Omega-3 and omega-6 fatty acids are categorized as polyunsaturated fatty acids (PUFAs) and are nutrients essential to human health: The two play a critical role in brain and heart function as well as growth and development. (And just what are fatty acids? Long story short: They are the building blocks of fats.) Because our bodies can’t produce either omega-3s or omega-6s, we must get them from food.

Omega-3 fatty acids are associated with a number of health benefits, including protection against heart disease, possibly stroke, and autoimmune diseases such as rheumatoid arthritis. There are two major types of omega-3s in our diets. One is alpha-linolenic acid (ALA), which is found in plants, especially leafy green vegetables. Flaxseed, walnuts, and their respective oils are high in ALA, as is an oil everyone loves to hate, canola.

The other type of omega-3s—eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)—is found in oily fish such as salmon (particularly wild), Atlantic and Pacific mackerel, sardines, anchovies, and herring, as well as in oysters and some aquatic plants. The human body partially converts ALA to EPA and DHA. (Fun fact: Life’s fastest tasks require DHA. Ours is concentrated in the brain and eyes, followed by sperm, then heart muscle.)

Health experts encourage increased omega-3 consumption (in place of, not in addition to, saturated fats and trans fats) because of their deficiency in the diets of most Americans. The newest (2010) Dietary Guidelines recommend at least 250 milligrams per day. Many omega-3 specialists think the target should be higher, approximately 500 mg per day.

Of course, this summer’s news that the second large study by scientists at the Fred Hutchinson Cancer Research Center confirmed a link between high blood concentrations of omega-3 fatty acids and an increased risk of prostate cancer does give one pause. Omega-3 authority William Harris takes issue with their findings here. The one thing I’ll bet they all agree on is that nutrition research and the interpretation of data are very difficult.

Omega-6 fatty acids, primarily composed of linoleic acid (LA), are found in nuts, seeds, many vegetable oils (thus almost any processed food you can think of), and meat. Most people today consume far more omega-6s, which are linked to inflammation, than omega-3s, and many experts think this dietary imbalance may be the cause of many chronic inflammatory diseases and metabolic disorders. Science writer Susan Allport gives a compelling account of the science and politics behind this theory in The Queen of Fats (2006). And a fresh interpretation of clinical trial data first collected in the 1960s and published in the British Medical Journal in February 2013 suggests that omega-6s may be linked with a higher risk of death among patients with heart disease.