A retrospective study released in JAMA Network Open found that antibiotic exposure in early childhood repeatedly caused substantially higher body mass index (BMI) compared with those not exposed. This raised the risk of obesity dramatically. The research recruited 6853 children between 2009 and 2010; 93 percent of the children had assessed their weight and height at the 54-month follow-up. The data analysis was performed between 2017 and 2018.

Results found that 95 percent of the 5128 singletons had antibiotic prescription by age 48 months and 9 percent had obesity at age 54 months. With the number of antibiotic dispensations, the modified mean BMI-for-age z scores increased dramatically, for 4 to 6, 7 to 9 and more than 9 dispensations.

Information from the New Zealand Pharmaceutical Collection database is collected for children whose parents consented to external data linking. Singletons with weight and height measurements of 54 months, population antibiotic dispensing and birth weight results, gestational age greater than 27 weeks and no congenital abnormalities were included in the analytic samples. The study results confirmed that Repeated antibiotic exposure in early childhood increased the likelihood of obesity and a higher mean BMI. The measures taken included the BMI-for-age z scores of the World Health Organization and the excess weight and obesity cutoff points of the International Obesity Task Force which pass through adult BMI values of 25 and 30.