No Need To Sugarcoat It: Lactose Is a Healthy Sugar

Written by: Lauren Milligan Newmark, Ph.D. | Issue # 126 | 2025

A healthy sugar might sound like an oxymoron, but not all sugars were created equal. The milk sugar lactose evolved as an energy source for infant mammals, and many of its attributes that benefit infants also extend to adult consumers. Although lactose is probably best known for causing digestive issues in some adults, growing evidence [1-3] for health benefits of moderate lactose consumption—even in individuals who might be genetically predisposed to lactose intolerance—suggests it could be time to rebrand milk’s main carbohydrate as a nutritional good guy rather than a digestive villain.

Origin Story

Lactose is a disaccharide exclusively made by mammary glands. During milk synthesis, the milk protein alpha-lactalbumin acts as an enzyme and binds together one glucose molecule and one galactose molecule. Mammalian milk expert Olav Oftedal proposed two evolutionary advantages that may explain why mammary glands synthesize lactose rather than passing along glucose and galactose as monosaccharides [4,5]. First, more is better: one lactose molecule has half the osmotic concentration of two monosaccharides. As a result, lactose exerts less osmotic pressure per unit mass than glucose (or galactose) and most mammalian milks end up having the same ion concentration as the mother’s plasma. This supports optimal milk production. Second, it’s good to be unique: glucose is prevalent in nature, as are the bacteria that feast on glucose. Only bacteria that evolved the ability to digest lactose could colonize milk. Synthesizing a novel sugar may have protected the mammary gland and infant digestive tract from a large number of microbes.

The very attributes that made lactose evolutionarily advantageous for ancestral mammalian mothers and infants—being complex and novel—are also what make many modern-day adults avoid milk and other dairy foods. Lactose needs to be broken down into its simple sugars during digestion. This is accomplished by the digestive enzyme lactase, made by cells that line the human infant’s small intestine. After infancy, these cells stop making lactase (presumably because it is energetically inefficient to produce an unneeded enzyme). However, shifts in the human diet to include animal milks approximately 10,000 years ago acted as a selective pressure to keep lactase turned on. Today, roughly one third of all modern-day adults have one or two copies of the lactase persistence (LP) gene and the ability to continue to make lactase throughout their lifetime.

The Break Down

In the other two-thirds of adults (who stop producing lactase), lactose passes through the small intestine undigested into the colon, where bacteria ferment and break it down. Some of the gut microbes that dine on lactose produce hydrogen, carbon dioxide, and methane as by-products. These by-products cause the gas, bloating, and discomfort commonly associated with lactose intolerance.

However, intolerance is not the inevitable fate of life without lactase. Research suggests that most lactase non-persistent (LNP) people can handle 12 grams of lactose without noticing symptoms [3], which is roughly equivalent to the lactose content of an eight-ounce glass of cow’s milk. Moreover, not all gut bacteria produce gas because of lactose metabolism. Some gut bacteria can metabolize lactose into lactic acid and short-chain fatty acids. These byproducts do not cause painful side effects, and production of short-chain fatty acids can even have beneficial effects on gut health and immune function, including increasing the integrity of the gut barrier and reducing inflammation in the gut [6, 7].

The ability of undigested lactose to “feed” lactic acid bacteria (e.g., Bifidobacteria), promote their growth, and produce metabolites that improve human health indicates lactose can act as a prebiotic in LNP individuals. Some researchers [3,8,9] have called it a “conditional” prebiotic because these actions depend on the quantity of lactose consumed and the microbe populations present in the gut. There is also evidence suggesting that LNP individuals can shift their gut microbe populations toward those that produce short-chain fatty acids and away from those that cause symptoms of lactose intolerance simply by adding small amounts (<12 grams) of lactose to their daily diet [10-13]. As contradictory as it might sound, prolonged lactose consumption could reduce the incidence of lactose intolerance and turn lactose from a gut-disrupting sugar to a health-promoting prebiotic.

