Lactose: The Sugar Commonly Found in Milk Explained

January 4, 2026 •

3 min read

Lactose, a disaccharide sugar, is the primary carbohydrate found naturally in milk and comprises about 2–8% of milk by weight, depending on the mammal. This is the sugar commonly found in milk, giving it a mildly sweet flavor and providing energy to growing mammals.

Quick Summary

Lactose, known as milk sugar, is the main carbohydrate in dairy products, formed from glucose and galactose. Its digestion requires the enzyme lactase, and its concentration varies across dairy foods. The article examines lactose's properties, nutritional impact, and intolerance issues.

Key Points

Lactose is Milk's Sugar: The carbohydrate most commonly found in milk is lactose, a disaccharide made of glucose and galactose.
Digestion Requires Lactase: For the body to digest lactose, it needs the enzyme lactase, which breaks the sugar down into its simpler components.
Intolerance is a Lactase Deficiency: Lactose intolerance is caused by insufficient lactase production, leading to undigested lactose and digestive issues.
Lactose Varies in Dairy: The concentration of lactose differs significantly across dairy products, with hard cheeses having almost none and milk containing more.
Lactose-Free Options Exist: Lactose-free milk is produced by adding the lactase enzyme to break down the lactose, making it digestible for intolerant individuals.
Offers Unique Health Benefits: For those who can digest it, lactose aids in the absorption of minerals like calcium and acts as a prebiotic.

What Exactly is Lactose?

Lactose is a disaccharide carbohydrate composed of two simpler sugars: glucose and galactose. It is found almost exclusively in mammal milk and provides essential energy, especially for newborns. The concentration varies by species, with human milk having a higher concentration than cow's milk. Lactose is less sweet than sugars like sucrose. When isolated during cheese production, it forms a crystalline powder used in food and pharmaceuticals.

The Role of the Enzyme Lactase

To be absorbed, lactose must be broken down by the enzyme lactase, produced in the small intestine. Lactase splits the bond between glucose and galactose. These are then absorbed for energy. Insufficient lactase causes lactose intolerance symptoms.

Digestion and Lactose Intolerance

Many mammals, and a significant portion of humans, experience a decline in lactase production after infancy. Undigested lactose in the large intestine is fermented by bacteria, causing gas and symptoms like bloating and diarrhea, known as lactose intolerance. This differs from a milk allergy, which is an immune response to milk proteins.

Lactose in Different Dairy Products

Lactose content varies in dairy products due to processing and aging.

Milk: An 8-ounce glass of cow's milk contains about 12 grams of lactose.
Yogurt: Fermentation reduces lactose in plain and Greek yogurt.
Hard Cheeses: Aged cheeses like cheddar and Parmesan contain very little to no lactose.
Lactose-Free Milk: Made by adding lactase to break down lactose, making it digestible for those with intolerance.

Comparing Lactose to Other Common Sugars


Sugar Type	Components	Primary Sources	Sweetness (Relative to Sucrose = 1.0)	Digestion
Lactose	Glucose + Galactose	Dairy products	~0.2-0.4	Requires the enzyme lactase
Sucrose	Glucose + Fructose	Table sugar, fruits, vegetables	1.0 (standard)	Digested by the enzyme sucrase
Fructose	Fructose (monosaccharide)	Fruit, honey, high-fructose corn syrup	1.5-1.8	Processed by the liver
Maltose	Glucose + Glucose	Malted grains, starches	~0.4-0.5	Digested by the enzyme maltase

The Health Benefits of Lactose (for those who can digest it)

For those who can digest it, lactose offers health benefits. It can act as a prebiotic, supporting beneficial gut bacteria. Lactose also helps absorb minerals like calcium and magnesium, important for bone health. Its low glycemic index means a slower rise in blood sugar.

The Rise of Lactose-Free Products

Increased awareness of lactose intolerance has led to the growth of lactose-free dairy. These products are made by adding lactase to milk, pre-digesting the lactose into sweeter glucose and galactose, while maintaining nutritional content. Another method is ultra-filtration to remove lactose. For more on navigating lactose sensitivity, explore resources from organizations like the International Food Information Council (IFIC).

Conclusion

Lactose is the primary sugar in milk and dairy products. This disaccharide of glucose and galactose provides natural energy. While most infants digest it with lactase, many adults become lactose intolerant due to decreased enzyme production. Lactose levels vary in dairy, with hard cheeses having minimal amounts. Lactose-free options and milk alternatives are available for those who cannot tolerate it. For those who can, lactose provides nutritional advantages, including mineral absorption and prebiotic effects. Understanding lactose helps consumers make informed dietary choices.

Frequently Asked Questions

The primary sugar in milk is lactose, also known as 'milk sugar'. It is a disaccharide, meaning it is composed of two simpler sugar units: glucose and galactose.

When you are lactose intolerant, your body produces insufficient amounts of the lactase enzyme needed to break down lactose. This undigested lactose ferments in the colon, leading to gas, bloating, and other digestive discomforts.

Not all dairy contains the same amount of lactose. Hard, aged cheeses like cheddar and Parmesan contain very little, while milk, soft cheeses, and ice cream contain higher levels.

Manufacturers produce lactose-free milk by adding the lactase enzyme to regular milk. This breaks down the lactose into glucose and galactose before it is consumed, making it easy to digest for people with lactose intolerance.

No, the lactose found in plain milk is a naturally occurring sugar, not an added one. Added sugars are those put into a product during processing, typically found in flavored milks, ice cream, and other sweet dairy products.

For those who can digest it, lactose offers benefits such as providing an energy source, aiding in the absorption of calcium and magnesium, and promoting healthy gut bacteria. The benefits of milk as a whole food generally outweigh concerns about its natural sugar content for most people.

Lactose-free milk contains the same amount of total sugar as regular milk, around 12 grams per 8-ounce serving. The only difference is that the lactose has already been broken down into the simpler sugars, glucose and galactose.