Pagan Kennedy, writing in the New York Times Sunday Review things use of antibiotics by children might be a contributor the rise in obesity in recent decades:
In 2002 Americans were about an inch taller and 24 pounds heavier than they were in the 1960s, and more than a third are now classified as obese. Of course, diet and lifestyle are prime culprits. But some scientists wonder whether there could be other reasons for this staggering transformation of the American body. Antibiotics might be the X factor — or one of them.
Of course, while farm animals often eat a significant dose of antibiotics in food, the situation is different for human beings. By the time most meat reaches our table, it contains little or no antibiotics. So we receive our greatest exposure in the pills we take, rather than the food we eat. American kids are prescribed on average about one course of antibiotics every year, often for ear and chest infections. Could these intermittent high doses affect our metabolism?
As the article points out, we know low-dose feeding of antibiotics to animals increases weight gain. Is the same thing happening to humans? It is possible, but it seems a little speculative to me at this point. Are there really enough antibiotics prescribed in childhood to produce the kinds of weight gains we've seen? The US has one of the highest rates of obesity in the world - but there are several countries (including France) that prescribe antibiotics more frequently than do American doctors. In some countries like Mexico and Brazil, a consumer can by antibiotics over the counter. It would be useful to flesh out some of these sorts of issues before reaching strong conclusions.
Even if the relationship is true, how many parents would be willing to trade-off making a baby's ear ache or strep throat go away against a potential 10 extra pounds at age 30?