Farmers may have played their part in the obesity epidemic by fattening their livestock with antibiotics, a study suggests.
By altering the fine balance of gut bacteria which influence our metabolism, even small amounts of the drugs entering the food chain could have caused obesity rates to rise, researchers claim.
Although the use of antibiotics on farms is now banned in the EU due to the risk of germs becoming drug-resistant, it was commonplace in the 1950s and is still permitted in the US.
Prof Martin Blaser of New York University, who led the study, said: "The rise of obesity around the world is coincident with widespread antibiotic use, and our studies provide an experimental linkage.
"It is possible that early exposure to antibiotics primes children for obesity later in life."
For decades farmers in Britain and around the world fed low doses of antibiotics to cattle, sheep, pigs and chickens because the drug caused the animals to gain weight.
In the new study, researchers studied the effects of penicillin and other common antibiotics on weaning mice, using doses similar to the non-medical amounts used by farmers.
Their results, published in the Nature
journal, showed that the drugs altered the balance of bacteria in their gut, causing metabolic changes which led them to gain 10 to 15 per cent more fat than untreated mice.
Although antibiotics were already known to cause weight gain, the role of gut bacteria in causing the effect was previously unclear.
Co-author Dr Ilseung Cho said: "By using antibiotics, we found we can actually manipulate the population of bacteria and alter how they metabolise certain nutrients.
"Ultimately, we were able to affect body composition and development in young mice by changing their gut microbiome through this exposure."
A related study
published earlier this week by the same authors showed that young children who had taken small amounts of antibiotics were more likely to have higher amounts of body fat.
Prof Brendan Wren of the London School of Hygiene & Tropical Medicine, who was not involved in the study, 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.
"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, of University College Cork, added: "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."