A hydrogen breath test is an objective measure of lactose digestion.

“So, there's a standard test for lactose intolerance. It's objective. It's a hydrogen breath test. And so you have to drink 16 ounces of milk in one setting fairly fast. And then you every half hour breathe into a tube, capture the gas, and put it into this breathalyzer. And it'll tell you if there's hydrogen there. And if you have not digested the lactose, it'll go to your colon. The microbes will eat it up. It'll generate hydrogen. You'll absorb that and you'll exhale it. So, it's a very objective test of whether you are or aren't digesting your lactose.”