DHA IS BRAIN FOOD
Most of the dry weight of the brain is lipid (fat) because brain activity depends greatly upon the functions provided by lipid membranes. Compared to other body tissues, brain content of DHA and arachidonic acid is very high. DHA is particularly concentrated in membranes that are functionally active, namely in synapses and in the retina.
The ability of enzymes to produce the omega-6 and omega-3 family of products of linoleic and alpha-linolenic acid declines with age. One experiment showed that desaturase enzyme function in old rats was only 44% of the desaturase function in young rats. Because DHA synthesis declines with age, as we get older our need to acquire DHA directly from diet or supplements increases.
Because of the decline in DHA synthesis, it is not surprising that DHA content of brain cell membranes declines. DHA is also reduced when the brains of rats are experimentally exposed to high oxygen levels. Free-radical oxidation probably causes the depletion in both cases. Vitamin E treatment protected the rats from neuron damage from the oxygen. This suggests that Vitamin E may be important for prevention of neurodegeneration in humans.
The greatest dependence on dietary DHA occurs in the foetus during the last third of pregnancy and (to a lesser extent) in the infant during the first 3 months after birth. It is during this period that brain synapses are forming most rapidly, and an infant's demand for DHA exceeds the capacity of the enzymes to synthesize it. The additional requirements are fulfilled by mechanisms believed to concentrate DHA absorption from the mother's placenta.
After birth, the additional needed DHA comes from the nursing mother. Rapid brain growth in the human infant requires large amounts of omega-3 and omega-6 essential fatty acids. Human milk contains (in total fatty acids by weight) 12% linoleic acid, 0.5% alpha-linolenic acid, 0.6% arachidonic acid and 0.3% DHA. Infant formulas frequently have not contained arachidonic acid or DHA. One study showed that by (or just before) age 8, children who had been breast-fed as infants had an 8.3-point IQ advantage over children who had received formula. The study corrected for the education and social class of the mother.
Further support for the idea that DHA is critical for brain development came from an experiment which studied the effects of adding DHA (in the form of fish oil) to infant formula. At both 16 and 30 weeks of age the breast-fed and supplement-formula-fed infants showed significantly better visual acuity than the placebo-formula-fed infants. Arachidonic acid supplementation is also needed because DHA supplementation given alone lowers arachidonic acid levels and because arachidonic acid is essential for growth. Deficiency of arachidonic acid during brain development is less reversible than deficiency of DHA. Recent reviews have firmly recommended the inclusion of both arachidonic acid and DHA in the formula of premature babies .
Even in the best formulations the efficiency of DHA and arachidonic acid absorption by an infant is inferior to what is seen for breast milk. Therefore, the best way to ensure adequate DHA and arachidonic acid would be for a pregnant/nursing mother to take a DHA supplement. The content of DHA and EPA in human milk has been increased experimentally by giving fish oil supplements to lactating women. The diet of the mother may contain enough omega-6 fat to allow her to synthesize sufficient arachidonic acid. DHA supplementation would be particularly important for mothers who have consumed excessive alcohol, because alcohol inhibits the desaturase enzymes necessary for DHA synthesis.
Arachidonic acid is similar to glutamate (glutamic acid) in that it can be harmful in conditions of restricted blood circulation (ischemia), but it is essential for normal brain function. It is the EPA (not DHA) in fish oil that can reduce arachidonic acid synthesis. Where pure DHA, rather than fish oil, has been used in infant formulas, inhibition of growth has been much less. The best infant formula should contain both DHA and arachidonic acid, however, because arachidonic acid improves growth.
An experiment studying maze-learning in rats demonstrated that, after training, the rats showed less cholesterol and more membrane fluidity in the hippocampal and cortical regions of the brain . Adult mice fed fish oil for 12 months showed more brain DHA, less brain arachidonic acid, more synaptic membrane fluidity and higher maze-learning ability.
Fatty acid in phosphatidylethanolamine of human gray matter cell membrane is roughly 25% DHA, 25% stearic acid, 14% arachidonic and 12% oleic acid. In the outer segments of retina photo-receptors of the eye more than 50% of the fatty acid content is DHA. It is DHA's special properties of permeability and perhaps fluidity that probably accounts for this high concentration.
Epidemiological studies have shown that consumption of DHA is associated with reduced risk of Alzheimer's Disease [ARCHIVES OF NEUROLOGY 60:940-946 (2003)]. DHA has also been shown to induce a 10-fold increase in transcription of the amyloid-©?scavenger transthyretin [PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES (USA) 100(4):1580-1585 (2003)] -- which may explain the part of the protective effect of DHA.
In contemporary diets, omega-6 fatty acids typically exceed all omega-3s (alpha-linolenic, EPA or DHA) by 6 or 7 times. A study on guinea pigs showed that both insufficient and excessive dietary DHA resulted in less than optimal visual acuity. The problems with excessive DHA were attributed to oxidative damage. This result is not too surprising because DHA, with six double-bonds, is the most highly unsaturated fatty acid found in significant quantities in the human body. The researchers did not include Vitamin E in their experiment, which is unfortunate because Vitamin E would be expected to reduce lipid peroxidation. Nonetheless, another study has shown that DHA reduces lipid peroxidation in the cerebral cortex.
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