Yes. Either. Both. But perhaps more on the foe side.
Briefly, naturally-occurring antibiotics have been around for a long time, but the modern antibiotic era really started in 1928 when the researcher Fleming accidentally discovered penicillin. Production methods were refined during World War II, and by 1945 mass production and distribution was underway.
At first, antibiotics were used on humans to cure infections, and shortly after were applied to the same use in animal care. However, in 1950 a group of scientists discovered that adding antibiotics to animal feed caused the animals to put on weight faster– up to 50% faster.
This side effect of normal curative uses quickly became a primary use of antibiotics, leading to significant amounts of antibiotics being introduced into the food chain, either through the meat of animals eating the antibiotics or indirectly through runoff from feeding operations or seepage into groundwater.
Why do antibiotics cause weight gain in animals? Unknown, exactly, but antibiotic use appears to alter the makeup of the microbiome– microbes living in the intestines. There seems to be a connection between the microbiome and obesity. So the huge influx of antibiotics into our food chain and escaping into our overall environment may be contributing to our obesity epidemic.
You see… antibiotics cause weight gain in animals.
And people are, well– animals.
But there is more to the story…