Milk and cookies, macaroni and cheese, ice cream: For many people, milk and milk-based items are the comfort foods they turn to in times of stress as well as celebration. However, for some individuals, those foods instead produce discomfort in the form of gastrointestinal distress. The explanation for this misfortune lies with one enzyme: lactase.

Enzymes are proteins that provide the power behind chemical reactions. In the human body, digestive enzymes break food into tiny particles that can pass the brush border, which is a chemical boundary that food must cross for absorption by the intestines. During the digestion of milk products, lactase is the enzyme that breaks down the milk sugar lactose into simpler sugars called glucose and galactose. This enables the intestines to absorb those sugars for immediate use or storage as energy for powering the body's movements and processes.

The gene LCT controls lactase production. Mutations in this gene can produce a heritable deficiency in lactase, resulting in lactose intolerance, or the inability to properly digest foods containing lactose. This condition also occurs with age in much of the human population. While the majority of infants produce copious amounts of lactase to enable the digestion of an all-milk diet, lactase production declines as humans age and begin consuming other types of food. For people with lactose intolerance, lactose not broken down by lactase moves into the colon, where gut bacteria ferment it. This fermentation produces gas and results in considerable discomfort for the lactase-deficient individual.

Of course, some people who are lactose-intolerant still prefer to eat or drink foods that contain lactose. One option is to use dairy products that contain added lactase, which breaks down the lactose in the carton or container before consumption. Non-dairy alternative milk derived from plant products such as soy, rice, nuts or hemp is another option since this milk is naturally lactose-free and therefore does not require lactase.