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By PM
Kris-Etherton, Denise Shaffer Taylor, et. al.
Nearly 90% of the polyunsaturated
fat is the omega-6 fat linoleic acid (LA). The omega-3 fat linolenic acid
(ALA) contributes the majority of the rest of polyunsaturated fat in the
diets of the adult population.
Americans consume 0.2 grams (200 mg)
of EPA and DHA per day which is less than 2% of energy from polyunsaturated
fat.
Sources of Omega-3 Fats in the US Diet
The predominant sources of omega-3 fats in the diet are vegetable oils
and fish. Fish are the major source of EPA and DHA, whereas vegetable
oils are the major source of ALA. Other sources include nuts and seeds,
vegetables and some fruit, and egg yolk, poultry, and meat, all of which
collectively contribute minor quantities of omega-3 fats to the diet.
Of the commonly consumed oils in the United States, soybean and canola
oil are the primary sources of ALA. The contents of ALA in soybean and
canola oil are 7.8% and 9.2%, respectively. Flaxseed oil is a particularly
rich source of omega-3 fats (ie, ALA) although it is not a commonly used
food oil.
Fats and oils contribute 87% of the ALA in the US diet (7), it is apparent
that the contribution of other sources is currently minor.
Purslane, a vegetable used in soups and salads along the Mediterranean
basin and in the Middle East, is unique because it is the richest source
of ALA of any green leafy vegetable examined (6, 8). Moreover, it is one
of the few plants known to be a source of EPA. Although not typically
consumed in the US diet, purslane is nonetheless found in all 50 states
and certainly could be developed as an important source of dietary omega-3
fats.
Dietary Omega-3 Fats Supplements
A variety of omega-3 fat supplements are available to consumers. Many
of these supplements are derived from marine oils and contain 180 mg EPA
and 120 mg DHA per capsule. Another source of omega-3 fats is cod-liver
oil in some capsules that contain 173 mg EPA and 120 mg DHA.
Industry estimates indicate that 300 tons of fish oil are used yearly
for fish-oil supplements in the United States. The
average yearly contribution of EPA and DHA from fish-oil supplements to
the US diet is 0.6-0.9 mg/person. Thus, fish-oil supplements
currently are not an important source of omega-3 fat in the US diet.
Manipulation of Omega-3 Fats in Animal
Products
On the basis of what is known about the effect of diet on the amount of
omega-3 fats in animal products, researchers are manipulating animal feed
in an attempt to increase the omega-3 content of eggs, milk, and meat.
Animal feed enriched with algae, fishmeal, or fish oil correspondingly
increases EPA and DHA concentrations in tissues (eg, muscle and egg yolk).
Accordingly, feeding animals diets rich in flaxseed or flax oil, which
are good sources of ALA, results in increased amounts of ALA in eggs,
milk, pork, chicken, and beef. Major obstacles to this innovative technology
include the tendency of these fats to oxidize, producing "off"
flavors in food products, as well as the added expense of enriching animal
feed with omega-3 sources.
Increasing the vitamin E content of a hen's diet when feeding it omega-3
fats helps control oxidation and off flavors in eggs and meat (19), but
increases the cost of feeding the animals.
Of the animal products enriched with omega-3 fats, eggs are currently
the only products available on the market.
Eggs were probably targeted first because a large percentage of the omega-3
content of the hen's diet is transferred to the egg yolk. The Flax Council
of Canada notes that one omega-3 fat-enriched
egg has about the same amount of omega-3 fats as 85 g (3 oz) fish (20).
Changes in the Ratio of Omega-6 to Omega-3
Fats over Time
Over the course of evolution there appears, on the basis of estimates
from studies of Paleolithic nutrition and modern-day diet assessment,
to have been a remarkable change in the fat content and fat profile of
the human diet (21, 22). The Paleolithic diet was likely much lower in
total fat (21% of energy) and saturated fat (7-8% of energy) than our
present-day diet (21, 22).
Moreover, the diet of our hunter-gatherer ancestors contained approximately
the same quantities of omega-6 and omega-3 fats (ie, the ratio is thought
to have been 1:1).
Sources of omega-6 and omega-3 fats were wild plants, animals, and fish
(23, 24). Plant seeds are good sources of omega-6 fats and the green leaves
of wild plants are good sources of ALA. The wild animals and birds that
ate these food sources were sources of these fats in the human food chain.
Whereas EPA accounted for 4% of fats
in the fat of wild animals (18), domestic animals raised for meat production
had undetectable amounts of EPA in their tissues.
At the onset of the industrial revolution,
140 years ago, there was a marked shift in the ratio of omega-6 to omega-3
fats in the diet.
Omega-6 fat consumption increased at the expense of that of omega-3 fats
(25). This change reflected the advent of the modern vegetable oil industry
as well as the increased use of cereal grains for domestic livestock.
In 1935 the omega-6 to omega-3 fat ratio was 8.4:1 (estimated by annual
per capita food use 7). From 1935 to 1985, this ratio increased to 10.3
(23% increase).
Meeting Dietary Recommendations for
Omega-3 Fats
To date, no official dietary recommendations have been made for omega-3
fats in the United States.
Although the United States has not established official dietary recommendations
for omega-3 fat intake, Canada (32) and the United Kingdom (33) have.
Canada recommends a total omega-3 fat intake of 1.2-1.6 g/d, which is
similar to the recommendation made by nutrition scientists in the United
States but does not distinguish between individual omega-3 fats. The United
Kingdom does distinguish between omega-3 fats and recommends that 1% of
energy be from ALA and 0.5% be from EPA and DHA combined.
Japan (39) has recently changed its recommendation from 4:1 to 2:1 (W
Lands, personal communication, 1998).
The recommended ratio of omega-6 to omega-3 fats is 2.3:1 and has been
made to maximize the conversion of ALA to DHA (40). Because of competition
between omega-6 and omega-3 fats for desaturase and elongase enzymes,
the quantity of linoleic acid in the diet can affect the extent to which
ALA is converted to EPA and DHA in vivo.
The mean ratio of omega-6 to omega-3 fat intake in the United States is
9.8:1 which is much higher than that recommended (2.3:1). Sixty percent
of the population consumes a ratio of 8-12:1 (Figure 2 ). However, it
may be as high as 20-25:1 in some individuals (20).
American
Journal of Clinical Nutrition, Vol. 71, No. 1, 179-188, January 2000
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