The emerging biological functions of lactose: A narrative review

Lactose, the main carbohydrate found in milk, is a disaccharide composed of glucose and galactose linked by a glycosidic bond. Sometimes referred to as ‘milk sugar’, this compound plays a role in various facets of health, from early infancy to adulthood, extending beyond its basic function as a carbohydrate source.¹

To further investigate alternative biological functions of lactose, a 2025 narrative review assessed its role in various physiological processes and health-related outcomes, including calcium absorption and bone health, satiety regulation, cariogenic potential, athletic performance, and potential prebiotic effects.¹ The authors also considered current global patterns of dairy and lactose intake.

Lactose digestion and potential prebiotic effects

Lactose digestion relies on the enzyme lactase-phlorizin hydrolase to split lactose into glucose and galactose. This function peaks after birth and can either remain stable or decline following the weaning phase.

Lactase persistence describes the continued expression of the enzyme lactase-phlorizin hydrolase into adulthood. People with lactase persistence are able to hydrolyze large amounts of lactose and thus can consume large quantities of fresh milk (which is higher in lactose than many other milk products) without issue.

In contrast, lactase non-persistence is a genetically regulated trait in which lactase activity progressively declines after the weaning phase. This reduced enzyme activity can limit lactose digestion, though the extent varies between individuals. It is important to distinguish between lactase non-persistence and lactose intolerance, the latter being the symptomatic outcome of insufficient lactose digestion.

Understanding lactase expression phenotypes is key to interpreting individual responses to dietary lactose and its broader impact on the gut microbiome.

Beyond its role as a carbohydrate, lactose may offer prebiotic benefits. Even in individuals with normal lactase activity, some lactose escapes digestion in the small intestine and is metabolized by gut microbes in the ileum and proximal colon. Factors such as intestinal disorders (e.g., infection, inflammation), lactose amount, timing and frequency, and co-consumption of other foods can influence its digestion.

Long-term lactose intake can support the growth of lactose-digesting bacteria (e.g., bifidobacteria), which produce beneficial metabolites like short-chain fatty acids and lactate. This shift, referred to as colonic adaptation, may reduce the intensity and frequency of gastrointestinal symptoms in lactose malabsorbers. This process yields various bioactive compounds, including short-chain fatty acids, which have been shown to regulate intestinal transit, support gut barrier integrity, stimulate the immune system, and influence gut–brain signaling. However, while evidence suggests potential prebiotic benefits of lactose, scientific consensus has yet to be reached.

Glycemic index and satiety

Dairy foods have widely been recognized for their satiating effects. While these effects are often attributed to their protein content, increasing evidence suggests a potential influence of lactose on satiety.

The authors propose two potential pathways for lactose’s role in satiety regulation:

1. The low glycemic index of lactose and the slow digestion of its galactose component contribute to lower postprandial blood glucose, insulin, hunger, and subsequent lower energy intake compared to glucose.
2. Lactose may help regulate satiety by suppressing ghrelin (also known as the “hunger hormone”) more effectively than glucose, leading to lower appetite and reduced energy intake.

Promoting bone health through increased calcium absorption

The specific role of lactose in supporting bone health through enhanced calcium absorption remains under discussion. Positive effects have been observed in infants, likely because lactose digestion produces organic acids that lower the intestinal pH, which helps increase calcium solubility and its absorption.² However, these benefits have not been shown in lactose-tolerant adults.

Overall, the impact of lactose on bone health appears to be multifactorial, influenced by variables like dosage, food source, age, and both genetic and epigenetic regulation of lactase activity. The well-established benefit of lactose on calcium absorption in infants underscores its distinct role across the lifespan.

Cariogenicity

Cariogenicity refers to the ability of a substance to cause dental caries, the process underlying tooth decay. Different sugars affect dental caries risk in distinct ways, as highlighted in the World Health Organization’s 2022 Global Oral Health Status Report.³ Sucrose, also known as table sugar, is the most cariogenic due to its rapid fermentation in the mouth, increasing acidity in the mouth and creating an environment conducive to tooth decay.

In contrast, the review highlights that lactose is significantly less cariogenic than sucrose, primarily due to its lower acidogenic potential, resulting in slower and reduced acid production and a relatively higher oral pH. The authors also note that other beneficial components of dairy products may contribute to their protective effects against dental caries, including calcium, phosphorus, casein, lactoferrin, lysozyme, and lactoperoxidase.

Sports Performance

Carbohydrates are recognized as an important fuel source before, during, and after exercise.

Before exercise: Carbohydrate intake before endurance exercise is recommended to increase carbohydrate stores. However, the differential effects of specific types of carbohydrates on this process remain under investigation.

• Lactose has been proposed as an effective pre-exercise fuel option due to its oxidation rate being comparable to that of glucose.

During exercise: The ingestion of readily oxidizable carbohydrates is recommended during prolonged bouts of exercise (>45 mins) to optimize performance. While glucose, glucose polymers, and glucose-fructose mixtures are commonly highlighted in the literature, moderate lactose intake (48g/hr) is emerging as a viable option, as it is readily oxidized and supports fat oxidation while preserving endogenous carbohydrate.

Post-exercise: Lactose may support postexercise recovery by supplying glucose and galactose to help restore glycogen stores, with galactose specifically promoting liver glycogen resynthesis.

Although lactose is not yet widely incorporated into sports nutrition guidelines, growing evidence highlights its potential as a beneficial addition to the diets of athletes and recreationally active individuals.

Current global trends in dairy products and lactose consumption by continent

The extent to which the benefits of lactose can be realized depends largely on both dairy consumption patterns and effective lactose digestion. Global dairy consumption is projected to increase by 1.2% annually.

High-dairy intake regions: Regions with high dairy intake such as North America, Europe, and Oceania (notably Australia) show the highest per capita consumption of dairy. Patterns vary considerably among South American countries, often influenced by household income. In these regions, relatively higher prevalence of the lactase persistence phenotype is reported, often influenced by ethnicity, age, and dietary patterns. Notably, dairy intake patterns are shaped by sociocultural and socioeconomic factors as well as milk product accessibility and availability.

Low-dairy intake regions: Overall, Asia and Africa have lower dairy consumption, with generally low lactose persistence rates but varying significantly across countries and population groups. Despite lower overall intake, demand for dairy, particularly lactose-free and low-lactose products, is expected to grow. The authors note that consumption patterns remain highly dependent on factors including lactase persistence/ lactase non-persistence phenotypes, age, sex, sociocultural and economic status, availability, accessibility, and the country of residence.