What are the carbs in lactose?

December 6, 2025 •

4 min read

Approximately two-thirds of the world's adult population experiences a decrease in their ability to digest lactose over time. This highlights the importance of understanding what are the carbs in lactose and how they are processed by the body. Lactose is a disaccharide, or a simple carbohydrate, composed of two smaller sugar units joined together.

Quick Summary

Lactose is a simple carbohydrate or disaccharide found naturally in milk and dairy products. It is composed of two smaller sugar molecules: glucose and galactose. This carbohydrate requires the enzyme lactase for proper digestion in the small intestine, breaking it down into its constituent parts for absorption.

Key Points

Lactose is a disaccharide: This simple carbohydrate is composed of two single sugar molecules, glucose and galactose.
Lactase is the key enzyme for digestion: The lactase enzyme, produced in the small intestine, is necessary to break down lactose into absorbable glucose and galactose.
Lactase production can decline with age: A majority of the global population naturally experiences reduced lactase production after infancy, which can lead to lactose intolerance.
Lactose intolerance is not an allergy: It is a digestive condition resulting from insufficient lactase, which is different from a milk allergy, an immune response to milk proteins.
Undigested lactose causes symptoms: When not properly broken down, lactose ferments in the colon, producing gas, bloating, and diarrhea.
Lactose content varies in dairy products: Fermented and aged dairy products like hard cheeses have much lower lactose levels than milk.
Options are available for the intolerant: Lactose-free products and over-the-counter lactase supplements can help manage symptoms.
Undigested lactose has prebiotic effects: For those with reduced lactase, the undigested lactose can serve as a prebiotic, fostering beneficial gut bacteria.

Understanding Lactose as a Carbohydrate

To understand what are the carbs in lactose, one must first recognize its chemical composition and its place within the carbohydrate family. Carbohydrates are categorized based on their chemical structure and the number of sugar units they contain. These categories include monosaccharides (single sugar units), disaccharides (two sugar units), and polysaccharides (multiple sugar units).

Lactose as a Disaccharide

Lactose is classified as a disaccharide, meaning it is made up of two single sugar molecules, or monosaccharides, joined together. This specific structure is why it is often referred to as 'milk sugar,' since it is the primary carbohydrate found in the milk of mammals. The two monosaccharide units that form lactose are glucose and galactose, which are linked by a β-1→4 glycosidic bond.

The Role of Lactase in Digestion

For lactose to be absorbed and used for energy, the body must first break this bond. The enzyme responsible for this process is lactase, which is produced in the small intestine. Lactase hydrolyzes the lactose molecule, splitting it into one molecule of glucose and one molecule of galactose. These two simpler sugars are then easily absorbed into the bloodstream for energy.

Infant Digestion: Nearly all infants are born with high levels of lactase to digest their mother's milk, which is rich in lactose. This is a crucial adaptation for early nutrition.
Adult Adaptation: After weaning, lactase production typically decreases significantly in a large portion of the global population.
Lactase Persistence: However, some populations, particularly those with a history of pastoralism in Northern Europe, have developed a genetic trait known as lactase persistence, allowing them to continue producing lactase into adulthood.

The Breakdown and Effects of Lactose Carbohydrates

When lactase production declines, a person may become lactose intolerant. This means they cannot efficiently break down lactose, which then passes undigested into the large intestine. The undigested lactose draws water into the intestine, leading to watery stools. In the large intestine, bacteria ferment the lactose, producing gases and short-chain fatty acids. This fermentation process is what causes the classic symptoms of lactose intolerance.

Symptoms of Undigested Lactose

If lactose is not properly digested, it can trigger a range of gastrointestinal issues. The severity of these symptoms depends on the amount of lactose consumed and the individual's level of lactase deficiency.

Bloating and Gas: The fermentation of lactose by intestinal bacteria produces excessive gas, leading to bloating and flatulence.
Abdominal Cramps: Gas and water retention can cause pain and cramping in the abdominal area.
Diarrhea: The osmotic effect of undigested lactose pulls water into the bowel, resulting in diarrhea.

Health Implications Beyond Digestion

Interestingly, lactose also has other physiological effects. As a slower-digesting carbohydrate, it can provide a more sustained energy release. Furthermore, undigested lactose can act as a prebiotic, serving as a food source for beneficial bacteria in the gut and potentially promoting a healthier gut microbiome.

Comparison of Dairy Products and Lactose Content

The amount of lactose varies significantly between different dairy products, which is important for individuals managing lactose intolerance. Some products undergo fermentation or aging, which naturally reduces their lactose content. A comprehensive guide to lactose content in common dairy items is provided below. For those with significant lactose intolerance, opting for products with lower lactose or consuming lactase enzyme supplements can help manage symptoms.


Dairy Product	Lactose Content	Notes
Milk	High	Contains approximately 12g of lactose per 250mL serving.
Yogurt (with live cultures)	Low to Medium	The live bacterial cultures help pre-digest the lactose, making it easier to tolerate for many individuals.
Hard Cheeses (e.g., Cheddar, Swiss)	Very Low	The aging process breaks down most of the lactose, leaving minimal residual amounts.
Cottage Cheese	Medium	Contains more lactose than hard cheeses but less than milk.
Butter	Very Low	Has a negligible amount of lactose due to the fat content.
Lactose-Free Milk	None	Processed to remove or break down lactose using added lactase enzyme.

Conclusion: The Final Word on Lactose Carbs

In conclusion, the carbohydrates in lactose are not a single unit but a compound of two simple sugars: glucose and galactose. While most infants can easily digest this milk sugar, the ability to do so often decreases in adulthood due to a natural reduction in the lactase enzyme. For those with lactose intolerance, consuming undigested lactose leads to fermentation in the colon, causing a range of uncomfortable digestive symptoms. However, understanding the lactose content in different dairy products allows for informed dietary choices, enabling many individuals to manage their intolerance effectively and even benefit from the prebiotic effects of unabsorbed lactose. For more on this topic, exploring resources like the International Food Information Council (IFIC) can provide further clarity on how lactose impacts health.

Frequently Asked Questions

Lactose is a disaccharide, or a 'double sugar,' made of two smaller sugar units linked together: glucose and galactose. It is the main carbohydrate naturally found in milk.

Lactose is a simple carbohydrate. This is because its molecule contains only two sugar units, classifying it as a disaccharide, unlike complex carbohydrates (polysaccharides) which contain three or more sugar units.

People who have trouble digesting lactose have insufficient levels of the enzyme lactase in their small intestine. Without enough lactase, the lactose cannot be broken down into glucose and galactose for absorption.

When lactose is not digested in the small intestine, it travels to the large intestine where it is fermented by gut bacteria. This fermentation process produces gas, causing symptoms like bloating, cramping, and diarrhea.

Products like hard, aged cheeses (e.g., Cheddar, Swiss) and yogurt with live cultures are typically low in lactose. The aging and fermentation processes break down much of the lactose.

Many people with lactose intolerance can consume smaller portions of dairy, especially aged or fermented products. Over-the-counter lactase enzyme supplements can also be taken with dairy to aid in digestion.

Yes, lactose is the primary carbohydrate found in human milk. It is a vital energy source for infants and provides about 40% of their energy needs.

Lactose intolerance is a digestive problem caused by a lactase deficiency, while a milk allergy is an immune system response to the protein in milk. The symptoms and management strategies are very different for each condition.

When properly digested by the lactase enzyme, lactose is broken down into glucose and galactose, which the body can use as a source of energy. It provides a slower, more sustained energy release compared to some other simple sugars.