What Type of Carb is Lactose? Understanding This Milk Sugar

December 5, 2025 •

2 min read

Lactose is the sole natural source of this specific carbohydrate for infant mammals, with human milk containing a higher concentration than cow's milk. Understanding what type of carb is lactose, the primary sugar found in milk, is key to comprehending its role in both digestion and nutrition.

Quick Summary

Lactose is a simple carbohydrate classified as a disaccharide. It consists of two smaller sugar units, glucose and galactose, and is the main sugar in milk.

Key Points

Disaccharide Classification: Lactose is a simple carbohydrate classified as a disaccharide, meaning it is a 'double sugar' composed of two smaller sugar units.
Component Sugars: This milk sugar is formed from one molecule of glucose and one molecule of galactose, which are chemically linked together.
Enzyme Dependency: Digestion of lactose requires the enzyme lactase, which breaks the bond between glucose and galactose in the small intestine.
Energy Source: Lactose is an important source of energy, particularly for infant mammals, and can aid in the absorption of minerals like calcium.
Lactose Intolerance Cause: A decrease or absence of the lactase enzyme leads to lactose malabsorption, resulting in common symptoms of lactose intolerance.
Prebiotic Potential: If not fully digested, lactose can travel to the large intestine where it can be fermented by gut bacteria, acting as a prebiotic.
Nutritional Variation: The concentration of lactose varies among different mammal milks, impacting its nutritional role depending on the source.

The Classification of Carbohydrates

Carbohydrates are one of the three main macronutrients found in food, serving as the body’s primary source of energy. They are broadly classified into three types based on their chemical structure: monosaccharides, disaccharides, and polysaccharides.

Monosaccharides: The Simplest Sugars

Monosaccharides, or “single sugars,” are the basic building blocks of all other carbohydrates. They include glucose, fructose, and galactose, all of which have the same chemical formula ($C6H{12}O_6$) but differ in their atomic arrangement. Your body absorbs these simple sugars directly into the bloodstream for immediate energy use.

Disaccharides: The Double Sugars

Disaccharides are formed when two monosaccharides are chemically bonded together. The most common disaccharides in our diet are:

Sucrose: Formed from glucose and fructose, commonly known as table sugar.
Maltose: Formed from two glucose molecules, often found in malted beverages and grains.
Lactose: Formed from a molecule of glucose and a molecule of galactose, famously known as milk sugar.

Polysaccharides: Complex Carbohydrates

Polysaccharides are “many sugars” composed of long chains of monosaccharides. Examples include starch, a plant-based energy storage molecule, and glycogen, the animal equivalent stored in the liver and muscles. These complex carbohydrates require more time for digestion than simple sugars.

Lactose: The Disaccharide of Milk

Lactose is a simple carbohydrate, more precisely a disaccharide composed of a glucose molecule linked to a galactose molecule via a β-1,4-glycosidic bond. This structure makes it the primary sugar found in milk.

Digestion and Absorption of Lactose

Digesting lactose requires the enzyme lactase, produced in the small intestine. Lactase breaks the bond between glucose and galactose, allowing these monosaccharides to be absorbed into the bloodstream.

The Challenge of Lactose Intolerance

Many individuals lose the ability to produce sufficient lactase after infancy, leading to lactose malabsorption. When undigested lactose reaches the large intestine, gut bacteria ferment it, causing gas, bloating, and diarrhea characteristic of lactose intolerance.

The Nutritional Role of Lactose

Lactose provides energy, particularly for infants. It may also enhance the absorption of minerals like calcium and can act as a prebiotic, supporting beneficial gut bacteria.

Comparison of Common Dietary Carbohydrates


Feature	Lactose (Disaccharide)	Sucrose (Disaccharide)	Starch (Polysaccharide)
Classification	Simple Carbohydrate	Simple Carbohydrate	Complex Carbohydrate
Composition	Glucose + Galactose	Glucose + Fructose	Chains of Glucose
Source	Milk, Dairy Products	Sugar Cane, Sugar Beets	Grains, Potatoes, Rice
Digestion Speed	Rapid (Requires lactase)	Rapid (Requires sucrase)	Slower (Requires amylase)
Sweetness	Mild	Very Sweet	Not Sweet
Digestion Issue	Lactose Intolerance	Minor Issues	Not Applicable

Conclusion: Lactose's Impact on Health and Digestion

Lactose is a simple carbohydrate and a disaccharide made of glucose and galactose. Its digestion relies on the lactase enzyme. Decreased lactase production leads to lactose intolerance in many people. Lactose serves as an energy source, aids calcium absorption, and has prebiotic potential. For those with intolerance, lactose-free options are available to enjoy the nutritional benefits of dairy. Understanding what type of carb is lactose is important for managing diet and digestive health. The National Institutes of Health offers more information on carbohydrate metabolism.

Frequently Asked Questions

Lactose is a simple carbohydrate. Simple carbohydrates include monosaccharides (single sugars) and disaccharides (double sugars), and because lactose is a disaccharide, it falls into this category.

Lactose is formed from two monosaccharide units: glucose and galactose.

The body digests lactose in the small intestine using the enzyme lactase, which breaks the glycosidic bond connecting the glucose and galactose units so they can be absorbed into the bloodstream.

Both lactose and sucrose are disaccharides, but they are composed of different monosaccharide units. Lactose consists of glucose and galactose, while sucrose consists of glucose and fructose.

The lactose content in cheese varies significantly depending on how it is made and aged. During the cheesemaking process, much of the lactose is removed with the whey. In aged cheeses, remaining lactose is further broken down by bacteria, resulting in very low levels.

The severity of lactose intolerance varies. Some individuals can tolerate small amounts of lactose, while others might tolerate fermented products like yogurt or aged cheese, where bacteria have already broken down much of the lactose.

When not fully digested in the small intestine, lactose travels to the large intestine where gut bacteria ferment it. This process produces gas and acids that can cause bloating, pain, and diarrhea.