Treating babies with antibiotics risks making them overweight as they grow up, say scientists.
A study of more than 11,000 British children found those given the drugs before they were six months old had a bigger BMI (body mass index) than their peers.
By the time they were just over three exposed children were 22 per cent more likely to be overweight.
But those given antibiotics between the age of six and 14 months did not have significantly higher body mass than children who did not receive any in that period.
Prof Leonardo Trasande, of New York University, said: ‘We typically consider obesity an epidemic grounded in unhealthy diet and exercise, yet increasingly studies suggest it’s more complicated.
‘Microbes in our intestines may play critical roles in how we absorb calories, and 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.’
The study found on average babies given antibiotics from birth to five months weighed more for their height.
Between the ages of 10 to 20 months, this translated into small increases in body mass after taking into account impacts of diet, physical activity and parental obesity.