If you have your own lactase, lactose is broken into simpler sugars, and then those sugars pass through your intestinal wall into your blood before bacteria can reach them. When your cells eventually burn those sugars for energy, you breathe out the resulting carbon dioxide.
If you don’t have your own lactase, bacteria in your gut supply the lactase to break down lactose, so they get to burn the simple sugars for energy. When bacteria in your gut make carbon dioxide by burning sugar, they don’t have a way to send it to your lungs, so it builds up in your gut and makes you sick.
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