You can find statements like the following in a hundred places:
Both fish and flax are good sources of omega-3. Flaxseed oil is less expensive, which can be an important consideration. The main difference is that flaxseed oil contains only alpha-linoleic acid (ALA), which is the parent compound from which other omega-3 fatty acids are derived. This leaves it to your body to do the conversion to the other forms it needs, eicosapentaonoic acid (EPA) and docosahexaenoic acid (DHA). The problem is that the conversion is not always that efficient, and the body often uses the ALA for extra energy, leaving less for conversion to the other types. The body uses various enzymes to convert ALA to other omega-3s, and the process is not very efficient, especially as one gets older. Estimates of the rate of conversion range from 5% to 25%. In order to make sufficient amounts of EPA and DHA, one needs to consume 5-6 times more ALA than if one relies on fish oil alone. Fish oil, on the other hand, contains the other forms and delivers them directly to your body with no conversion necessary.
It isn’t that simple. Here are three things I rarely see mentioned:
1. The conversion rate is not fixed. It depends on the amount of the conversion enzyme, which increases with ALA exposure. The body makes the enzymes it needs and doesn’t make the enzymes it doesn’t need. You don’t have enzymes to digest food you don’t eat but if you start to eat them you will build up the enzymes. The measurements of ALA conversion rate I have seen measured the conversion rate without giving the subjects extensive exposure to ALA before the test. (Measurements of glycemic index have the same problem.) They should be considered lower bounds of what would happen with long exposure.
2. Time release is good not bad. When you take a dose of flax oil, its ALA is converted to DHA and EPA, which takes time, thus smoothing out the supply versus time function. If you take a dose of fish oil, the brain receives a sudden burst of DHA and EPA. It is likely that a smoother supply is better.
3. ALA (omega-3) conversion blocks AA (omega-6) conversion. Omega-3 fatty acids and omega-6 fatty acids are almost identical. The enzyme that converts short-chain omega-3 to long-chain omega-3 also converts short-chain omega-6 to long-chain omega-6. Long-chain omega-6 probably has bad effects in your brain, at least in large amounts, because it displaces long-chain omega-3. For industrial reasons, our diets are high in short-chain omega-6. Having ALA in your system, which flax oil provides but fish oil does not, slows down the conversion of short-chain omega-6 to long-chain omega-6 by occupying the conversion enzymes.