Why Can't We Digest Lactose? The Science of Lactose Intolerance

November 16, 2025 •

4 min read

Globally, an estimated 65% of adults have a reduced ability to digest lactose after infancy, a condition known as lactase non-persistence. The inability to properly digest lactose, the sugar in milk, is due to a deficiency of a specific enzyme. This article explores the biological reasons behind this common digestive issue and its surprising evolutionary history.

Quick Summary

This article explains how the lack of the lactase enzyme in the small intestine causes lactose intolerance. It details the digestive process, different types of lactase deficiency, genetic background, and resulting symptoms. The content also provides practical dietary management strategies.

Key Points

Lactase Deficiency is Key: The inability to digest lactose is caused by a lack of the lactase enzyme in the small intestine, which is needed to break down milk sugar.
Undigested Lactose Ferments: Without lactase, unabsorbed lactose travels to the large intestine, where bacteria ferment it and produce uncomfortable gas, bloating, and other digestive issues.
Lactase Non-Persistence is Common: For a majority of the world's population, lactase production naturally decreases after infancy, which is the ancestral human condition.
Lactase Persistence is an Adaptation: The ability to digest lactose throughout life is a recent evolutionary trait, strongly selected for in populations with a long history of dairy farming.
Multiple Types of Deficiency Exist: In addition to the common primary form, lactase deficiency can be temporary (secondary) due to intestinal illness or present from birth (congenital).
It Differs from a Milk Allergy: Lactose intolerance is a digestive problem, not an immune-system-based allergy to milk proteins, and is generally less severe.
Several Management Options are Available: Dietary control, lactose-free products, and lactase enzyme supplements can effectively manage symptoms for most people.

The Core Problem: A Lack of the Lactase Enzyme

At the heart of why many people can't digest lactose is a simple, biological issue: the small intestine doesn't produce enough of the digestive enzyme called lactase. During digestion in a person with sufficient lactase, the enzyme breaks down the complex milk sugar lactose into two simpler, absorbable sugars: glucose and galactose. These smaller sugar molecules are then easily absorbed into the bloodstream from the small intestine.

When lactase production is low, the lactose remains undigested and travels to the large intestine. Here, the resident gut bacteria ferment the unabsorbed sugar. This bacterial fermentation is the cause of the common and unpleasant symptoms associated with lactose intolerance, including bloating, gas, stomach cramps, and diarrhea.

The Three Types of Lactase Deficiency

Lactose intolerance is not a single condition but rather a symptom of several types of lactase deficiency, each with a different cause.

Primary Lactase Non-Persistence

This is the most common form of lactose intolerance and is genetically determined. All humans are born with the ability to digest lactose to process breast milk. However, in most global populations, this ability naturally decreases after infancy and weaning. In these individuals, the gene responsible for lactase production is gradually switched off. This is a normal and ancestral state for humans, with the ability to digest milk into adulthood being a relatively recent evolutionary adaptation.

Secondary Lactase Deficiency

Unlike the genetic, permanent primary form, secondary lactase deficiency is caused by an injury or disease affecting the small intestine where lactase is produced. Conditions that can damage the intestinal lining and cause a temporary reduction in lactase include gastrointestinal infections, celiac disease, bacterial overgrowth, or inflammatory bowel diseases like Crohn's disease. Treating the underlying condition can often restore lactase levels over time.

Congenital and Developmental Lactase Deficiency

This is a very rare form of genetic lactose intolerance where a baby is born with no lactase enzyme activity at all. It is a serious condition that must be diagnosed shortly after birth. Developmental lactase deficiency occurs in premature infants, as the small intestine does not fully develop the lactase-producing cells until late in the third trimester. This is usually a temporary issue that resolves as the infant matures.

The Evolutionary Trait of Lactase Persistence

The ability to digest lactose into adulthood, known as lactase persistence, is a genetic mutation that spread in populations with a history of dairy farming. Strong evolutionary selection favored this trait in early farming communities, particularly in Northern European populations, where milk was a reliable source of nutrition. Genetic analyses show that this beneficial mutation arose independently in several different locations and populations across the world. Today, the prevalence of lactose intolerance is significantly lower in areas with a long history of dairying, like Northern Europe, and much higher in populations in Asia and Africa with no historical ties to dairy farming.

