The modern American diet is typically low in omega-3 and high in omega-6 fatty acids (another form of polyunsaturated fatty acid). Too much omega-6 causes inflammation, but interestingly enough, too much omega-3 can also be bad; it's all about keeping the ratio of omega-3:omega-6 in check.
A well-balanced diet with natural sources of ALA, DHA, and EPA is fundamental to maintaining a healthy ratio that prevents inflammation and promotes long-term health.
ALA comes from plants and is the most largely consumed form of omega-3 in the typical American diet. It serves as an energy source for our cells, and a small percent is converted into DHA and EPA. Dietary sources of ALA include flaxseeds and flax oil, walnuts and walnut oil, chia seeds, pumpkin seeds, hemp seeds and hemp oil, mustard oil, most leafy green vegetables, and tofu. Some research is being done on algal oil, as it may be more bioavailable, meaning it will be easier to absorb and converts easily to DHA in the body. One study found micro-algae were able to increase blood erythrocyte and plasma DHA, a marker of absorption. For vegans and vegetarians, this may be a promising alternative to fish oils. Some supplement brands have already started creating vegetarian EPA/DHA supplements from algal oil for those who prefer not to take fish oils.
DHA and EPA are found in fatty, cold-water fish, such as salmon, trout, cod liver, herring, mackerel, and sardines; shellfish such as shrimp, oysters, clams, and scallops. These sources of omega-3 are most bioavailable. However, it's important to purchase wild caught fish and check out seafoodwatch.org to see which fish are lowest in mercury and other environmental toxins. A balance of sources from plants and seafood will help you find the right balance of these essential fatty acids.
When diets don't meet the mark, supplementation may be recommended but should always be done under the supervision of a doctor and dietitian for these reasons: