DHA is one of the omega-3 essential fatty acids, the good fats that can protect your eyes, brain and heart, and found in fish oil and algae.

Another important omega-3 essential fatty acid is ALA ( alpha-linolenic acid), in chia, flax and walnuts, which the body eventually, but in small quantities, converts to DHA.

A third essential fatty acid is EPA, also found in fish oil.

Walnuts and chia seeds are sources of ALA and do not contain any DHA. Nuts and seeds are not nutritional equivalents to fish oil or algae oil with respect to omega-3 content.

Humans have a limited ability to convert ALA to DHA: Less than 1 percent of it is turned into DHA (although women convert it better than men do). Western diets limit their ability to convert it even more because they eat too many omega-6 fats from corn and soybean oils (Americans average 18 grams a day of omega-6 fat from these oils).

The trouble is that these omega-6 fatty acids compete with ALA for conversion. So, for instance, if a person consumed an ounce of walnuts per day, they'd get approximately 2,500 mg of ALA. But it would, at best, convert to just 25 mg of DHA, a small fraction of the DHA necessary (600 mg) for optimal health.

ALA itself isn't bad: Some research shows that it can have positive effects on heart health (DHA does too). But for brain development and health, research has shown only DHA to be beneficial.

The only way to achieve the DHA blood and tissue levels essential for optimal health and disease risk reduction is to get at least 600 mg of preformed DHA per day through fish, foods fortified with DHA or DHA supplements from algae oil or fish oil.