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Children who regularly use antibiotics gain weight faster than those who have never taken the drugs, according to new research that suggests childhood antibiotics may have a lasting effect on body weight well into adulthood.

The study, published in the International Journal of Obesity, examined the electronic medical records of 163,820 children ages 3 to 18, counting antibiotic prescriptions, body weight and height. The records, which covered pediatric exams from 2001 through 2012, showed that one in five — over 30,000 children — had been prescribed antibiotics seven or more times. By the time those children reached age 15, they weighed, on average, about 3 pounds more than children who had received no antibiotics.

While earlier studies have suggested a link between antibiotics and childhood weight gain, they typically have relied on a mother’s memories of her child’s antibiotic use. The new research is significant because it’s based on documented use of antibiotics in a child’s medical record.

“Not only did antibiotics contribute to weight gain at all ages, but the contribution of antibiotics to weight gain gets stronger as you get older,” said Dr. Brian S. Schwartz, the first author and a professor in the department of environmental health sciences at the Johns Hopkins Bloomberg School of Public Health.

Scientists have known for years that antibiotic use promotes weight gain in livestock, which is why large food producers include low doses of antibiotics in the diets of their animals.

While researchers don’t know exactly why frequent use of antibiotics is associated with weight gain in children, it may be that the drugs wipe out the healthy bacteria in a child’s body. These may lead to permanent changes in the microbiome — the many and varied organisms that live in our gut. Shifts in the microbiome may change how food is broken down in our bodies, how food is absorbed and how many calories are released from foods.

In July, a study of nearly 10,000 Danish schoolchildren found that a mother’s use of antibiotics during pregnancy was associated with a higher risk that her child would become overweight or obese.

The latest study linking frequent antibiotic use and weight gain doesn’t mean that parents should never give a child these medications. Some bacterial illnesses can be life-threatening without antibiotic treatment. However, researchers have documented that parents often want — and pressure pediatricians to give them — antibiotic prescriptions for ear infections and viruses that can’t be helped by the drugs.

For years the main concern about the overprescribing of antibiotics has been the fear that the drugs would eventually lose their effectiveness as bacteria develop drug resistance. The new research opens the door to a potential new strategy for curbing antibiotic use, warning parents that overuse has an immediate detrimental effect on a child’s health and puts them at risk for becoming fat.

“We’ve got to totally dissuade parents from advocating for antibiotics,’’ said Dr. Schwartz. “As parents we want to feel like we’re doing something active for our kids, but I think we’re doing our kids damage. If your doctor says you don’t need them, don’t take them. ”

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