Many people cannot digest milk properly, in spite of its place as a staple food in the Western diet. Lactose is a naturally occurring sugar found in milk, and lactase is an enzyme that helps people digest it. After infancy, your body produces less lactase to digest milk. A few factors -- including individual tolerance to milk and what other foods are consumed in the same meal -- determine how long it takes you to digest milk, so the transit time can vary widely, and there's no definitive study developing a timeline for milk digestion.
Since milk contains all six nutrients - protein, fat, carbohydrates, vitamins, minerals and water -- it is digested and absorbed throughout the digestive system. Milk passes through your stomach, then small intestine before arriving in your large intestine or colon. It’s estimated that food remains in the stomach for four to five hours, with higher fat milk staying in the stomach longer than lower fat milk. The milk passes into in the small intestine where most of the nutrients are digested and absorbed. A mixed meal may take three to five hours to pass through the small intestines. What's left of the milk then passes through the colon over a period of up to 24 hours or so, where some water and a few vitamins and minerals are absorbed. These estimates are based on a standard meal, and the digestion time for a liquid, such as milk, may vary considerably.
Digestion Time Factors
Symptoms of lactose intolerance -- which include bloating, gas, diarrhea, stomach pain and rumbling -- occur in the colon. It’s estimated that symptoms begin between 30 minutes and two hours after consuming milk. This can give you an idea of how long your body takes to digest milk. Your diet also affects the duration of milk's bowel transit. For example, adding fiber-rich foods, such as green vegetables and whole grains, to a meal including milk will shorten transit time and soften and bulk up your stool, according to the Linus Pauling Institute.