Fish, fortified foods and fish oil supplements differ in the form and amounts of EPA and DHA they contain. Most fish have more DHA than EPA, while fish oil supplements usually have more EPA than DHA. For general health purposes, these differences are not of great significance. Both EPA and DHA have important jobs in keeping the body healthy.
There is one other omega-3 fatty acid found mainly in plants, the “short-chain” alpha-linolenic acid. This omega-3 is shorter, less unsaturated and has less potent health effects than EPA and DHA. It is not used by the brain, so it cannot substitute for DHA in brain function or infant development. The body can convert alpha-linolenic acid to EPA, but does so very inefficiently. Almost no DHA is made from alpha-linolenic acid. Thus, the health effects of plant and fish oil omega-3s are not equivalent. Oils such as flax, canola and soybean, as well as walnuts and flaxseeds contain the most alpha-linolenic acid. These items may be used in foods to support the claim, “contains omega-3s,” but the term alpha-linolenic acid seldom appears. Find out more about omega-3s here.
FORMS OF OMEGA-3S
In the body, EPA and DHA are located almost exclusively in phospholipids-complex derivatives of fat-that make up cell membranes. To a small extent, EPA and DHA are stored in fat cells in the form of triglyceride, the technical name for fat. Both triglycerides and phospholipids protect omega-3s by providing a stable structure.
The body oils of fatty fish, the richest source of EPA and DHA, contain mainly triglycerides. Fish body oils are commonly used in fish oil supplements and have been purified to remove contaminants. Triglycerides are broken down during normal digestion, the same as other food fats, making EPA and DHA readily available for absorption and transfer to tissues. DHA added to infant formula is in triglyceride form.
EPA and DHA are also found in foods and supplements as phospholipids, mainly in egg yolk and fish liver oils. These, too, are highly available and easily digested. Omega-3s may be slightly more available from phospholipids than from triglycerides, but differences are small. When we eat fish, we obtain omega-3s in both triglyceride and phospholipid forms.
: Purified omega-3s are available as “concentrates” in certain dietary supplements and by prescription. These fatty acids are in the form of ethyl esters. Studies on the health effects of omega-3s using either fish oil or omega-3 ethyl esters have shown that both forms are highly bioavailable and have similar health effects.
ABSORPTION OF OMEGA-3S
The absorption of omega-3s from fish oil is increased if other fat-containing foods are consumed with them. Absorption is greater as the amount of omega-3s increases.
Interestingly, EPA and DHA appear to be better absorbed from fish and fortified foods than from fish oil, although there are few studies comparing the two. Absorption from omega-3-fortified liquid eggs appears equivalent to fish and fish oil concentrates, as reflected in the improvement of several heart risk factors. Some fish products are also fortified. In a study comparing regular salmon patties with fish oil-fortified salmon patties having twice the EPA and DHA content, omega-3 levels in the blood were more than twice as high with the fortified patties compared with the unfortified ones. This observation suggests that EPA and DHA absorption from foods is increased with higher doses still in the range considered safe by the U.S. Food and Drug Administration.
In contrast, the absorption and subsequent metabolism of EPA and DHA are reduced when large amounts of other polyunsaturated fatty acids are present. Meals rich in vegetable oils, such as soybean, corn, sunflower, safflower and cottonseed oils, provide substantial linoleic acid, an omega-6 fatty acid that competes with omega-3s. Vegetable oils rich in monounsaturates, such as canola and olive oils, have much less linoleic acid, so they do not interfere with omega-3 metabolism. Western diets contain 10 to 20 times as much omega-6s as omega-3s. Widespread use of vegetable oils, especially soybean oil, contributes to this highly imbalanced intake of these two fatty acid families.
For strict vegetarians, supplements produced from microorganisms or marine algae are sources of EPA and DHA. Marine algae vary widely in their content of omega-3s ranging from 17% to nearly 50%. The content of alpha-linolenic acid compared with EPA also varies widely (3% to 24%). Marine algae are not commonly available in the U.S.
A referenced version of this article is available from the editor.