There is a persistent myth about infant gut development that comes up in nearly every online discussion of starting solid foods. It’s the myth that infants have a “virgin” or “open” gut until around 6 months of age. I’ve received so many emails, Facebook posts, and comments about the virgin gut over the last few years that I thought it was finally time to take a look at the science – and lack thereof – behind this myth.

I have written before, in my book and on my blog, about the controversy a...