Children who are given antibiotics frequently are far more likely to become overweight and obese, according to new Irish research. Birth by Caesarean section also greatly increases the risk of obesity, the research suggests.

The study of almost 9,000 Irish children found that early-life exposure to antibiotics may lead to weight gain by age five while being delivered by C-section has an “additive effect”.

A child exposed to six or more courses of the drug each year is almost three times more likely to be obese, according to the research, which will be presented at the Royal College of Physicians of Ireland today.

Worryingly, children are both the most prescribed age group for antibiotics and the most vulnerable, because their guts are in a “critical window of development”, according to Barrie Tyner of UCC’s department of epidemiology.

Mr Tyner warned of significant limitations to his work, notably a lack of specific information on antibiotic use in the first year of life. His research, which does not claim a causative link between antibiotic use and weight gain, is unpublished and has yet to be peer-reviewed.

Increased risk

A child exposed to three to five courses of antibiotics has a 23 per cent higher risk of being overweight and a 71 per cent higher risk of being obese compared with a child who has no exposure, he found.

Where a child is exposed to six or more drug courses, the risk of being overweight rises by 36 per cent, while the risk of obesity increases by 178 per cent.

Having six or more exposures to antibiotics and being born by Caesarean section carries twice the risk of obesity as having the same level of exposure in a child delivered normally.

The research was based on data from the wider Growing Up in Ireland study, with information gathered at nine months and three years, and weight measurements taken at the age of five.

Antibiotics have been used for decades as growth promoters in agriculture but it is only recently that international studies have raised the possibility of links with obesity. It is thought that antibiotics induce changes in bacteria living in the gut that favour weight gain. This change on the gut microbiome persists even after the persons stops taking the drugs.

The child’s exposure to antibiotics while still in the womb is also coming under closer scrutiny. Studies have suggested babies born via Caesarean section tend to be heavier and more susceptible to immune conditions such as allergies and asthma.