The 1950s was a time when people held fat babies and big men in high regard, so people were starting to consider that antibiotics may also cause weight gain in humans. What shocks me is that Pfizer sponsored a competition to see which animal feed salesman could gain the most weight in four months by taking antibiotics, and it was a public spectacle to see the men weigh themselves on scales as part of the competition.
Not everyone thought this was a good idea. "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.” "
More shocking stories have been exposed. Scientists fed antibiotics to Guatemalan schoolchildren for a year, and to mentally disabled children in Florida twice a day, and found they did gain more weight than control groups. Adult male Navy recruits also took antibiotics daily for seven weeks and gained weight.
Pharmaceutical companies began selling antibiotic feed supplements for farm animals, and when animals ate the feed, they also gained the ability to survive better than they used to in dirty environments. The animals started to be caged in factory farms.
Of course, now there is alarm that humans are taking too many antibiotics, especially due to drug resistance. (Although we do not take in antibiotics from the meat we eat, it is prescribed frequently for infections.) Scientists are investigating if antibiotics contribute to our obesity epidemic, and are altering our natural microbiomes in our bodies that are beneficial.