IF you walk into a farm-supply store today, you’re likely to find a bag of antibiotic powder that claims to boost the growth of poultry and livestock. That’s because decades of agricultural research has shown that antibiotics seem to flip a switch in young animals’ bodies, helping them pack on pounds. Manufacturers brag about the miraculous effects of feeding antibiotics to chicks and nursing calves. Dusty agricultural journals attest to the ways in which the drugs can act like a kind of superfood to produce cheap meat.

But what if that meat is us? Recently, a group of medical investigators have begun to wonder whether antibiotics might cause the same growth promotion in humans. New evidence shows that America’s obesity epidemic may be connected to our high consumption of these drugs. But before we get to those findings, it’s helpful to start at the beginning, in 1948, when the wonder drugs were new — and big was beautiful.

That year, a biochemist named Thomas H. Jukes marveled at a pinch of golden powder in a vial. It was a new antibiotic named Aureomycin, and Mr. Jukes and his colleagues at Lederle Laboratories suspected that it would become a blockbuster, lifesaving drug. But they hoped to find other ways to profit from the powder as well. At the time, Lederle scientists had been searching for a food additive for farm animals, and Mr. Jukes believed that Aureomycin could be it. After raising chicks on Aureomycin-laced food and on ordinary mash, he found that the antibiotics did boost the chicks’ growth; some of them grew to weigh twice as much as the ones in the control group.

Mr. Jukes wanted more Aureomycin, but his bosses cut him off because the drug was in such high demand to treat human illnesses. So he hit on a novel solution. He picked through the laboratory’s dump to recover the slurry left over after the manufacture of the drug. He and his colleagues used those leftovers to carry on their experiments, now on pigs, sheep and cows. All of the animals gained weight. Trash, it turned out, could be transformed into meat.

You may be wondering whether it occurred to anyone back then that the powders would have the same effect on the human body. In fact, a number of scientists believed that antibiotics could stimulate growth in children. From our contemporary perspective, here’s where the story gets really strange: All this growth was regarded as a good thing. It was an era that celebrated monster-size animals, fat babies and big men. In 1955, a crowd gathered in a hotel ballroom to watch as feed salesmen climbed onto a scale; the men were competing to see who could gain the most weight in four months, in imitation of the cattle and hogs that ate their antibiotic-laced food. Pfizer sponsored the competition.

In 1954, Alexander Fleming — the Scottish biologist who discovered penicillin — visited the University of Minnesota. His American hosts proudly informed him that by feeding antibiotics to hogs, farmers had already saved millions of dollars in slop. But Fleming seemed disturbed by the thought of applying that logic to humans. “I can’t predict that feeding penicillin to babies will do society much good,” he said. “Making people larger might do more harm than good.”

Nonetheless, experiments were then being conducted on humans. In the 1950s, a team of scientists fed a steady diet of antibiotics to schoolchildren in Guatemala for more than a year,while Charles H. Carter, a doctor in Florida, tried a similar regimen on mentally disabled kids. Could the children, like the farm animals, grow larger? Yes, they could.

Mr. Jukes summarized Dr. Carter’s research in a monograph on nutrition and antibiotics: “Carter carried out a prolonged investigation of a study of the effects of administering 75 mg of chlortetracycline” — the chemical name for Aureomycin — “twice daily to mentally defective children for periods of up to three years at the Florida Farm Colony. The children were mentally deficient spastic cases and were almost entirely helpless,” he wrote. “The average yearly gain in weight for the supplemented group was 6.5 lb while the control group averaged 1.9 lb in yearly weight gain.”

Researchers also tried this out in a study of Navy recruits. “Nutritional effects of antibiotics have been noted for some time” in farm animals, the authors of the 1954 study wrote. But “to date there have been few studies of the nutritional effects in humans, and what little evidence is available is largely concerned with young children. The present report seems of interest, therefore, because of the results obtained in a controlled observation of several hundred young American males.” The Navy men who took a dose of antibiotics every morning for seven weeks gained more weight, on average, than the control group.  (...)