In 1997, the American Plastics Council ran a New Yorker ad calling plastics “an important part of a healthy diet.”

The ad, which trumpeted plastic juice jugs and plastic-wrapped food as the “sixth basic food group,” wound up proving far more literal than the copywriters intended.

Nowadays, nearly all of our meals come seasoned with a flurry of tiny plastic particles and fibers. Scientists don’t know exactly how much we consume, but estimates range from five grams a week — the weight of a credit card — down to a minute fraction of that.

Weight, however, isn’t all that matters. Exceptionally tiny fragments, as small or even smaller than a single bacterium, can penetrate human cells and pass from the gut to the bloodstream.

What this “sixth basic food group” is doing to our digestive health is an open question — one that recent studies attempt to answer. Their findings, while preliminary, show microplastics can alter the permeability of the intestine and change how we digest food.

They also appear to shift the gut microbiome to an unhealthy state. This ecosystem of trillions of bacteria and other microbes helps us digest food and fend off infection and plays a major role in body processes from weight loss and immune function to heart and brain health.

“It’s important to highlight that microplastics are already in our bodies,” Mathilde Body-Malapel, an environmental health researcher at the University of Lille, in France, told Environmental Health News (EHN). “For most people, microplastic is a problem for the ocean, it’s an ecological problem. But now we know it’s a problem for human health.”

How much plastic do we eat?

Microplastics are smaller than five millimeters, about the size of a sesame seed. Researchers have found these persistent polymers virtually everywhere they’ve looked: in rice, sugar, seafood, vegetables, drinking water, rain and air.

Polyester clothes and rugs shed minute plastic fibers. Opening a soda bottle can release a spray of thousands of plastic particles.

Despite their ubiquity in our diet, little is known about how these elements affect their first stop in the human body — the digestive tract.

Microplastics have only become a research concern in the last decade or so, and the field is under-financed, particularly in the U.S., Philip Demokritou, the director of two environmental health labs at the Harvard School of Public Health, told EHN.

Microplastics are also notoriously difficult to study. “For microplastics, it’s not one pollutant,” Body-Malapel said. “It’s a lot of different pollutants.”

In addition to the wide range of plastic polymers in use — the polyethylene in a plastic bottle, the vinyl in a shower curtain — there’s an even wider range of chemicals added to plastics to give them special properties such as flexibility or ultraviolet resistance.

Evidence has shown that many of these additives, such as bisphenol-A and phthalates, pose health risks even at extremely low levels, such as parts per trillion.

Figuring out what kind of plastic a person or animal has consumed is “something that always sounds easier than how it is in reality,” Gloria Fackelmann, a microbial ecologist at Italy’s University of Trento, told EHN.

Scientists use specialized devices to analyze the wavelengths of light that bounce off a given piece of plastic and compare it to databases created by other researchers. This process is imperfect, Fackelmann said, because something as basic as the plastic’s age can alter how it reflects light.

Adding to the complexity are shape and size. A grain of plastic visible to the naked eye behaves far differently from a nanoscale particle that’s the size of a virus. Multiple studies have shown that the smallest plastics can pass from the gut into the blood, and from there into the brain, placenta, liver and other organs.

And the smaller something is, the more surface area it has, increasing a plastic particle’s ability to glom onto cells and leach a higher dose of chemical additives.

Body-Malapel said nanoplastics are so miniscule that there is currently no way to detect them in human tissues.

Researchers can’t feed human volunteers plastic-peppered food and see what happens.

Instead, they must pursue a variety of indirect routes: analyzing human feces, creating artificial stand-ins for digestive organs, using lab animals, or looking at wild creatures in plastic-polluted environments.

Plastic in birds

In her most recent study, published in March in Nature Ecology & Evolution, Fackelmann and her collaborators looked at how microplastics affected the gut microbiomes of two seabird species, Northern fulmars and Cory’s shearwaters.

The team measured bacteria in the birds’ stomachs and cloacas and flushed out their digestive tracts to count and weigh the microplastics inside.

