There are different types of omega-3 fatty acids, but the two most beneficial are the long-chain eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). The human marine sources – oily, cold-water fish like tuna, salmon, and mackerel – contain both EPA and DHA, which is why they’re preferable.
The human body isn’t great at converting short-chain fatty acids to long-chain fatty acids, so it’s best to ingest them directly. Try eating oily fish three times a week or supplementing with fish oil.
Some eggs are advertised as being rich in omega-3; that’s because chickens are pretty good at converting short-chain omega-3 to long-chain. Farmers feed their chickens with flaxseed, which they turn into long-chain fatty-acids – and therefore their eggs are good sources of omega-3. Just check the labelling on the carton (and be sure to buy free range)
Because B12 is primarily found in animal sources, it’s generally recommended to take a supplement if you’re following a vegan diet. Be sure to consult with your doctor if you have any concerns about your nutrient intake.