The first study, published Wednesday in Nature, used mice to investigate the effects of low doses of antibiotic treatment. The researchers, led by Martin Blaser, thereby mimicked the treatment farm animals have been receiving in the United States in recent decades. Since the 1950s, low doses of antibiotics have been widely used as growth promoters in animals, reportedly increasing their size of by up to 10 percent.
“People used antibiotics to treat infections, but then they were surprised that even in the absence of disease, this led to bigger animals,” said scientist Didier Raoult, who was not connected to the studies.
Aiming to explain these observations, Blaser’s team treated young mice with low doses of antibiotics and found that while the treated mice did not become larger overall, they were, in fact, more obese.
The researchers suspected that changes in the intestine bacteria were responsible. They surveyed the bacteria using genetic methods and found that while treated and untreated mice had a similar total count of bacteria species, their compositions seemed to diverge. Some species of bacteria, previously shown to be associated with obesity in mice, were found at a higher concentration.
The scientists then did a genetic analysis of the bacteria’s metabolism and found that some genes responsible for fat synthesis showed a higher activity in the treated mice. Overall, the scientists concluded that the treated mice had bacteria that were more efficient in digestion.
The second study, published in the International Journal of Obesity this week, aimed to corroborate these findings in humans. The research led by Leonardo Trasande looks at statistical data collected from nearly 11,000 children in Avon, England. The researchers found that children, who were given antibiotics in their first six months, had a higher incidence of obesity later. The difference was most pronounced at about 3 and tended to teeter off at about 7, the latest time analyzed. Obesity at that age correlated slightly with antibiotics during the toddler stage. The differences remained even when the scientists took additional factors such as diet and activity into account. Overall, the research corroborated earlier findings by a Danish study while providing additional detail.
Still, the scientists considered their results preliminary. “What we can show here is correlation, rather than causation,” Trasande said. But Blaser, who also participated in the second study, said that these two investigations combined with other data strengthen the idea that obesity might depend on the composition of intestinal bacteria.
Blaser stressed that bacteria play a crucial role in the uptake of nutrients from food, and that their perturbation — by antibiotics, for example — should be considered more carefully. Antibiotics are poisons for bacteria and normally affect beneficial bacteria in the gut together with the pathogens they are supposed to target.