DHA is an omega-3 fatty acid found in fish oil and cold-water fish like wild salmon and sardines.

It is also commercially manufactured from microalgae.

Your body can manufacture small amounts the consumption of alpha-linolenic acid (ALA), an omega-3 fatty acid found in chia, flax, and many other seeds and nuts. But that conversion is very inefficient. In fact, only 8% of ALA is converted to EPA in men (with 0-4% converting to DHA) and 21% of ALA is converted to EPA in women (with 9% converting to DHA). Also, the older we get, the less we convert. To supply your body with the DHA it needs, you must eat cold-water fish and supplement, ideally.

DHA is metabolized to form the docosanoids—several families of potent hormones. It is a major fatty acid in sperm and brain phospholipids, and especially in the retina. DHA is the most abundant essential fatty acid (polyunsaturated fatty acids, PUFAs) in the brain and retina. It comprises 40% of the PUFAs in the brain and 60% of the PUFAs in the retina. 50% of the weight of the neuron's plasma membrane is composed of DHA. Of all the fatty acids, DHA has the largest effect on brain PUFA composition.

Studies have shown the numerous health benefits of DHA including the potential to reduce the risk of heart disease and maintain levels of serotonin (the "feel good" neurotransmitter) in the brain. While low levels have been associated with ADHD, Alzheimer's disease, and depression, among other diseases.

References

Cambridge Applied Polymers Ltd. (4 April 2006). Latest Publications on the Link between DHA and Intelligence / The Brain.. Retrieved on April 4, 2006.