Lactose is the primary type of sugar found naturally in the breast milk of all mammals, including humans. It’s also widespread within various commercially made dairy products, but milk is the only source of it found in nature. Different types of milk contain different percentages of lactose. Lactose intolerance is a widespread digestive problem, especially among people of African and Asian ancestry.
Lactose is classified as a disaccharide because it’s made of two simple sugars called galactose and glucose. Glucose is the end-product of all carbohydrate metabolism, and it’s quickly absorbed into the bloodstream and transported around your body to all your cells. Your cells burn glucose to do work and store energy. For lactose to be broken down into galactose and glucose in your intestines, an enzyme called lactase is needed. Lactose is only produced in the mammary glands of mammals and then secreted within milk, which explains why it’s often called milk sugar. Compared to sucrose, which is common table sugar, lactose is less soluble in liquids and less sweet.
Lactose makes up between 2 and 8 percent of mammalian milk by weight, with the amount varying among different species and individuals. For example, the percentage of lactose in human milk ranges from about 6.5 to 7.5 percent, depending somewhat on the health and diet of the woman. In contrast, the percentage of lactose in both cow’s and goat’s milk hovers around 5 percent.
Lactose intolerance is a widespread digestive problem caused by a lack of lactase secretion in the small intestine. Lactose needs to be split in half by lactase before it can be absorbed through the intestine. Without enough lactase, the undigested lactose ends up in the large intestine where it becomes a meal for friendly bacteria. The bacteria ferment the lactose and produce carbon dioxide as a by-product, which leads to bloating, abdominal pain, flatulence and diarrhea. Lactose intolerance affects up to 70 percent of people of African and Asian ancestry, but only about 30 percent of people with European heritage.
Fresh milk always contains lactose, but fermented varieties such as buttermilk have almost none because they are subjected to bacterial fermentation which breaks down the lactose into glucose and galactose. Lactose-free cow’s milk is commonly available in grocery stores and it’s made by adding some lactase enzyme. Dairy products made from milk that contain quite a bit of lactose include creamy butter, whipping cream, ice cream and non-aged cheeses such as mozzarella.
- Advanced Nutrition and Human Metabolism; James L. Groff et al.
- Textbook of Functional Medicine; David S. Jones
- Public Health Nutrition: From Principles to Practice; Mark Lawrence and Tony Worsley
Sirah Dubois is currently a PhD student in food science after having completed her master's degree in nutrition at the University of Alberta. She has worked in private practice as a dietitian in Edmonton, Canada and her nutrition-related articles have appeared in The Edmonton Journal newspaper.