Farmers may have caused the obesity epidemic by using antibiotics to fatten up livestock, according to a study.
Researchers found low exposure to antibiotics can upset the delicate balance of gut bacteria which in turn alters metabolism.
The findings indicate a possible link between rising rates of obesity and modern farming methods.
For decades since the 1950s, farmers have used low non-therapeutic doses of antibiotics to increase the body weight of cows, sheep, pigs and chickens.
The practice has sparked fears of spreading antibiotic resistance as the drugs get into the food chain, leading to a tightening of the rules.
Using antibiotics to fatten up farm animals is now banned in the EU but still allowed in the US and other countries.
The new research suggests antibiotic use on farms may pose other risks besides breeding resistant bugs.
Exposure to the medicines may seriously impair the metabolic development of children, setting them up for a lifetime struggle with their weight.
As well as being exposed to antibiotics indirectly through the food chain, children are commonly given the drugs to treat minor ailments.
In the US, the average child now receives one antibiotic course per year.
"The rise of obesity around the world is coincident with widespread antibiotic use, and our studies provide an experimental linkage," said lead researcher Professor Martin Blaser, from New York University School of Medicine in the US. "It is possible that early exposure to antibiotics primes children for obesity later in life."
The scientists administered common antibiotics such as penicillin and vancomycin to weaning mice at similar doses as those used in agriculture.
The treatment altered the composition of gut bacteria in the mice which in turn led to metabolic changes, such as increased production of fatty acids. After about six weeks the mice had gained about 10% to 15% more fat mass than untreated mice.
"By using antibiotics, we found we can actually manipulate the population of bacteria and alter how they metabolise certain nutrients," said co-author Dr Ilseung Cho, also from New York University.
"Ultimately, we were able to affect body composition and development in young mice by changing their gut microbiome through this exposure."
The findings are published in the journal Nature.
Although it was known that antibiotics could fatten up animals, previously the mechanism involved was unclear.
Commenting on the results, British expert Professor Brendan Wren, form the London School of Hygiene & Tropical Medicine, said:
"The role of the composition of our gut microflora is increasingly recognised as being important and has been linked to inflammatory bowel disease, metabolic disorders, immunity and obesity.
"It has been considered as the forgotten super organ in humans, and it is only now through advances in genome sequencing that we can assess the composition of the complex population of microbes in the gut.
"Indiscriminate use of antibiotics for livestock (often used to fatten animals), not only promotes the spread of antibiotic resistance, but can get in our food chain and affect the homeostasis of our gut microflora."
Dr Cormac Gahan, from University College Cork in the Irish Republic, said: "These studies support an emerging body of evidence linking gut bacteria with the development of obesity.
"Other research has identified specific subgroups of gut bacteria that play a role in energy extraction from the diet and influence the production of hormones in the host. Disrupting this finely balanced ecosystem clearly has consequences for host metabolism and weight gain."