But intervention trials, where people are randomly assigned to a group and an intervention such as taking omega-3 supplements is measured, have problems, too. Analysing potential health impacts of EPA and DHA deficiency, for example, is difficult, Calder says, because people start trials with varying levels of omega-3 in their systems.
In addition, research shows that fish might impact everyone’s health to varying degrees, depending on how well they can convert precursor forms of EPA and DHA. This difference could come down to a person’s overall diet and lifestyle, Calder says, but genetic differences could also play a role.
Another reason the health benefits of fish may vary is because of how fish are raised.
Marine ecosystems are full of omega-3: little fish eat marine plankton, and get eaten by bigger fish, and the whole food chain passes on omega-3 to humans. But the system is different for farmed fish, which is what most of us eat. “In a fish farm, it’s just thousands of fish in a cage. They eat what they’re given by the fish farmer,” Napier says.
As they would in the wild, farmed fish normally are fed smaller fish species. In the wild, however, fish would eat a variety of smaller fish. In farms, fish are often fed fish meal made from Peruvian anchovies.
But these anchovies are already being fished at the maximum level at which the industry can be sustained, Napier says – even as global aquaculture is expected to keep growing. According to the UN’s Food and Agricultural Organization, growing demand for fish oil supplements means that the fish oil contained in the fish meal fed to farmed fish is diminishing. That means the amount of omega-3 in the fish we consume is declining, too.