Online Reporter
LONDON — 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?
Fish has a reputation for being one of the healthiest foods we can eat.
But the rising availability of plant-based alternatives, and increasing concerns about seafood’s sustainability and carbon footprint, have led some to question whether we need it in our diets.
Since 1974, the UN’s Food and Agricultural Organization reports, fish stocks within biologically sustainable levels have declined from 90 percent to just under 66 percent today. Meanwhile, concerns over mercury and other pollutants mean women who are pregnant or breastfeeding, for example, are told to limit their consumption of some species.
So does eating fish provide more health benefits — or health risks?
Are fish high in heavy metals?
In recent decades, one of the biggest concerns about fish has been its potentially harmful levels of pollutants and metals.
One concern is polycholorinated biphenlys (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.
But 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.
In 2023, an expert panel of scientists concluded that evidence relating to the health effects of PCB exposure from eating in fish is lacking, but there is consistent evidence for an association between exposure to dioxins — another environmental pollutant found in fish — and reduced semen quality.
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.
The NHS recommends that pregnant and breastfeeding women limit their intake of fish species more likely to contain PCBs, as well as other pollutants like dioxins, to two portions per week.
These fish include oily fish like salmon and sardines, as well as non-oily fish including crab and sea bass.
Another worry is mercury, a neurotoxin that has been thought to pass through the placenta and affect child development.
However, a 2022 review of studies says the evidence base showing this link is weak.
There are also 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 have 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.
Fish and omega 3 fatty acids
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).
Some plant-based sources of omega 3, such as flax seeds and walnuts, are rich in a third type – ALA. A 2022 study concluded that the heart health benefits of plant-based omega 3 may be comparable to EPA and DHA, but there needs to be more evidence before this can inform dietary recommendations, researchers say.
However, you can find both EPA and DHA in algae supplements and in edible seaweed.
DHA is abundant in our brains, retinas and other specialised tissues.
Along with EPA, it helps to fight off inflammation in the body, which is linked to higher risk of heart disease, cancer and diabetes.
“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 carried out in 2018 on behalf of the World Health Organisation (WHO) looking at omega-3 supplements across a range of health outcomes found they don’t have the same effect as eating oily fish.
On the other hand, a population study conducted two years later using data from more than 400,000 people found an association between habitual fish oil supplement use and lower risk of death from cardiovascular disease, and death from all causes.
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. — BBC
Mathew Masinge