Unfortunately, seeds and nuts only provide one type of omega-3 fatty acid: Alpha-linolenic acid (or ALA) These fatty acids are much (much) less useful than the DHA and EPA fatty acids that come from algae.
In fact, studies show that ALA fatty acids have to be converted into EPA or DHA before your body can use them. And that’s a big problem, because experts estimate that just 5-20% of dietary ALA ends up being successfully turned into longer EPA chains (1, 2) and it’s even more bleak when we consider ALA to DHA conversion.
One study, conducted in 2002, found that just 0.5-4% of ALA could be successfully turned into DHA fatty acids (3), which means that people who prioritise ALA Omega-3s are highly unlikely to be getting enough DHA fatty acids to support good health.
Taking the example above, we can see that a tablespoon of flaxseed contains a total of 2.4 grams of Omega-3 ALA, which sounds like a lot until you consider the conversion rate. Those 2.4 grams of ALA will get converted to 120 - 480 mg of EPA and between 12 - 96 mg of DHA.
This means that if you rely on nuts and seeds for your Omega-3, you may not be getting as much as you think. You should be able to manage getting enough EPA, but you're highly unlikely to be getting enough DHA.
It’s also important to note that the ALA found in seeds, beans and nuts is often bound up with insoluble dietary fibre, which means that it’s very difficult for your body to digest.
SUMMARY: ALA is the most common plant-based Omega-3 and is what is usually recommended for those who don't want to eat fish. However it's a poor source of Omega-3, as the body needs to convert it into DHA and EPA before it’s actually used. The conversion process is very inefficient in humans, so only 0.5 - 4% actually gets converted into DHA. This means that if you rely on nuts and seeds for Omega-3, you're not getting anywhere near as much as you might think.