Importantly, the prebiotic actions of lactose may not be exclusive to LNP individuals. In healthy LP individuals, some lactose may resist being broken down by lactase in the small intestine and could reach the colon if the amount of lactose consumed exceeded the individual’s lactase activity [1,3]. Data to support this hypothesis primarily come from infants, making extrapolations to older consumers difficult. Additional research on lactose digestion among LP children, adolescents, and adults is needed to fully understand the prebiotic potential of lactose across all human consumers.

Sugar Without the Spike

Whether you are LP or LNP, lactose must be broken down into smaller, easier to absorb simple sugars so the body can use them for energy and building blocks for molecules such as glycoproteins [1-3]. So much of the focus on lactose digestion is on how the disaccharide is broken up that the when is often overlooked. Complex molecules like lactose take longer to digest than simple molecules. As a result, glucose and galactose are released more slowly into the bloodstream than if they were delivered as monosaccharides.

Lactose has a lower glycemic index (GI, a measure of how quickly a carbohydrate raises blood glucose levels) than glucose. (For reference, the GI of glucose is 100 whereas the GI of lactose is 46)  [1-3]. Carbohydrates with a slower release of glucose into the bloodstream offer several health benefits including improving blood sugar control, lowering the risk of diabetes, and potentially contributing to weight management by helping the consumer feel fuller for a longer period.

Lactose may also help consumers feel full by regulating the concentration of the hormone ghrelin, often referred to as the “hunger hormone” because its production tells the brain that it is time to eat [1,3]. Although much of milk’s satiating effects have been attributed to its protein content, studies have found that consuming lactose reduces ghrelin levels and appetite more than consuming glucose [1]. Apart from experimental studies like these, lactose is not usually consumed as a single ingredient but instead as part of the milk matrix. The observed satiating effects of milk likely result from multiple nutrients, including whey and casein proteins and potentially even lactose.

Bone of Contention

Milk’s matrix also includes essential minerals, such as calcium. Compared with other foods, calcium from milk (and other dairy) has high bioavailability, meaning that much of the calcium that is present in milk can be absorbed and used by the body. Again, milk proteins (particularly casein) may be partly responsible for this effect, but lactose may also have a hand in increasing calcium uptake.

There are two possible mechanisms that could explain lactose’s influence on calcium absorption. First, lactose digestion is associated with a decrease in the pH of the digestive tract. This more acidic environment increases the solubility of calcium, which in turn can increase the passive uptake of this mineral by the intestines [1-3. 14]. Another (not mutually exclusive) possibility is that lactose’s influence on the composition of the gut’s microbe population could lead to increases in intestinal absorption of calcium [14].

Many older adults are in negative calcium balance, meaning they lose more calcium (in urine or stool) than they absorb and deposit into their skeleton. Foods with high bioavailability would be especially important to aging adults looking to maintain strong bones. Although studies from human infants and non-human animals suggest lactose does increase the intestinal absorption of calcium [1-3, 14, 15], whether or not lactose has this effect in humans beyond infancy is a point of contention across researchers [1-3, 14]. Although some studies have found that lactose intake increased calcium absorption among adult study subjects, many have found no effect and others have even reported that lactose impairs calcium absorption [16]. There are also arguments that lactose consumption may increase calcium intake in LNP individuals more so than LP individuals as a result of its prebiotic effect on the gut microbiome [3, 16], providing one more reason why moderate consumption of lactose among LNP adults could be nutritionally beneficial.

No Need to Be Lactose-Free

With the growing popularity of plant-based milks and low-carbohydrate diets, it is important for consumers to understand that lactose is unique among sugars. Whereas some sugars can promote the growth of harmful gut bacteria and increase blood sugar levels, lactose has opposite effects. Consuming too much sugar, especially those found in sugar-sweetened beverages, can negatively impact bone health by increasing calcium excretion. But drinking one glass of lactose-containing milk a day could improve bone health by increasing calcium absorption. Individuals with LNP may be avoiding lactose because of the digestive issues it can cause, but emerging research suggests that they may reap more health benefits and can avoid many of the distressing symptoms by consuming small amounts throughout the day. Lactose may have evolved to provide energy for growing mammalian infants, but it provides a nutritional advantage beyond infancy.

References

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