Lactose is the primary sugar found in milk. It’s a compound made of two simpler sugars, glucose and galactose, bonded together. Your body needs a specific enzyme to split that bond and absorb the energy from lactose, which is why it plays a central role in infant nutrition and why so many adults have trouble digesting dairy.
How Lactose Is Built
Lactose is classified as a disaccharide, meaning it’s a double sugar. Its two building blocks, glucose and galactose, are locked together by a specific chemical link. Until that link is broken, your body can’t absorb either sugar. This makes lactose different from simple sugars like glucose, which your cells can use immediately. The chemical formula is C₁₂H₂₂O₁₁, identical to table sugar (sucrose), but the internal arrangement of atoms is completely different, which is why they behave differently in your body and taste different on your tongue. Lactose is only about 20% as sweet as table sugar.
Where Lactose Shows Up Naturally
Milk is the only significant natural source of lactose. Cow milk contains roughly 4.5 grams per 100 grams, while goat milk comes in slightly lower at about 4.1 grams. Human breast milk actually contains more lactose than either, typically around 7 grams per 100 grams, making it the dominant calorie source for nursing infants.
Any food made from milk carries some lactose, but the amount varies enormously depending on processing. A 150-gram serving of yogurt contains about 4.8 grams, while buttermilk has around 6 grams per 150 ml. Aged hard cheeses tell a very different story. Parmesan, Emmentaler, Gouda, and Edam contain essentially zero lactose per serving because bacteria consume the lactose during the long aging process. Softer, fresher cheeses like mozzarella and cottage cheese retain more, with mozzarella holding about 3.3 grams per 100-gram serving.
How Your Body Breaks It Down
Digestion of lactose happens in the small intestine, where cells lining the intestinal wall produce an enzyme called lactase. Lactase splits the bond between glucose and galactose, freeing both sugars so they can pass through the intestinal wall and into your bloodstream. From there, your cells use glucose directly for energy, while galactose travels to the liver and gets converted into glucose as well.
During infancy, lactase production is high. This makes biological sense: breast milk is the sole food source, and lactose is its main carbohydrate. In most of the world’s population, lactase production declines after weaning, sometimes dramatically. This is the normal mammalian pattern. Populations with a long history of dairy farming, particularly those of Northern European descent, are more likely to maintain lactase production into adulthood.
What Happens When Lactose Isn’t Digested
When your small intestine doesn’t produce enough lactase, undigested lactose passes intact into the large intestine. There, gut bacteria ferment it, producing a mix of gases and acids. The gases include hydrogen, carbon dioxide, and methane. The acids include short-chain fatty acids like acetate, propionate, and butyrate, along with other compounds like lactate and formate.
This fermentation is what causes the familiar symptoms of lactose intolerance: bloating, gas, cramping, and diarrhea. The undigested lactose also draws water into the colon through osmosis, which contributes to loose stools. The severity depends on how much lactase you still produce and how much lactose you consumed. Most people with reduced lactase can handle small amounts of dairy, especially when eaten with other foods, without significant symptoms.
Diagnosis typically involves a hydrogen breath test. You drink a lactose solution, then breathe into a device at intervals. If hydrogen levels in your breath rise by 20 parts per million or more above your baseline, it confirms that lactose is reaching your colon undigested and being fermented by bacteria there.
Lactose in Processed Foods and Medications
Lactose appears in far more products than most people realize. In food manufacturing, it contributes to browning. When lactose reacts with proteins during heating, it triggers a chain of chemical reactions that produce the golden-brown color and complex flavors in baked goods, coffee, bread, and cocoa products. Food manufacturers also use it as a mild filler and flavor carrier because of its low sweetness.
The pharmaceutical industry relies heavily on lactose as a filler in pills and capsules. It appears in roughly 20% of prescription medications and 6% of over-the-counter drugs. When a tablet’s active ingredient is tiny, a filler is needed to bring the pill up to a manageable size, and lactose is the go-to choice because it’s chemically stable, compresses well into tablets, and is inexpensive. A single tablet or capsule typically contains 100 to 200 milligrams of lactose, though some contain up to 400 milligrams. Lactose is also used as a carrier in dry powder inhalers. For most people with lactose intolerance, these amounts are too small to cause symptoms, but individuals with severe sensitivity should check inactive ingredient lists.
Lactose Content in Common Dairy Foods
If you’re managing your intake, knowing the actual numbers helps more than vague labels. Here’s how common dairy products compare:
- Whey (150 ml): 7.1 g lactose
- Buttermilk (150 ml): 6 g
- Kefir (150 g): 5.4 g
- Yogurt (150 g): 4.8 g
- Mozzarella (100 g): 3.3 g
- Cottage cheese (30 g): 1 g
- Cream cheese (30 g): 0.9 g
- Parmesan (30 g): 0 g
- Aged hard cheeses like Gouda, Edam, Emmentaler (30 g): 0 g
The pattern is straightforward: the longer a dairy product has been aged or fermented, the less lactose remains. Bacteria do the work of breaking it down before it ever reaches your digestive system. This is why many people who struggle with a glass of milk can eat aged cheese without any issues.