The birds with more individual pieces of plastic had a higher diversity of bacteria, while those with a greater weight of plastic had lower diversity.

Greater diversity isn’t always beneficial, Fackelmann said. “Maybe you have more microbes, but the excess are all pathogens, in which case, that wouldn’t be good.”

This finding “lends some credence to the idea of plastics as a vector” for bacteria, she added. Bits of seafaring plastic are thought to accumulate bacteria, which then make a home in a bird’s stomach and intestines.

As the researchers discovered, these bacteria weren’t a representative sample of what’s normally found in seawater. Instead, there was a preponderance of plastic-eating microbes, along with antibiotic-resistant and disease-causing bacteria.

While the team didn’t assess the seabirds’ health, the microbial shift suggests this petrochemical diet is doing the birds no favors.

“The very fact that we were able to measure the effects of microplastics on the gut microbiome in this natural system, using just naturally found concentrations of microplastics,” Fackelmann said, is “striking.”

She added: “I wouldn’t be surprised if the situation were to be similar for humans.”

Artificial organs, real problems

In France, a team of researchers took a radically different approach. Unable to directly observe microplastics in a living person’s gut, they devised a fermentation chamber to mimic the conditions in the large intestine.

The team seeded the fermentation chamber with donated poop and fed the resulting bacterial ecosystem with nutrients and vitamins. “It does smell a bit,” study author Lucie Etienne-Mesmin, a microbiologist at Université Clermont Auvergne, told EHN. “After a while you get used to it.”

Every day for two weeks the team dosed the system with tiny beads of polystyrene — the most common type of plastic — and measured changes to the artificial gut.

Per the study, published in January in the Journal of Hazardous Materials, beneficial bacteria declined, while two disease-associated strains increased, as did the production of skatole, the substance that gives feces their nose-wrinkling odor.

Notably, the study’s authors say, these changes mirror what medical researchers see in inflammatory bowel diseases such as Crohn’s and celiac, where the body’s immune system mistakenly attacks healthy gut cells.

Many factors are likely to blame for the global rise in bowel diseases, from air pollution and ultra-processed food to antibiotic overuse.

However, this study, and several others like it, suggest that microplastics may also have a role.

A 2021 study from China’s Nanjing University, for example, found a correlation between the severity of a patient’s inflammatory bowel disease and the number of plastics in their feces.

A Tufts study published this June, which utilized 3D intestinal cell cultures called organoids, found that plastic exposure prompted cells to secrete inflammatory molecules.

The French researchers cautioned that it was only a preliminary study, meant to guide more research. Still, Etienne-Mesmin said, the results warrant concern, particularly because harmful changes were seen after only two weeks.

“In real life, you are exposed on a daily basis,” she said. “So maybe the effect will be even stronger than the one we observed.”

Plastics and obesity

The use of pristine plastic beads, such as those used in the French study, is a “good starting point,” Demokritou said, but he cautions that their utility is limited, particularly from the perspective of regulators and health agencies.

“We need to use more environmentally relevant microplastics, the ones that you and I consume in our food and the water we drink,” he said. “Otherwise, it’s a dangerous thing to generate data that may or may not be relevant to human health.”

Demokritou and his colleagues are developing ways to shorten 40 or 50 years of plastic degradation down to a few weeks through incineration and other methods.

Evidence suggests that this process, called weathering, induces physical and chemical changes that can make plastic more dangerous.

An August study from South Korea’s Daegu Gyeongbuk Institute of Science and Technology, for example, found that weathered microplastics induced noticeably more brain inflammation than their pristine counterparts.

In a study published this March, Demokritou’s team applied a slurry of weathered nanoplastics to a cell culture of a human intestinal wall.

The team then “fed” the cells a high-fat solution and discovered that the microscopic plastics increased fat digestion by 33% and fat absorption by a whopping 145%.

Plastics and intestinal membranes are both hydrophobic, meaning they repel water, Demokritou said.