There are three types of omega-3 fatty acids: alpha-linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA).

ALA is an 18-carbon long chain polyunsaturated fatty acid (SCPUFA), sourced primarily from plants. ALA provides a source of energy when metabolized, and because the body can’t make ALA, it’s considered an essential nutrient.

In humans, ALA undergoes an elongation step into the second form of omega-3, EPA. However, it is a poor source of EPA due to a low conversion efficiency of only about 5 to 10 percent.

EPA, meanwhile, is a 20-carbon long chain (LCPUFA) known for its biologic activity in humans. Supporting a variety of functions including brain health and heart health, EPA can also elongate into the third form of omega-3, known as DHA.

DHA is a 22-carbon long chain polyunsaturated fatty acid known as a physiologically-essential LCPUFA. While possible, conversion efficiency of ALA through to DHA is extremely low, typically occurring with an efficiency of just 0.2 to 2 percent.

Therefore, the best way to incorporate these essential nutrients into your diet is to eat fish rich in omega-3s. However, not all fatty fish are created equally. SMASH fish — that is, salmon, mackerel, anchovies, sardines, and herring — are not only packed with omega-3s, they’re also safer to eat thanks to low amounts of mercury.

Comparatively, high-mercury fish such as tuna, swordfish, orange roughy, and shark are typically larger in size because they  tend to live longer — two factors that increase their exposure to mercury.

While good sources of omega-3s, these fish should be eaten in moderation (or not at all) due to their mercury levels.

Throughout history, humans consumed diets heavy in fish. Today, fish consumption is low. Thanks to western diets, these essential nutrients have become a nutritional deficit for many. Thus, a dietary solution is needed.