If you’re lactose intolerant, you should read this article: Genetics of food intolerance. He’s basically saying you can become lactose tolerant fairly quickly by exposing yourself to small amounts of lactose digesting bacteria, like those found in kefir or yoghurt. They exchange genes with your indigenous bacteria, and then you have everything you need to digest lactose. This is how pathogens quickly develop resistance to antibiotics. If your gut flora is so badly damaged that there aren’t the right kinds of bacteria to pass genes onto, then it may not work, but it’s certainly worth a try.