When it comes to looking for an EPA and DHA supplement, nothing comes close to cod liver oil. Cod liver oil supplements are the best source of EPA and DHA, and also contain vitamins A and D. Fish oils are comparable to cod liver oils, but they are open to abuse, and don’t usually contain vitamins A or D. The term ‘fish oil’ is extremely vague, and although there are some very good fish oil producers out there, there are also some more dubious ones. This makes the EPA and DHA content of fish oils quite variable. Cod liver oils are much more consistent, and are backed with thorough research. There is still quite some variation in the quality of cod liver oil supplements though, and you can read a detailed analysis here.
There are many many brands to look at when it comes to cod liver oil. My personal preferences are Dropi cod liver oil from Iceland, or Green Pastures fermented cod liver oil from America. I have spoken at length with both companies, and they produce a clean and nutritional product that I am confident in.
Other nutritional oils such as plant oils are poor sources of EPA and DHA. Flaxseed oil for example, is extremely high in the omega-3 fatty acid ALA, but contains no EPA or DHA. ALA can be converted into EPA and DHA though, which means it can still offer an indirect source of these fats. This is important for vegans/ vegetarians who may struggle to get enough omega-3 fats in their diet. However, this conversion is extremely inefficient, and is inhibited by omega-6 fatty acids which are abundant in modern diets. This makes plant based oils very poor source of these nutrients.
You can read more about how flaxseed oil compares to cod liver oils here if you wanted to know more about the two.