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Early Antibiotic Use May Affect Weight

New research suggests that early-life exposure to antibiotics affects gut microbes and changes how food is metabolized. The findings may explain how antibiotics fatten farm animals. They may also have implications for childhood obesity.

For more than 50 years, the agricultural industry has used antibiotics to promote growth in livestock. Low doses fed over time to cattle, swine, sheep, chickens and turkeys can boost their weight by as much as 15%. Past studies have shown that, when administered at high doses to fight infection or disease, antibiotics change how the digestive system metabolizes nutrients. The effects of treating animals with these lower doses, called subtherapeutic antibiotic therapy, haven't been carefully examined.

A new study led by Dr. Martin Blaser at NYU School of Medicine investigated the effects of low-dose antibiotic therapy on the bacteria that live in the intestines—the gut “microbiome”—of young mice. The gut microbiome is a complex network of microorganisms that play active roles in maintaining our immune health, hormone production and growth. Treatment with high doses of antibiotics alters the population and structure of the microbiome. The researchers expected long-term lower doses to have similar effects.

To mimic the steady, low dose of antibiotics fed to farm animals, scientists added antibiotics to the mice's drinking water. They studied the total fat mass, percent body fat and bone density of the mice. The work, funded in part by NIH's National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) and National Center for Advancing Translational Sciences (NCATS), appeared in the August 30, 2012, issue of Nature.

After 7 weeks of exposure, the treated mice weighed the same but had 10% to 15% more fat mass than control mice. They also grew at a faster rate and had increased bone density. Hormones related to metabolism were higher in the treated mice. The composition of their gut microbiomes was altered, as was their metabolism of carbohydrates and fats.

In related work, Dr. Leonardo Trasande, Blaser and colleagues analyzed data from over 11,000 children in a British study, examining body mass and antibiotic exposures during infancy. The results appeared on August 21, 2012, in the online edition of the International Journal of Obesity.

The researchers found that children who were given antibiotics during the first 6 months of life were more likely to have a higher body mass and were more likely to be overweight by 3 years of age than those who weren't given the drugs. Exposure between 6 and 14 months wasn't associated with body mass index at any time point. While exposure to antibiotics between 15 and 23 months appeared to affect body mass index at 7 years of age, none of the exposures were linked to being overweight or obese at 7.

Taken together, these results suggest that antibiotics given early in life might affect the risk for becoming overweight. More work will be needed to confirm this connection.

“Microbes in our intestines may play critical roles in how we absorb calories,” Trasande says. “Exposure to antibiotics, especially early in life, may kill off healthy bacteria that influence how we absorb nutrients into our bodies, and would otherwise keep us lean.”

—by Meghan Mott, Ph.D.