It will come as no surprise that taking antibiotics alters the gut flora of infants, however a recent study confirms the findings of several other studies in suggesting that antibiotics may have more profound longer term effects on infants, and probably children (and then adults). One way of thinking about this, is to appreciate that in farming, antibiotics were widely used as growth promoting agents.
In this current study by Saari et al (Pediatrics 2015 135:617-626) they followed up a large group (6114 boys and 5984 girls) and looked at their growth and obesity (BMI) over the first 2 years, and examined these changes in relation to antibiotic exposure before 6 months of age. BMI is not a very accurate measure of the amount of fat you have: think of a body builder – they have lots of muscles and almost no fat, but still have a high BMI. But BMI is the only thing you can really measure when you follow up thousands of children (you can't do expensive MRI scans on them all). The study found that children who had received antibiotics were heavier (more so in boys, than girls) and this was particularly strong for certain types of antibiotics (macrolides). The authors suggest that antibiotic exposure may play a role in the growing [sic] epidemic of obesity . Of course, a study like this doesn’t prove that antibiotics cause obesity, just that there is an association. Maybe families who have obese children have a different approach to use of antibiotics i.e. maybe they attend GPs more often? Either way, it emphasises that we should be cautious of the unnecessary use of antibiotics, and especially in infancy. http://pediatrics.aappublications.org/content/135/4/617.abstract