Drugs might impact body composition, weight by altering mix of gut microbes

By Tina Hesman Saey

Web edition: August 22, 2012

Antibiotics may plump up cows, pigs, chickens, mice and humans alike, new research suggests.

For decades, low doses of antibiotics have been given to livestock to make the animals grow and bulk up faster, but no one really knew how the drugs promoted growth. Now, researchers led by microbiologist Martin Blaser at the New York University School of Medicine report online August 22 in Nature that antibiotics alter the mix of bacteria in the intestines of mice and cause the rodents to build up more fat than normal.

In a separate study, published online August 21 in the International Journal of Obesity, Blaser’s group found a link between antibiotic use in babies younger than 6 months old and being overweight at age 3. Together, the studies suggest that medications that alter the mix of friendly bacteria in the gut may have lasting effects on body weight.

Mice served as stand-ins for harder-to-study cows and other livestock animals in the Nature study. Genetically identical mice were divided into groups and given low doses of antibiotics in drinking water for seven weeks after weaning. One group of mice received no antibiotics. The mice all weighed roughly the same at the end of the experiment, but those that drank antibiotic cocktails had more body fat than mice that didn’t get the drugs.

Although antibiotics are meant to kill bacteria, the researchers found that the total number of bacteria in the mice’s guts didn’t change. But the mix of the microbes did. Mice on antibiotics had more of a type of bacteria called Firmicutes and fewer Bacteroidetes bacteria than did mice in the no-antibiotics group. That shift in the composition of gut bacteria is similar to the mix of microbes previously found in obese people, says study coauthor Ilseung Cho, a gastroenterologist at NYU.

Gut microbes in antibiotic-ingesting mice produced more short-chain fatty acids, a type of fat that cells use for energy. “Essentially you’re getting more fuel from the same amount of starting material,” Cho says.

While the study may help explain how chronic exposure to antibiotics boosts growth in animals, it doesn’t necessarily mean that the medicines’ use for short stints in people can be blamed for weight gain.

 “I think the work described in the Nature manuscript is provocative and intriguing, but I would not jump to conclusions about mechanisms and about the translatability of the work to children,” says David Relman, a microbiologist at Stanford University. “Short-term exposures in children do not necessarily cause effects at all like this.”

To determine how antibiotic use might affect people’s weight, NYU researchers examined medical records of more than 11,000 children born in Avon, England, between April 1991 and December 1992. By 6 months old, about a third of the children had received antibiotics. Those children had a 22 percent greater chance of being overweight at age 3 than did kids who didn’t take such medication early on. But by age 7 the lingering effect of antibiotics on body weight appeared to dissipate, says study coauthor Leonardo Trasande, a pediatrician and epidemiologist at NYU. Antibiotic use after the age of 6 months was not consistently linked to differences in children’s later weight.

Babies pick up microbes from their mothers during birth and nursing, and from the environment. Generally the bacterial mix in the gut is flexible during the first year of life, and there was no previous reason to expect that giving antibiotics early would have enduring consequences, says Amanda Thompson, a biological anthropologist at the University of North Carolina at Chapel Hill.

Although the researchers did not examine the gut microbes in the British children, the mouse study by the NYU group suggests that antibiotic use may contribute to toddlers’ weight gain by skewing the microbe mix in infancy, Thompson says. “There is a good arrow pointing toward the microbiome.”