Two fold: Even though you can’t digest lactose and turn it into energy or fat stores, the bacteria living in your colon can and break those carbon chemical bonds. This provides energy to the bacteria those causes that carbon to bond with oxygen and form CO2. The excess CO2 causes pain, flatulence and bloating. Second, sugar is osmotically active meaning that is draws water to itself, causing diarrhea.

Therefore, Lactose intolerance commonly leads to a combination of pain, bloating, flatulence and diarrhea.