The diet of our ancestors was less dense in calories, being higher in fiber, rich
in fruits, vegetables, lean meat, and fish. As a result, the diet was lower in total
fat and saturated fat, but contained equal amounts of n-6 and n-3 essential fatty
acids. Linoleic acid (LA) is the major n-6 fatty acid, and alpha-linolenic acid (ALA)
is the major n-3 fatty acid. In the body, LA is metabolized to arachidonic acid (AA),
and ALA is metabolized to eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA).
The ratio of n-6 to n-3 essential fatty acids was 1 to 2:1 with higher levels of the
longer-chain polyunsaturated fatty acids (PUFA), such as EPA, DHA, and AA, than today's
diet. Today this ratio is about 10 to 1:20 to 25 to 1, indicating that Western diets
are deficient in n-3 fatty acids compared with the diet on which humans evolved and
their genetic patterns were established. The n-3 and n-6 EPA are not interconvertible
in the human body and are important components of practically all cell membranes.
The N-6 and n-3 fatty acids influence eicosanoid metabolism, gene expression, and
intercellular cell-to-cell communication. The PUFA composition of cell membranes is,
to a great extent, dependent on dietary intake. Therefore, appropriate amounts of
dietary n-6 and n-3 fatty acids need to be considered in making dietary recommendations.
These two classes of PUFA should be distinguished because they are metabolically and
functionally distinct and have opposing physiological functions; their balance is
important for homeostasis and normal development. Studies with nonhuman primates and
human newborns indicate that DHA is essential for the normal functional development
of the retina and brain, particularly in premature infants. A balanced n-6/n-3 ratio
in the diet is essential for normal growth and development and should lead to decreases
in cardiovascular disease and other chronic diseases and improve mental health. Although
a recommended dietary allowance for essential fatty acids does not exist, an adequate
intake (AI) has been estimated for n-6 and n-3 essential fatty acids by an international
scientific working group. For Western societies, it will be necessary to decrease
the intake of n-6 fatty acids and increase the intake of n-3 fatty acids. The food
industry is already taking steps to return n-3 essential fatty acids to the food supply
by enriching various foods with n-3 fatty acids. To obtain the recommended AI, it
will be necessary to consider the issues involved in enriching the food supply with
n-3 PUFA in terms of dosage, safety, and sources of n-3 fatty acids.