What is Lactose and Its Functions in the Body?

Lactose, often called milk sugar, comprises approximately 2–8% of milk by weight across various mammals. As a disaccharide found naturally in milk and dairy products, it serves as a vital energy source, especially for infants. However, its role extends far beyond basic nutrition, influencing everything from bone development to gut health and immune function.

Quick Summary

Lactose is a milk sugar composed of glucose and galactose, which is broken down by the enzyme lactase for energy. Its functions include aiding mineral absorption, supporting the gut microbiome as a prebiotic, contributing to satiety, and fueling athletic performance. The body's ability to digest it varies, impacting tolerance.

Key Points

• Source of Energy: Lactose is a disaccharide broken down into glucose and galactose, providing energy, especially crucial for infants.

• Enhances Mineral Absorption: It aids in the absorption of key minerals like calcium, magnesium, and zinc, which are vital for bone health.

• Supports Gut Health: Undigested lactose acts as a prebiotic, promoting the growth of beneficial bacteria like Bifidobacterium and Lactobacillus in the colon.

• Contributes to Brain Development: Its galactose component is necessary for synthesizing important neural components, including cerebrosides and gangliosides.

• Lower Cariogenic Potential: Lactose is less likely to cause dental cavities compared to other sugars because it is fermented more slowly by oral bacteria.

• Regulates Satiety: It has a relatively low glycemic index and may help regulate appetite by influencing the hunger hormone ghrelin.

• Determines Milk Volume: During synthesis in the mammary gland, lactose's osmotic properties play a significant role in determining the overall volume of milk produced.

The Composition and Synthesis of Lactose

Lactose is a disaccharide, meaning it is a sugar composed of two simple sugar units, or monosaccharides, linked together. Specifically, lactose consists of one molecule of D-galactose and one molecule of D-glucose joined by a $\beta-1\rightarrow4$ glycosidic bond. In the body, this bond is broken down by the enzyme lactase, which resides in the small intestine.

Unlike other common sugars, lactose is synthesized exclusively in the mammary glands of mammals. This process is controlled by an enzyme complex called lactose synthase, which combines glucose and UDP-galactose within the mammary epithelial cells. The concentration of lactose in milk is a key factor influencing milk volume, as lactose is a primary osmotic component. Human milk contains a high percentage of lactose, providing a significant portion of an infant's energy needs.

The Multifaceted Functions of Lactose

Lactose plays several critical roles in the body, particularly during infancy, and offers various benefits for those who can digest it. These functions go beyond merely serving as a source of energy:

• Provides a vital energy source: When broken down by lactase, lactose yields glucose and galactose, both of which can be readily absorbed and used for energy. For infants, especially, lactose provides a crucial source of calories for growth and development.
• Enhances mineral absorption: Lactose has been shown to improve the absorption of essential minerals like calcium, magnesium, and zinc, particularly in infants. By lowering the pH of the gut, lactose increases the solubility of these minerals, making them more available for the body to absorb.
• Acts as a prebiotic: Some ingested lactose, particularly in individuals with reduced lactase activity, reaches the colon undigested. Here, it is fermented by beneficial gut bacteria, such as Bifidobacterium and Lactobacillus, which supports a healthy gut microbiome. This prebiotic effect can aid digestion and contribute to immune system function.
• Supports brain development: The galactose component of lactose is crucial for the formation of certain macromolecules, such as cerebrosides and gangliosides, which are important constituents of nerve cell membranes. Galactose is particularly important for the myelination process during infancy.
• Low cariogenic potential: Compared to other sugars like sucrose, lactose is significantly less cariogenic, meaning it is less likely to cause dental cavities. It is fermented more slowly by oral bacteria and does not lead to the same drastic drop in oral pH that promotes tooth decay.
• Regulates satiety: Some evidence suggests that lactose may have a stronger satiating effect than other carbohydrates, possibly due to its lower glycemic index and potential influence on hunger hormones like ghrelin.

The Issue of Lactose Intolerance

The ability to digest lactose is not universal. Lactose intolerance is a condition characterized by a deficiency of the enzyme lactase. For individuals with this condition, undigested lactose travels to the large intestine, where it is fermented by bacteria, producing gases and other byproducts. This leads to uncomfortable gastrointestinal symptoms such as bloating, gas, diarrhea, and abdominal pain. While many adults worldwide experience a natural decline in lactase production after weaning, the severity of symptoms can vary greatly.

Lactose vs. Other Carbohydrates

Feature	Lactose	Sucrose (Table Sugar)	Glucose	Fructose
Classification	Disaccharide	Disaccharide	Monosaccharide	Monosaccharide
Composition	Glucose + Galactose	Glucose + Fructose	Single Unit	Single Unit
Source	Milk and dairy products	Sugarcane, sugar beets	Grains, fruits	Fruits, honey
Sweetness (relative to sucrose)	0.2 to 0.4	1.0 (baseline)	0.6 to 0.7	1.3
Glycemic Index	46 to 65	68 to 92	100 to 138	19 to 27
Cariogenic Potential	Low	High	Medium to High	Medium
Primary Digestion	Lactase in the small intestine	Sucrase in the small intestine	Absorbed directly	Absorbed directly

Conclusion

Far from being just a simple sugar, lactose is a complex carbohydrate with a diverse range of functions that support overall health. From its fundamental role in providing energy for infants to its capacity to enhance mineral absorption and promote a healthy gut microbiome, lactose is a valuable component of dairy. While those with lactose intolerance need to manage their intake, understanding its composition and multifaceted biological functions highlights its importance in both human development and nutrition. For most, moderate consumption can offer significant health benefits throughout life.

Alternative biological functions of lactose: a narrative review

Frequently Asked Questions

Is lactose a natural sugar?

Yes, lactose is a natural sugar found almost exclusively in the milk of mammals, and is composed of two simpler sugar units: glucose and galactose.

What is the primary function of lactose in milk?

The primary function of lactose in milk is to serve as a vital energy source for the newborn mammal. As an osmotic component, it also helps determine the volume of milk produced.

How does the body break down lactose?

The body breaks down lactose using the enzyme lactase, which is produced in the small intestine. This enzyme splits lactose into its two monosaccharides, glucose and galactose, which are then absorbed into the bloodstream.

Can lactose be beneficial for adult health?

Yes, for lactose-tolerant adults, moderate intake can offer several benefits. These include aiding mineral absorption, acting as a prebiotic for gut health, contributing to satiety due to its lower glycemic index, and potentially supporting athletic performance.

What happens if the body doesn't produce enough lactase?

If the body does not produce enough lactase, undigested lactose travels to the colon. There, it is fermented by gut bacteria, which can lead to symptoms of lactose intolerance, such as gas, bloating, and diarrhea.

Is lactose-free milk really lactose-free?

Lactose-free milk is made by adding the lactase enzyme to regular milk during production. This breaks the lactose down into glucose and galactose, which is why it often tastes sweeter, but the product itself contains virtually no lactose.

How is lactose different from sucrose?

Lactose is a disaccharide made of glucose and galactose, found in milk, with a low sweetness and glycemic index. Sucrose is made of glucose and fructose, found in table sugar, and has a higher sweetness and glycemic index.