DHA (or docosahexaenoic acid) is an omega 3 fatty acid which is found in large quantities in the brain and retina. Theories suggest that high levels of DHA prevent or slow neuronal (brain cell) death; along those same lines, some studies have shown low levels of DHA lead to or correlate with decline of mental functioning. Thus, theoretically supplementing your diet with DHA may protect neurons, slow the rate of neuronal cell death and slow cognitive decline. Of note, a National Institute of Health study in 2007 did not show a benefical effect of DHA in patients who were already diagnosed with Alzheimer's disease. The take home message is supplementation of DHA may prevent or slow the onset of Alzheimer's disease however is not helpful once you already have the disease.
More Answers from Darshan Shah, MD