Isegoria

Strange things have been happening to the human body over the last few decades:

Why have human body temperatures declined in the United States over the last 150 years? Or why has the age of first puberty been declining among teenagers since the mid-nineteenth century, from 16.5 years in 1840 to 13 years in 1995?

Or—to take a more troubling and immediate case—why have rates of autism been increasing so dramatically? After having been very rare a few decades prior, the rate has grown from about 1 in 150 children in 2000 to 1 in 44 in 2018, according to the Center for Disease Control. The standard explanation for this increase—changing diagnostic criteria and increased awareness—simply does not explain how sustained the uptick has been, nor does it explain the first-hand accounts of the increase by teachers. In fact, studies have found that changing diagnostic criteria account for only one-fourth of observed increases. Something else is causing the rest.

Or consider something as seemingly straightforward as obesity. In 1975, about 12 percent of American adults were obese; now that figure sits above 40 percent. The standard explanation of the remarkable increase in obesity over the last few decades—the “big two,” more calories and less physical exertion—have an intuitive appeal, but they do not seem to capture the full picture. Between 1999 and 2017, per capita caloric intake among Americans did not change, while the rate of obesity increased by about a third. The increase is so dramatic that a drop-off in physical exertion in so brief a period is unlikely to be the sole explanation, especially since the majority of human energy expenditure is non-behavioral.

Obesity thus remains, in the words of an article in the American Journal of Clinical Nutrition, an “unexplained epidemic.” This is why many scientists have sought to locate contributing factors to the secular increase in obesity, from the decline in cigarette use to increases in atmospheric CO2 levels.

There are many conditions like this: allergies, irritable bowel syndrome, eczema, and autoimmune conditions like juvenile arthritis are other notable examples. These are not the well-known “diseases of modernity,” like heart disease or Type 2 diabetes, whose causes are reasonably well-known. Disturbingly, there seem to be connections between all of these conditions: the “autistic enterocolitis” gut disorder that resembles Crohn’s disease in autistic children, the obesity-asthma link, the irritable bowel syndrome-eczema link, the eczema-allergies link. These “diseases of postmodernity” appear to be a package deal: autistic children report higher rates of stomach pain, and obese people be more likely to develop eczema-like skin diseases. There is some common root underlying these conditions.

A wealth of scientific work mostly done in the last decade by scientists like Martin Blaser of Rutgers may point to the answer. The origin lies in the extraordinary pressure we have been placing on a part of the body about which we know and think little: the microbiome of the human gut.

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We have known for a long time that antibiotics induce rapid weight gain in everything from mice to humans. The specific dynamic—antibiotics cause gut dysbiosis, and gut dysbiosis leads to obesity and other diseases—is now becoming increasingly clear. Similar studies for conditions like asthma or juvenile arthritis, all conducted only in the last few years, have found the same link.

This is especially worrying because antibiotics are everywhere.

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Consider animal agriculture, the main force for antibiotic pollution in the United States. Antibiotics are now crucial to the industrial production of chicken, pig, and cow protein; in recent years antibiotics have even begun to be used in aquaculture. The reasons are simple: antibiotics used prophylactically can prevent and suppress infectious diseases, like bovine footrot and anaplasmosis, that are common in the claustrophobic quarters of concentrated animal-feeding operations (CAFOs). More insidiously, antibiotics can make livestock larger by disrupting their gut biomes and metabolisms, allowing them to be slaughtered at younger ages and at greater weights. In 2019, of the antibiotics sold in the United States, only about a third went to humans, with the rest consumed by livestock.

Antibiotics have been used in American animal agriculture since the late 1940s. It was then that Thomas Jukes, a biologist for the pharmaceutical company Lederle Laboratories, discovered that treating chickens with even trace amounts of the antibiotic chlortetracycline—a drug that had been discovered in 1945 at Lederle—caused them to gain much more weight. The more chlortetracycline the birds got, the larger they were; the chickens that had gotten the highest doses weighed two and a half times more than the ones that hadn’t gotten anything.

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Per capita consumption of chicken—once a rare and expensive kind of meat, typically consumed as a Sunday treat—more than tripled between 1960 and 2020, growing from a relatively marginal part of the American diet in the first few decades of the twentieth century into the country’s premier staple protein.

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As with chickens, the biological effects on cows were significant. The year that monensin was licensed, the average weight of cows at slaughter was 1,047 pounds; by 2005, it had grown thirty percent, to 1,369 pounds. By 2017, American cattle producers used about 171 milligrams of antibiotic per kilogram of livestock—four times as much as in France, and six times as much as in the United Kingdom.

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As a result of this mass pharmaceutical use in animal agriculture, natural bodies of water now contain remarkable amounts of antibiotic waste. One study of a river in Colorado found that “the only site at which no antibiotics were detected was the pristine site in the mountains before the river had encountered urban or agricultural landscapes.” Antibiotics like macrolides and tetracyclines have been found in chlorinated drinking water, while the antibiotic triclosan has been found in rivers and streams around the world. This effluent trickles into everything else: research has detected uptake of veterinary antibiotics in carrots and lettuce, as well as in human breast milk.

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It was not until 2017, well after European countries had strictly limited the use of antibiotics, that the FDA was finally able to ban the use of antibiotics for growth promotion in livestock, mandating that all antibiotics given to cattle needed a prescription. After peaking in 2015, antibiotic use on farms has declined by about 40 percent, with most of the effect taking place in the year of the ban.

But antibiotic use remains elevated, above an average of 100 milligrams per kilogram per year—far more than the 50 milligram limit that reports on antibiotic resistance have proposed, and several times more than is normal in European countries like France or Norway. The reason, Lewis believes, goes back to his anaplasmosis episode. He believes that anaplasmosis is commonly used as a pretext for administering growth-promoting antibiotics, and that this is an open secret among farmers and livestock veterinarians. The “motorway veterinarian,” dependent on the business of growth-hungry farmers, remains alive and well.

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One study in Science found that 42 percent of lots that were certified by the Department of Agriculture as “Raised Without Antibiotics” actually contained cattle that had been given antibiotics, with five percent of lots being composed entirely of cattle raised on antibiotics.

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