DHA doesn't stop Alzheimer's, but it seems to help age-related memory loss, and it it also seems to slow the reproduction of colon cancer cells. And in some cases it seems to ease depression.
You can also get DHA by eating oily, cold-water fish, such as herring, mackerel, sardines, or wild Alaskan salmon. Three 3-1/2 oz (100 g) servings a week is enough. Grass-fed beef and free range chicken and eggs contain DHA, although in much small amounts.
DHA for Infant Formula
There is a product known as DHASCO, or DHA single cell oil, made from algae grown in specialized incubators. DHASCO is added to infant formula.
The only time DHASCO is better than the EPA/DHA complex found in fish oil is when the use is in infant formula.
For other health applications, either an EPA/DHA supplement or even ALA found in flaxseed and other plant oils is actually superior.
Infant formulas supplemented with DHASCO and ALA have been sold in Europe and Japan for 30 years. In the USA, however, babies are usually given an ALA-only supplement. Girls get slightly better nutrition from this formula than boys. Infants of both sexes should be breastfed whenever possible.
DHA for Adults
If you are an adult in good health, you can benefit from either a DHA/EPA supplement such as fish oil, or from an ALA supplement such as flaxseed oil, borage seed oil, or evening primrose oil.
All omega-3 fatty acid supplements can occasionally cause burping, acid reflux, or indigestion, if taken in excess, but this is only common with cheaper fish oil products that have not been molecularly distilled.