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TYPES OF OMEGA-3S

The term "omega-3s" is shorthand for omega-3 fatty acids.* They are polyunsaturated - more so than most others commonly found in foods - and differ from all other fatty acids in the placement of their first double bond. This structure is impossible for humans to make from scratch or from other fatty acids, so, we must obtain omega-3s from foods.

There are 3 main omega-3s in foods. The shortest is alpha-linolenic acid, with 18 carbons and 3 double bonds. It is the only omega-3 found in plants. The richest sources are the oils of flax, canola, and soy along with English walnuts and flax seeds. Less common sources are hemp, echium, perilla, black currant and chia in their respective oil or seed forms.

The other major omega-3s are EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid). These are critical for various functions in the brain, blood vessels, immune system, and elsewhere. EPA and DHA are sometimes described as "long-chain" because they have 20 or more carbons. They are found mainly in fish and shellfish, fish oil capsules, and in smaller amounts in omega-3 eggs. They may also be added to some foods. However, if a label says it contains "omega-3" it is not clear whether it has alpha-linolenic acid or the long-chain types. This could be important if you are seeking EPA and DHA. Most foods in the U.S. with added omega-3s contain only alpha-linolenic acid.

WHAT DIFFERENCE DOES IT MAKE?

The differences among omega-3s could be very important, depending on who you are.

Pregnant women: If you are a pregnant woman, your fetus requires DHA and cannot get as much as it needs from relying only on alpha-linolenic acid. DHA is a critical for proper brain development and function and is important for the retina of the eye and developing vascular system. Because the fetus strongly prefers DHA ready-made to alpha-linolenic acid, which has to be converted to DHA, it is important that pregnant women eat plenty of DHA-rich foods, especially in the last trimester of pregnancy. That requires consuming fish regularly, at least weekly, or taking fish oil supplements. Omega-3 eggs also have small amounts of these omega-3s.

Infants: Below the age of 2, infants need DHA ready-made as well. That is because the brain and nervous system are still growing. EPA and DHA, especially the latter, are needed for proper learning, visual function, and other aspects of neurodevelopment. Infants also have limited ability to convert alpha-linolenic acid to EPA and DHA.

Breast-fed infants obtain DHA from their mother's milk. They will obtain more DHA if their mother eats fish during pregnancy and nursing. The mother's intake of DHA is the main factor determining how much DHA is in her milk. The richest sources of these omega-3s are fish and shellfish, fish oil supplements, and omega-3 eggs. When infants begin to eat solid foods, egg yolk from omega-3 eggs and fish at the appropriate ages are the best food sources. Toddlers digest these foods easily, too.

Formula-fed infants will receive DHA only if they are fed formula supplemented with AA and DHA. If these fatty acids have been added to the formula, this information will appear on the label. It is wise to use these supplemented formulas for bottle-feeding infants. These infants, too, can consume egg yolk from omega-3 eggs and fish, at the appropriate ages.

Adults: For most adults, consuming foods with alpha-linolenic acid and the long-chain omega-3s makes good sense. Both types of omega-3s have heart-protective properties. It is uncertain, however, whether alpha-linolenic acid behaves as strongly or in as many ways as EPA and DHA. In most studies, EPA and DHA have been more potent and quicker-acting than alpha-linolenic acid. The latter has not been evaluated or compared directly with EPA and DHA in many health conditions where EPA and DHA have health benefits. Neither do we know whether alpha-linolenic acid itself or the omega-3s derived from it account for its effects. What we know is that EPA and DHA are the omega-3s with the widest array of health benefits including heart disease, immune and brain function, vision, and mental health.

CONVERSION OF ALPHA-LINOLENIC ACID TO EPA AND DHA

Humans can convert alpha-linolenic acid to EPA and to a lesser extent, DHA, but they do so very inefficiently. The most recent estimates of how much is converted range from less than 1% to about 5%. The amount converted is influenced by sex, age, and background diet, particularly the intake of other polyunsaturated fatty acids. Women appear to be somewhat better converters than men.

Conversion is reduced by the consumption of competing fatty acids, such as linoleic acid, high levels of alpha-linolenic acid, EPA and DHA. Western diets are very high in linoleic acid compared with alpha-linolenic acid - from 10 to 20 times greater - so conversion is already at a disadvantage. Simply eating more foods rich in alpha-linolenic acid discourages rather than increases its conversion.

Most of the alpha-linolenic acid we eat is oxidized or "burned" for energy, but some is found in body fat and other tissues. Consuming foods with alpha-linolenic acid helps offset the high intake of linoleic acid, too. In countries with low fish consumption, such as the U.S., people consume most of their omega-3s as alpha-linolenic acid.

FOODS RICH IN OMEGA-3S

Long-chain omega-3s, EPA and DHA, are most abundant in fish and shellfish, particularly fatty fish such as salmon, rainbow trout, tuna, mackerel, herring, sardines, and pilchards. All seafoods have some EPA and DHA. Fish oil capsules are a rich source of EPA and DHA, too. Egg yolks from hens fed flaxseed have alpha-linolenic acid and some EPA and DHA. These are usually sold touting "omega-3" on the label. Newly developed yogurt, margarines, spreads, and other foods may have omega-3s added to them. Check the label to see which fatty acids they contain.

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.

*Words in blue are described in the glossary.