Tuna are magnificent fish. They are large, stealth predators with powerful, stream-lined bodies that can out-perform even the most sophisticated machine (of comparable size) in strength, resilience, and vigor.
Unfortunately for them, they are also incredibly tasty and one of the most desirable ocean catches worldwide. Unfortunately for humans, they are not nearly as abundant as they once were, and nor are they as nutritious, because over-fishing threatens their survival and because they are becoming increasingly contaminated with pollutants such as heavy metals.
Heavy metals are so called because they literally are heavier than other elements (actually, the term “heavy metal” is rather ambiguous … there are other definitions of what constitutes a heavy metal, but this is the most common). Almost all heavy metals are toxic to humans. Some, such as iron, zinc, and copper, are essential for human health—but only in small quantities, otherwise they too can be toxic.
Others, such as mercury, cadmium, and lead, are toxic at any quantity. These heavy metals accumulate in tissues—especially fatty tissues including the brain—and poison the body. Our bodies lack the enzymes and cellular “machinery” needed to quickly remove these toxic metals from our system.
The result is a gradual buildup of heavy metals in our cells as we age. The health impacts can be profound, especially because heavy metal toxicity causes inflammation. Chronic inflammation is at the root of many common diseases.
Heavy metals also tend to accumulate in the tissues of most animals and plants (this is referred to as bioaccumulation). Higher concentrations of heavy metals are found in animals that:
-are top predators (and eat smaller, fatty animals)
-are large and fatty
-live in environments contaminated with heavy metals.
When we eat animals that are full of heavy metals, we ingest poison along with nutrients. The key to minimizing toxicity in your body is to be very selective of which animals you eat. It’s now common knowledge that food harvested from marine and freshwater sources is contaminated with pollutants such as heavy metals.
Enter tuna: a large, fatty, long-lived carnivorous fish that is the top or close-to-the-top predator in most marine food chains. With a ravenous appetite and an incredibly high metabolic rate that forces them to devour the largest and fattiest fish they find, plus an ocean environment that becomes more polluted each day, tuna accumulate more heavy metals than the majority of other fish species humans eat.
Numerous studies have measured pollutant levels in fish to determine the safest species to eat. Tuna has notoriously topped every list as one of the most toxic species, and heavy metal toxicity varies among tuna species.
A study published this month measured the concentrations of mercury, cadmium, and lead in 45 popular brands of canned tuna sold in Italy. The researchers also measured heavy metal concentrations in freshly caught tuna from the Tyrrhenian Sea (off the west coast of Italy). What they found wasn’t surprising.
Canned and freshly caught tuna were contaminated with all 3 heavy metals. The good news is that compared to the safe weekly intake set by the World Health Organization (WHO) for cadmium and lead, tuna contained an average of 0.40% of the safety levels of cadmium and 0.56% of the safety levels of lead.
The bad news is that 9% of the canned tuna samples and 20% of the freshly caught tuna samples had mercury levels exceeding the safety guidelines set by the European legislation.
This may not seem like a lot, but it’s difficult to know how much mercury is contained in the tuna you eat because several factors affect this. Tuna that are larger, older, living in polluted waters, and eating contaminated fish will contain higher levels of mercury. These are details that you rarely know about the fish you purchase.
Previous studies have suggested that canned tuna may contain lower levels of pollutants than fresh fillets because most small catches and smaller tuna species such as skipjack and albacore are canned whereas the larger fish such as bluefin tuna are sold fresh or frozen.
However, keep in mind that a small tuna is still a big fish—tuna average body lengths greater than 1 m and life spans of more than 10 years. What’s more, the names “albacore” and “skipjack” can refer to a number of different tuna species around the world, depending on whom you’re talking to.
There’s something else that’s very important and alarming. The WHO, setting a standard for the vast majority of countries, has set safety levels of mercury in fish at 0.50 mcg/g wet weight of fish. This means that only fish containing less than 0.5 micrograms of mercury per gram of uncooked meat are safe to eat. One exception is Japan, where any fish with more than 0.40 mcg of mercury/g wet weight of meat is not fit for human consumption.
However, the safety levels for predatory fish, such as tuna and sharks, are set at 1.0 mcg/g wet weight of fish. This is twice the amount of mercury that is allowable in non-predatory fish! This double standard has been set because few predatory fish contain less than 1.0 mcg/g wet weight, let alone less than 0.5 mcg/g wet weight.
BUT, mercury is toxic no matter where it comes from. If the WHO regards most fish to be unsafe when tissue levels exceed 0.50 mcg/g wet weight, then that same standard should be applied to predatory fish. Mercury is not less toxic to human health if it comes from predatory fish. That means if predatory fish rarely contain less than 0.5 mcg/g wet weight, they should not be eaten.
So what to do? If you eat fish, be very mindful of the species and sources you choose. Fortunately, the internet has become a sea of information to help you choose safe and sustainable fish.
WWF Seafood Guides, listed by country
CITES (Convention on International Trade in Endangered Species of Wild Fauna and Flora)