DHA (docosahexaenoic acid) is one of the two important fatty acids in fish oils (the other is EPA, eicosapentaenoic acid). We all need these nutrients in our diet for long-term physical and mental health and, as recent research confirms, to reduce our risks of inflammation, heart disease, cancer, and depression. If you're healthy, I recommend getting them by eating at least three servings per week of oily fish (wild Alaskan salmon, mackerel, sardines, or herring).
If you do take supplements, fish oil is a better source of DHA than algae because it contains these fatty acids in the form that the body requires. The only instance of which I'm aware that algae is regarded as a superior source of DHA is for supplementation of infant formula.
DHA and another fatty acid, AA (arachidonic acid) are found naturally in breast milk, but if mothers can't nurse, their babies are deprived of these nutrients because infants, especially those born prematurely, can't manufacture enough in their own bodies for optimal brain development. A number of studies have shown that formulas supplemented with DHA and AA enhance visual acuity in babies; intellectual development appears to be better, too.
Fish oil can't be used to supplement infant formula because it contains other fats that slow babies' growth, but in 2001 the FDA approved adding DHA and AA from algae and fungal sources to formulas. DHA and AA- supplemented infant formulas have been sold in Europe and Asia for years.
Taking fish oil supplements usually doesn't cause side effects but can sometimes cause belching, flatulence, nausea, and diarrhea. Very high doses may result in a slightly fishy body odor. I've also heard that taking the capsules can result in a fishy odor on the breath, but I don't think this is common.
Andrew Weil, M.D.