Babies who are prescribed antibiotics are at significant risk of being overweight at age 7, according to a study published in the March 2011 issue of the International Journal of Obesity.

Copenhagen researchers looked at data from the Danish National Birth Cohort study featuring 28,354 children, which included information about antibiotic administration in infancy. When they followed up with the kids at age 7, the researchers found that the children who were given an antibiotic during the first six months of life had an increased risk of being overweight compared to infants who did not get an antibiotic. Interestingly, the researchers noted this to be true even if the mother was not overweight.

Antibiotics are known to kill good bacteria in the human gut, as well as the bad bacteria that are being targeted. Several years ago, animal studies done by Jeffrey Gordon, MD, of Washington University found that the ratio of bad to good bacteria in the gut of obese animals was significantly different than the ratio in their lean counterparts. Since then, human studies have also shown that certain bacteria are linked with being lean, while other bacteria are aligned with weight gain.

Because antibiotics alter the ratio of good to bad bacteria, researchers have speculated that the drugs contribute to obesity. While his new study draws a clear line between early antibiotic use and obesity, more research is needed to make a more direct cause-and-effect link in both children and adults.

It’s well documented that antibiotic overuse is leading to antibiotic resistance, which poses many threats. Will  this new information make doctors think twice about giving the drugs to children or stop parents from requesting antibiotics? The Center for Disease Control and Prevention (CDC) reports that childhood obesity has more than tripled in the past 30 years, with more than one third of children reported as being overweight in 2008. Children who are obese have a higher likelihood of heart disease, diabetes, joint problems and sleep apnea, according to the CDC.