarily lead to high levels of DHA in various body tissues; neither do levels of DHA in the blood (plasma) correlate to DHA levels in various tissues. For example, high levels of DHA tend to concentrate in the retina and brain. (5)

Generally, studies do suggest that dietary ALA converts to adequate levels of DHA in the body, especially if LA in the diet is reduced. This is an especially important concept for vegans. However, the conversion process of ALA to EPA and DHA is slow and not nearly as effective as consuming EPA- and DHA-containing fish or fish oils.

There are several reasons why ALA conversion rates are so low. First, dietary ALA is "sidetracked" to be deposited in adipose tissue, skin and, at least in animals, fur. ALA also is diverted to [beta]-oxidation.

However, of particular importance is the omega-6 LA. It is the major dietary PUFA and, by several mechanisms, inhibits the metabolism of ALA to DHA. For example, LA uses a desaturase enzyme that also is required for ALA's metabolism to DHA. On this point, it has been shown that a good way to increase long-chain omega-3s in the body is through consumption of blackcurrant or alpine currant seed oil. These are high in the omega-3 fatty acid (18:4n-3), which also converts to DHA, but, being one step further along in the metabolic pathway toward DHA than ALA, do not require the use of the rate-limiting enzyme. Again, this is important for vegetarians with diets low in fish.

Helping Health

A great many studies support the benefits of omega-3 PUFAs in human health. Here is just a small sampling.

* Blood pressure. In a 1999 study, DHA significantly reduced both sleeping and waking systolic and diastolic blood pressures in humans. (6) A rat study showed n-3 deficiency early in life resulted in high blood pressure later.

* Lipoprotein lipids. Both LA and plant ALA reduce LDL and total cholesterol levels. And, n-3 PUFA, especially from marine sources, decrease triglyceride levels. (7)

* Thrombosis. A 14-year study with over 79,000 nurses showed that those with higher intakes of long-chain n-3 PUFA from fish had a lower risk of total strokes. (8)

* Cardiovascular disease, secondary prevention. The GISSI-Prevention Trial conducted in Italy with 11,324 patients with known coronary heart disease provides strong evidence that marine n-3 fatty acids help in the prevention of acute coronary syndromes. One theorized mechanism is the ability of n-3 fatty acids to prevent cellular damage during periods of ischemic stress. (9)

* Cancer. Much research supports the ability of long-chain n-3 PUFA to reduce cancer rates in animal models. However, there are few human studies. One Swedish study of 6,272 men correlated consumption of fatty fish with high levels of these fatty acids and lower prostate cancer rates. (10)