Is eating fish healthy?

We know of fish as a healthy food, but pregnant women are told to limit consumption. Do the health benefits of eating fish outweigh the risks, particularly as stocks grow more depleted?

In recent decades, one of the biggest concerns about fish has been its potentially harmful levels of pollutants and metals.

One concern is polychlorinated biphenyls (PCBs). Although they were banned by the 1980s, these industrial chemicals were used worldwide in huge quantities and still linger in our soil and our water. They’ve been associated with a range of negative health effects on everything from the immune system to the brain. While PCBs are present in everything from dairy products to drinking water, the highest levels tend to be found in fish.

The solution for limiting your intake of PCBs from fish may be counterintuitive, says Johnathan Napier, science director at Rothamsted Research in Hertfordshire, England.

“The possible problem of the accumulation of toxic compounds is likely to be more of concern for wild species that are caught for direct human consumption,” he says. Because the marine-derived ingredients that farmed fish are fed are cleaned or scrubbed to remove toxins, farmed fish is often safer than wild.

That isn't always the case, however, and PCB content also fluctuates seasonally.

While they are generally viewed as better for our health and the environment, large-scale aquaculture has its own problems, such as polluting the oceans with waste and becoming breeding grounds for diseases that can spill over into the wild.

Another worry is mercury, a neurotoxin that could pass through the placenta and affect child development. There are numerous links between mercury ingestion and cancer, diabetes and heart disease. While mercury can be found in other foods, such as vegetables, one study found that 78% of participants' mercury intake came from fish and seafood.

In fish, mercury levels are high enough for the US Food and Drug Administration (FDA) to recommend that pregnant people limit their intake of some popular fish, including halibut and tuna, to one serving a week.

But concerns around the accumulation of heavy metals in fish has been overexaggerated, says Napier. He says it’s only a problem when it comes to species that live a particularly long time – like swordfish, which can live for 15 to 20 years. Swordfish has a mercury concentration of 0.995 PPM, while salmon, which lives on average for four to five years, has around 0.014. While research is still ongoing, the US's Environmental Protection Agency currently states that for pregnant women, the highest allowable average mercury concentration per serving, if eating one serving a week, is 0.46 PPM.

But the issue is set to worsen, as there’s evidence to suggest that levels of mercury found in the ocean may rise as the planet warms. Research has found that as Arctic permafrost melts, it releases mercury that was trapped in frozen ground into waterways.

While mercury poses a small risk, Napier says there stands to be much more to gain from fish – particularly marine omega 3.

Consumption of oily fish, including salmon, tuna, sardines and mackerel, has been linked to a lower risk of cardiovascular disease, thanks to its marine omega 3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA).

“Both EPA and DHA play a plethora of important roles in human metabolism, but we can’t make them very effectively in our bodies, so it’s really important to have them as part of our diet,” Napier says.

“Population data looking at the effects of marine omega 3 on health is consistent and strong, and shows that people with a higher intake of EPA and DHA have a lower risk of developing common diseases, particularly heart disease, and dying from them,” says Philip Calder, head of human development and health at England’s University of Southampton.

One way to avoid potential damage from mercury exposure while still getting omega 3 is to take fish oil supplements. However, research recently carried out on behalf of the World Health Organization (WHO) looking at omega-3 supplements across range of health outcomes found they don’t have the same effect as eating oily fish.

“Our findings suggest a very small beneficial effect [in terms of lowering the risk] of dying of coronary heart disease,” adds Lee Hooper, a reader at the University East Anglia and one of the WHO study’s researchers.  

Around 334 people would have to take omega-3 supplements for four or five years for one person not to die from coronary heart disease, she says.

But there’s an issue with population studies like Hooper’s. While some oily fish, such as sardines, aren’t relatively expensive, fish is generally associated with a more expensive diet. It’s widely accepted that socioeconomic status affects health outcomes – so it’s possible that families who eat more fish also have higher incomes and healthier lifestyles in general.

Normally, researchers will take into account such confounding factors, Calder says, but they might not think of everything that could skew a study’s results. The WHO report was a review of 79 studies, which each will have differed in how they controlled for participants' socioeconomic status.

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.

“There are finite levels of omega fish oils that come out of the ocean each year – that’s all we’ve got,” he says. “If aquaculture is expanding but the most important input you need to put into people’s diets, the fish oil, is completely static, you’re diluting how much is fed to the fish.”

Research from 2016 found that levels of EPA and DHA in farmed salmon decreased by half over a decade. Even so farmed salmon still has more omega-3 than wild salmon, Napier says.

“Wild salmon swims back forth across the Atlantic; it’s a lean animal. It’s not laying down fat because it’s burning everything it consumes,” he says.

Still, because there isn’t robust research suggesting major health inadequacies for people who don’t eat fish, Calder says it’s difficult to definitively say that fish is essential to overall human health. However, he adds, it is clear that omega-3 promotes health and reduces the risk of disease.

But getting to the bottom of how healthy fish really is may be a moot point after long. “Since fish isn’t a sustainable food source, research now will probably focus on solutions to that – such as how to grow algae and harvest omega-3 oil, instead of more studies into fish,” Calder says.

Individuals can help by choosing the most sustainable fish species available. Guides like the one by the Marine Conservation Society show which fish are the best, with 50 of the 133 species listed coming up as mostly sustainable, “good” choices – including, fortunately, popular favourites like farmed salmon, prawns, cod, mackerel, mussels, oysters and farmed halibut.

https://www.bbc.com/future/article/20201211-is-eating-fish-healthy