Fatty Acids are Needed for Growth
The lipids of our central nervous system contain high proportions of arachidonic acid (20:4 n-6, AA) and docosahexaenoic acid (22:6 n-3, DHA) which are the two most important polyunsaturated fatty acids in the brain. Levels of linoleic acid (18:2 n-6) and alpha-linolenic acid (18:3 n-3) are low, usually less than 1% to 2% of total fatty acids (Innis, S78-79).
Linoleic acid and alpha-linolenic acid are precursors to AA and DHA; they are elongated and desaturated to form AA and DHA (Clandinin, 27). The brain growth in humans begins early in the third trimester of pregnancy. This is the most important period for brain AA and DHA accumulation (Innis, S78-79). Therefore, the composition of fat in the brain during this period is very important for normal growth and development of the brain. Any changes in the balance of the fatty acids in the dietary fat intake will result in changes in the membrane functioning of brain cells.
During this prenatal life, lipids are mostly needed for structural and regulatory functions. After birth, the infant is challenged by changes in the lipid sources. The response to these changes can be altered by changes in the diet composition. For example, a newborn infant’s response to human milk is considered to be ideal, therefore baby formulas try to mimic that response (Biervliet, S101). Studies have shown that fatty acid accumulation in the brain increases as the pregnancy progresses. It reaches the maximum level of accumulation toward the end of the pregnancy. But there are differences in the rate of accumulation of these fatty acids. For example, DHA levels in the cerebrum increase as the percentage of the total fatty acid, whereas AA levels decrease during the last trimester (Martinez, S130-131). This reduction in arachidonic acid levels is caused by the addition of eicosapentaenoic acid (20:5 n-3, EPA) and DHA in the diet. DHA and eicosapentaenoic acid are known to inhibit the desaturation of n-6 fatty acids. Therefore, there is a reduction in AA synthesis (Innis, S81).
Eicosapentaenoic acid is the other precursor of DHA besides alpha-linolenic acid (18:3 n-3). Studies done on chicks show that addition of EPA to the diet increased the brain and retinal DHA as much as when DHA was added to the diet. These studies have also shown that dietary alpha-linolenic acid (18:3 n-3) is a less efficient precursor for DHA in animals with low delta-6 desaturase activity. Thus, it is possible that humans have a stronger preference for dietary DHA over dietary alpha-linolenic acid as the source of tissue DHA (Anderson, 90,95). During the postnatal development, DHA is the only n-3 fatty acid that is present in significant amounts in the brain. Adrenic acid
(22:4 n-6), which is the third most abundant fatty acid in the brain, increases very rapidly during this period (Martinez, S132). If the amount of these fatty acids in the diet is not sufficient to maintain an adequate supply of...