Brainpower is driven by two types of cells, neurons and glia (connective branched cells). Neurons send and receive signals, while glia provide fuel and support. These cells are unique in having two highly unsaturated fatty acids, AA and DHA, abundant in their cell membranes. DHA is about twice as plentiful as AA, an indication of its importance in brain function. Both fatty acids participate in neuronal function, but their actions vary with each brain region. We are just beginning to unravel how these fatty acids work in the brain.
Why AA and DHA?: These fatty acids confer fluidity to brain cell membranes, a quality that facilitates the transfer of substances in and out of cells. AA and DHA provide a reservoir for membrane proteins to use. AA is the basis for making other potent substances, such as prostaglandins, that help regulate blood vessels in the brain. DHA participates in the relay of cell signals along the membrane. It, too, is used to make other substances, such as neuroprotectin, a molecule that protects brain cells against destruction and promotes cell survival.
The retina of the eye, another part of the nervous system, has the highest concentration of DHA anywhere in the body. The retina contains photoreceptors, specialized neurons that convert light energy to chemical signals, which are relayed to the brain for processing as visual images. DHA is key to cell signaling in the retina and optimum visual function.
Cell Talk: AA and DHA assist the communication between neurons. Substances called neurotransmitters, contained in nerve endings, are discharged from one neuron and picked up by its target cell. AA and DHA increase the number of packets of neurotransmitters and affect their release. They can also be released from the membrane to stimulate the activity of certain membrane proteins. In turn, these affect cell function. DHA appears to strengthen the communications between neurons.
Where the Brain Gets AA and DHA: In fetal life, the brain acquires these fatty acids by concentrating them from the mother’s circulation via the placenta. The fetus strongly prefers these fatty acids ready-made, rather than as precursors. The precursor of AA, linoleic acid, can be converted to AA fairly readily. In contrast, less than 5% of alpha-linolenic acid, the precursor of DHA, is converted to DHA. Further, high levels of polyunsaturated fatty acids, including alpha-linolenic acid, diminish the conversion. This means that simply consuming more alpha-linolenic acid is more likely to reduce conversion than increase it.
After birth, the infant obtains AA and DHA from its own body fat stores and from breast milk or from supplemented infant formula. The AA content of breast milk is fairly similar among women, but DHA content varies widely. The highest DHA is found in mothers who consume fish regularly. Mothers who do not eat any fish have much less DHA in their milk. The best way to boost the DHA content of breast milk is to eat fish or other sources of DHA regularly.
Standard infant formula does not contain either of these fatty acids. Now, most manufacturers of infant formula add AA and DHA to their formula and tout it on the label. It makes sense to use formula enriched with AA and DHA rather than unsupplemented formula.
When There is Too Little DHA: Studies in omega-3 deficient animals show that brain content is significantly lower if DHA is not part of the diet during pregnancy. DHA in the retina also becomes depleted, and if not restored, visual acuity-the ability to discern fine detail-is impaired. Other aspects of infant neurodevelopment, including learning, may also be delayed or suboptimal when DHA is inadequate.
When DHA is unavailable, the body replaces it with a substitute from the omega-6 class of fatty acids. This replacement fatty acid maintains cell membrane fluidity, but cannot replace all of the functions of DHA.
Several clinical conditions in people are associated with substantially lower levels of DHA and EPA and sometimes AA compared with healthy people. It has not been proven that reductions in these fatty acids are responsible for the symptoms, but replenishing them is associated with significant improvements in several instances. Examples of these conditions include depression, bipolar disorder, and attention deficit hyperactivity disorder. How fatty acids are linked to these conditions is described in Mental Health.