Practical Management and Treatment Options

For most individuals with lactose intolerance, the condition is manageable and not a serious health risk. Management primarily involves controlling dietary intake of lactose to a level that can be tolerated without causing symptoms.

Dietary Adjustments: Many people can tolerate small amounts of lactose. Experimenting with different dairy products is key, as some contain less lactose than others. Hard cheeses (like cheddar and Swiss) and yogurts with live, active cultures are often better tolerated.
Lactose-Free Products: Supermarkets offer a wide range of lactose-free milk, ice cream, and other dairy items. These products contain the lactase enzyme added during processing to break down the lactose beforehand.
Lactase Supplements: Over-the-counter lactase enzyme tablets or drops, like Lactaid, can be taken just before consuming dairy to aid digestion and reduce symptoms.
Nutrient Awareness: Those significantly reducing dairy intake should ensure they get enough calcium and vitamin D from other sources to avoid deficiencies. Good sources include fortified plant-based milks, leafy greens, and fatty fish.

Comparison: Lactose Intolerance vs. Milk Allergy

It is critical to distinguish between lactose intolerance and a milk allergy. They are fundamentally different conditions with separate causes and consequences.


Feature	Lactose Intolerance	Milk Allergy
Cause	Inadequate production of the enzyme lactase, preventing proper digestion of the milk sugar, lactose.	An immune system response to the proteins in milk, casein and whey.
Physiological Reaction	A digestive issue. Undigested lactose ferments in the large intestine, causing gastrointestinal symptoms.	An allergic reaction involving the immune system. Can range from mild to severe, including life-threatening anaphylaxis.
Symptoms	Primarily gastrointestinal: bloating, gas, diarrhea, abdominal pain.	Can affect multiple systems: hives, swelling, wheezing, vomiting, or breathing difficulties.
Severity	Uncomfortable but generally harmless. Symptom severity depends on the amount of lactose consumed and individual sensitivity.	Can be severe and potentially fatal, even with very small amounts of milk protein.
Onset	Symptoms typically appear 30 minutes to 2 hours after consuming dairy.	Allergic reactions can occur within minutes to hours after ingestion.
Management	Dietary adjustments to limit lactose, use of lactase enzyme supplements.	Strict avoidance of all dairy protein. Requires an allergy action plan and possibly epinephrine.

Conclusion

Understanding why we can't digest lactose reveals a fascinating interplay of genetics and evolution. For many, lactase deficiency is the default biological state, while the ability to digest milk into adulthood is a genetic adaptation tied to a history of dairy consumption. While symptoms can be uncomfortable, they are manageable with informed dietary choices, lactase supplements, and by understanding the difference between a digestive intolerance and a potentially dangerous immune-system-based allergy. By adopting these strategies, those with lactose intolerance can successfully navigate their dietary needs while maintaining good nutrition.

Frequently Asked Questions

Lactose intolerance is a digestive issue caused by a lack of the lactase enzyme, which prevents the proper digestion of milk sugar. A milk allergy is an immune system reaction to milk proteins (casein and whey) and can cause more severe, potentially life-threatening symptoms.

The ability to digest lactose as an adult, known as lactase persistence, is a genetic trait. The majority of the global population naturally experiences a decrease in lactase production after weaning. However, in populations with a history of dairy farming, a genetic mutation emerged and spread, allowing continued lactose digestion.

Not necessarily. Many people with lactose intolerance can tolerate small amounts of lactose, and some dairy products contain less lactose than others. Hard, aged cheeses like cheddar and Swiss, as well as yogurt with active cultures, are often easier to digest.

Yes, over-the-counter lactase supplements contain the enzyme needed to break down lactose. Taking these tablets or drops just before consuming dairy products can significantly improve digestion and reduce symptoms for many individuals.

You can get sufficient calcium from other sources. These include calcium-fortified plant-based milks (soy, almond, oat), leafy green vegetables, canned salmon with bones, and tofu. Supplements for calcium and vitamin D can also be an option.

Diagnosis can be done through a lactose tolerance test (measuring blood glucose after consuming lactose), a hydrogen breath test (measuring hydrogen levels produced by gut bacteria), or by observing symptom changes after eliminating dairy from the diet. A doctor's confirmation is recommended.

Yes. Most cases of primary lactose intolerance begin gradually after childhood as lactase production naturally declines. Additionally, secondary lactose intolerance can develop suddenly as a result of an illness, injury, or surgery involving the small intestine.