Journal Name:
Brain Research
Article Title:
Dietary omega 3 fatty acids and the developing brain.
Date Written:
2008
Volume:
1237
Number:
NA
Page:
35
Author(s):
Innis, S.M.
Article:
This extensive review article describes the action of the omega 3 fatty acids in brain health and development. The omega 3 fatty acids are essential dietary nutrients and one of their important roles is providing the fatty acid with 22 carbons and 6 double bonds known as docosahexaenoic acid (DHA) for nervous tissue growth and function. Inadequate intakes of omega 3 fatty acids decrease DHA and increase omega 6 fatty acids in the brain. Decreased DHA in the developing brain leads to deficits in neurogenesis, neurotransmitter metabolism, and altered learning and visual function in animals. Western diets are low in omega 3 fatty acids, including the 18 carbon omega 3 fatty acid alpha linolenic acid found mainly in plant oils, and DHA, which is found mainly in fish. The DHA status of the newborn and breast-fed infant depends on the maternal intake of DHA and varies widely.
Epidemiological studies have linked low maternal DHA to increased risk of poor child neural development. Intervention studies have shown improving maternal DHA nutrition decreases the risk of poor infant and child visual and neural development. Thus, sufficient evidence is available to conclude that maternal fatty acid nutrition is important to DHA transfer to the infant before and after birth, with short and long-term implications for neural function. However, genetic variation in genes encoding fatty acid desaturases also influence essential fatty acid metabolism, and may increase requirements in some individuals. Consideration of omega 3 fatty acid to include brain development, optimizing omega 3 and omega 6 fatty acids in gestation and lactation, and in fatty acid nutrition support for intravenous and formula-fed neonates is important.
Prior to birth, all of the omega 3 fatty acids required for fetal development must be provided by placental transfer from mother's circulation. Transfer of DHA across the placenta involves fatty acid binding proteins, with release of DHA to the fetal circulation, followed by transport to liver where it is esterified and resecreted in lipoproteins. Circulating levels of DHA, linoleic acid (LA) and other unsaturated fatty acids in maternal and fetal plasma are significant and positively correlated. These studies show DHA is higher and 18:2ω-6 is lower in the fetus (newborn) than mother, but DHA and LA varies widely with the maternal fatty acid status directly affecting that of the infant before birth. LA was inversely related to DHA in pregnant women and young children, raising the possibility that high 18:2ω-6 may contribute to low circulating levels of DHA.
To summarize, placental fatty acid transfer shows considerable dependence on maternal plasma fatty acids, and is not regulated to protect the fetus from high maternal omega 6 fatty acids, or low DHA. An increase in circulating long chain omega 6 fatty acids in some pregnant women could be explained by inadequate intakes of omega 3 fatty acids or excessive intakes of LA, both of which are practically overcome by dietary DHA.
After birth, breast milk provides the sole source of omega 3 fatty acids, as well as the omega 6 fatty acids to support the growth and development of the breast-fed infant, continuing for much of the first year of life when low fat cereals, fruits and vegetables are added to the infant diet. DHA, other omega 3 fatty acids and LA vary widely in human milk, a variability explained largely by differences in the maternal dietary fat intake. Human milk fat DHA varies among and within populations, from 0.1% to over 1.0% DHA, explained largely by differences in the intake of preformed DHA, usually from fish and other sea-foods. Women following vegan and vegetarian diets usually have 0.1% or less DHA in their milk fat and human milk DHA decreased by 50% to less than 0.2% milk fat over the last 3 decades. The amount of DHA in human milk is increased by increasing the maternal intake of DHA. Human milk LA also varies, typically from 6 to 30% milk fatty acids, depending on the maternal intake of LA. To summarize, as for placental transfer, the secretion of DHA and LA in human milk is strongly influenced on the mother's fat intake, with mammary gland appearing to lack mechanisms to protect the breast-fed infants from maternal dietary deficiency or excessive intakes of unsaturated fatty acids.
In conclusion, DHA is a critical component of brain membrane lipids, the possibility that western diets poor in omega 3 fatty acids and rich in omega 6 fatty acids contribute to poor CNS development and function is becoming increasingly recognized. The evidence to show low rates of conversion of 18:3 omega 3 to DHA in individuals following western diets, and the demonstrated dependence of maternal to infant transfer of DHA and omega 6 fatty acids on the maternal polyunsaturated fat intake provides strong reason for dietary recommendations for omega 3 fatty acids that include consideration of brain development and function. Recent research to show genetic variations in the fatty acid desaturase pathway influences circulating and human milk essential fatty acids also raise the need to understand whether gene polymorphisms alter the requirements for particular omega 3 fatty acids, and if diet-gene interactions increase risk of poor neural development or disease in some individuals.
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