In the U.S., people eat more protein than they need to. And though it might not be bad for human health, this excess does pose a problem for the country’s waterways. The nation’s wastewater is laden with the leftovers from protein digestion: nitrogen compounds that can feed toxic algal blooms and pollute the air and drinking water. This source of nitrogen pollution even rivals that from fertilizers washed off of fields growing food crops, new research suggests.

When we overconsume protein—whether it comes from lentils, supplements or steak—our body breaks the excess down into urea, a nitrogen-containing compound that exits the body via urine and ultimately ends up in sewage. Maya Almaraz, a biogeochemist at the University of California, Davis, and her colleagues wanted to see how much of this nitrogen is being flushed into the U.S. sewage system because of a protein-heavy diet. The researchers combined population data and previous work on how much excess protein the average American eats and found that the majority of nitrogen pollution present in wastewater—some 67 to 100 percent—is a by-product of what people consume. “We think a lot about sewage nitrogen. We know that’s an issue,” Almaraz says. “But I didn’t know how much of that is actually affected by the choices we’re making way upstream—when we go the grocery store, when we cook a meal and what we end up putting in our bodies.”

Once it enters the environment, the nitrogen in urea can trigger a spectrum of ecological impacts known as the “nitrogen cascade.” Under certain chemical conditions, and in the presence of particular microbes, urea can break down to form gases of oxidized nitrogen. These gases reach the atmosphere, where nitrous oxide (N2O) can contribute to warming via the greenhouse effect and nitrogen oxides (NOx) can cause acid rain. Other times, algae and cyanobacteria, photosynthetic bacteria also called blue-green algae, feed on urea directly. The nitrogen helps them grow much faster than they would normally, clogging vital water supplies with blooms that can produce toxins that are harmful to humans, other animals and plants. And when the algae eventually die, the problem is not over. Microorganisms that feast on dead algae use up oxygen in the water, leading to “dead zones,” where many aquatic species simply cannot survive, in rivers, lakes and oceans. Blooms from Puget Sound to Tampa, Fla., have caused large fish die-offs.

Although it is possible to treat algal blooms, many of the current methods—such as spraying clay particles or chemicals over the surface of a bloom to kill and sink the algae—are not always effective at eliminating all of the harmful growth. Some of these methods can even lead to additional pollution. So the best strategy for dealing with the effects of nitrogen pollution is prevention, says Patricia Glibert, an oceanographer at the University of Maryland, who was not involved with the new study.

One option for preventing nitrogen from getting into the environment is improving wastewater treatment plants. The technology exists to remove 90 percent of nitrogen from wastewater, but only 1 percent of all U.S. sewage is currently treated this way, partly because the technology is so expensive. Equipping plants in China to remove nitrogen from three quarters of the country’s urban sewage cost more than $20 billion. Almaraz and her team suggest, however, that curbing nitrogen pollution could be approached more quickly with a change in eating habits that could save billions of dollars in the long term.

Their new study, published in Frontiers in Ecology and the Environment, broke down protein requirements by age (adults 50 to 70 years old need the most) for the current U.S. population and projected future populations out to 2055. By midcentury, the country’s population is expected to be larger overall and to have a greater percentage of older people. The researchers calculated the amount of nitrogen that would enter the environment if people ate today’s average American diet and if they instead reduced their protein intake to only what is nutritionally needed. This shift in diet alone could reduce the amount of nitrogen reaching aquatic ecosystems by 12 percent today and by nearly 30 percent in the future, according to the study’s results. Such a change could also help reduce damaging nitrogen pollution while wastewater infrastructure catches up.

“Many people think that we need to all switch to becoming vegetarians. Obviously, that’s not practical. That’s not something that is really ever going to happen,” Glibert says. Rather than cutting out any foods entirely, she suggests consumers could switch to a “demitarian” diet—an approach that focuses on reducing the consumption of meat and dairy, which currently make up about two thirds of the protein eaten in the U.S. “Enjoy your steak, enjoy your burger but go modest on your meat consumption in your following meal,” she says.

“One cool area that opens up here is how human behavior can influence our environment,” Almaraz says. “I think it can be really empowering to people to understand that, ‘hey, my choices—once those add up with other people making similar choices—can actually have a positive impact.’”