However, many don't know they're lactose intolerant until they eliminate it for about a month and then reintroduce and see what happens.
Even then it's not a certainty. Introducing (or re-introducing) ANY major dietary change will elicit the "body no likey" response at first. Any change at all. Whatsoever. Even if the change is 100% salubrious in the long run.
Like, if you've been Super Sizing Me for the last five years and suddenly you start throwing down tons of whole foods and fresh vegetables, you'll feel HORRIBLE for a couple of weeks. Gas, more gas, lethargy, diarrhea, maybe nausea... the whole 9 yards. Does that mean you're "vegetable intolerant"? Most likely not, it just means you are adjusting to a big delta.
This is also the reason lots of people get sick/diarrheic when traveling abroad. Sometimes it's the sketchy stuff in the water, but 9 times out of 10 it's just the result of eating foods that make your body say, "whoa wut".
This is not to say that lactose intolerance isn't a thing; clearly, it's a thing. Rather, this is just a warning against being a hypochondriac when it comes to nutrition. If something is really a problem, you should test it beyond just eliminating and re-introducing—unless it's a food that you don't like very much, and so can eliminate from your diet without much ado.
I.e., don't be That Guy who suddenly starts mortifying his friends by insisting on gluten-free vegan non-dairy hemp-based love&light&chakra&etc. meals at restaurants... unless you are actually the 1 in 1,000,000 who actually
needs that.