It's a well-known fact that some of our favourite seafoods come with an unsavoury dose of heavy metals like mercury. But there's another group of chemicals that sometimes lace our tuna steaks, and the latest findings on them are anything but appetising. They're called persistent organic pollutants (POPs), and they do a bang-up job obstructing the human body's chemical defence system. That's according to an analysis published today in Science Advances, examining how ten POPs found in both tuna and human bodily fluids interact with a gatekeeper protein that shuttles unwanted toxins out of our cells. The findings suggest that ingesting even very small quantities of POPs can do bodily harm.
"These chemicals interfere with our chemical defence system's ability to operate, which can make us more vulnerable to drugs and other toxins," Amro Hamdoun, a biologist at the Scripps Institute of Oceanography and lead author on the study, told Gizmodo.
POPs have been around for a long, long time. One of the most infamous, the pesticide DDT, was developed during World War II. DDT was banned in the 1970s, but other POPs, including flame retardants and non-stick coatings, are still widely used today. While these chemicals are damn useful in many applications, they're a toxicologist's nightmare. They degrade very slowly in the environment, they accumulate in biological tissues and they have been been linked to everything from impaired neural development to obesity.
Hamdoun's study sheds new light on POP toxicity by demonstrating, for the first time, exactly how these chemicals shut down a key cellular defence system. Normally, foreign molecules are kicked out of our cells by a special transporter known as P-glycoprotein (Pg-p). "Pg-p is a bit like a bouncer," Hamdoun said. "Initially, we thought that these chemicals sneak past the bouncer. But what we found is that they actually form very intimate interactions with Pg-p."
Laboratory tests revealed that ten of the most widespread POPs all bind tightly to this protective protein, inhibiting its ability to do its job. Even at levels where POPs themselves pose no hazard, "they're interfering with the very systems and pathways that have evolved to protect us," Amro said.
Levels of POPs in the environment are on the decline. But the ocean is a big place, and industry leaves a long legacy. "The bottom line is, there are still reservoirs of these in the ocean," Amro said. "And we are still being exposed to them through the consumption of seafood."
Which types of seafood and which part of the ocean are most contaminated is something Amro and his colleagues are still figuring out. But we know enough to start taking some common-sense precautions, including routinely testing seafood and tracking potentially contaminated stocks.
"Tuna are in the upper five per cent of highly contaminated fish," Amro said. "The good news is, we can reduce exposure by monitoring where they come from. The bad news is, we don't do that yet."
Seafood is typically a very healthy source of protein, and only a small fraction of it is likely to contain appreciable levels of flame retardants and pesticides. But food distributors and regulators owe it to consumers to figure out where that fraction comes from.
"These compounds persist in the environment for a long, long time," Hamdoun said. "We still see fish that have levels as high as they were 30 to 40 years ago. We need to find those fish, find those food products, and reduce or eliminate them in the food supply."