From Scientific American, July 27:
Protein-packed diets add excess nitrogen to the environment through urine, rivaling pollution from agricultural fertilizers
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....
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