Why Can't Some Adults Drink Milk? Understanding Lactose Intolerance

October 15, 2025 •

4 min read

According to the National Institutes of Health, an estimated 68% of the world’s population has lactose malabsorption, which is the underlying cause of why can't some adults drink milk. This common digestive issue stems from a reduced ability to break down the sugar found in dairy products, leading to a host of uncomfortable symptoms.

Quick Summary

This article explains why many adults develop difficulty digesting milk, detailing the role of the lactase enzyme. It covers the different types of lactase deficiency and provides guidance on symptoms, diagnosis, and management options for lactose intolerance.

Key Points

Primary Lactase Deficiency: The most common cause of lactose intolerance in adults is a genetically programmed decrease in lactase production after infancy.
Not an Allergy: Lactose intolerance is a digestive issue, not an immune-system-driven allergic reaction to milk protein.
Manageable Symptoms: Symptoms like bloating, gas, and diarrhea are caused by undigested lactose fermenting in the colon and can be controlled with diet or supplements.
Varying Tolerance Levels: Individual sensitivity varies greatly, meaning some people can tolerate small amounts of lactose, while others must be stricter.
Dietary Alternatives and Supplements: Lactase enzyme supplements, lactose-free dairy, and fortified non-dairy alternatives are all effective management strategies.
Ethnic Predisposition: The prevalence of lactose intolerance is highest in populations of East Asian, African, and Hispanic descent.
Risk of Nutritional Deficiency: Individuals avoiding all dairy must ensure adequate intake of calcium and Vitamin D from other sources to prevent bone health issues.

The Core Issue: What is Lactose Intolerance?

Lactose intolerance is not an allergy but a digestive condition caused by the body's inability to produce enough of the enzyme lactase. Lactase is crucial for breaking down lactose, the sugar found in milk and dairy products, into simpler sugars, glucose and galactose, which the body can easily absorb. Without sufficient lactase, undigested lactose travels to the colon. Here, bacteria ferment it, creating gasses and attracting water, which leads to the uncomfortable digestive symptoms characteristic of lactose intolerance.

Primary Lactase Deficiency: The Most Common Cause

The most common reason why some adults can't drink milk is primary lactase deficiency, also known as lactase non-persistence. This is a genetically determined condition where the body gradually produces less lactase after infancy. While all infants produce high levels of lactase to digest their mother's milk, this production naturally declines for most people worldwide after weaning. This age-related decline explains why an individual might tolerate dairy products perfectly well as a child but develop symptoms as an adolescent or adult. The prevalence of this genetic trait varies significantly among different ethnic groups; it is most common in populations of East Asian, West African, Arab, Jewish, Greek, and Italian descent, and least common in those of northern European heritage.

Secondary Lactase Deficiency

In some cases, the reduced lactase production is not due to a natural, genetic decline but is a side effect of an illness, injury, or surgery involving the small intestine. Damage to the lining of the small intestine, where lactase is produced, can temporarily or permanently impair its function. Potential causes of secondary lactase deficiency include:

Gastrointestinal infections, such as viral gastroenteritis.
Inflammatory bowel diseases like Crohn's disease or ulcerative colitis.
Celiac disease.
Certain medications, including long-term antibiotic use.
Chemotherapy or radiation therapy targeting the abdomen.

Symptoms and Diagnosis

Symptoms of lactose intolerance can vary widely in severity and usually appear 30 minutes to 2 hours after consuming lactose-containing foods. Common symptoms include abdominal cramping, bloating, gas, nausea, and diarrhea. Diagnosis often begins with a physician evaluating your symptoms and may involve a temporary lactose elimination diet. Confirmatory tests include:

Hydrogen Breath Test: The most common diagnostic method involves drinking a lactose-containing liquid and measuring the hydrogen levels in your breath over a period of time. Elevated hydrogen indicates that undigested lactose is fermenting in the colon.
Lactose Tolerance Test: After consuming a lactose solution, blood sugar levels are measured. If the levels do not rise sufficiently, it indicates the body isn't absorbing the lactose.
Genetic Testing: A genetic test can determine the likelihood of having primary lactase deficiency by analyzing specific gene variants associated with lactase persistence.

Managing Lactose Intolerance

There is no cure for genetically determined lactose intolerance, but the condition is highly manageable through dietary adjustments and other strategies. Many people can tolerate small amounts of lactose, and finding your personal threshold is key.

Comparison of Management Strategies


Strategy	Description	Pros	Cons
Dietary Modification	Limiting or avoiding high-lactose dairy products.	Simple and highly effective for symptom control.	Requires careful label reading; may lead to low calcium intake if not managed properly.
Lactase Supplements	Oral tablets or drops containing the lactase enzyme.	Allows consumption of dairy; convenient for dining out.	Efficacy can vary; must be taken at the right time before eating.
Lactose-Free Products	Dairy products where lactase has been added during processing.	Taste and texture are similar to regular dairy; provides calcium and nutrients.	Some brands may still contain trace amounts; can be more expensive.
Probiotic Therapy	Ingesting beneficial bacteria that may help digest lactose.	Can improve overall gut health; found in some yogurts with live cultures.	Effectiveness varies by strain and individual; less immediate effect than supplements.

Nutritional Considerations and Hidden Lactose

For those who must significantly reduce or eliminate dairy, it is vital to ensure adequate intake of calcium and Vitamin D to prevent nutritional deficiencies that could lead to bone health issues like osteoporosis. Excellent non-dairy sources of calcium include fortified plant-based milks, fortified cereals, leafy green vegetables, and some nuts and seeds.

It's also important to be aware of 'hidden lactose' in processed foods. Manufacturers often use milk solids, whey, or lactose as an ingredient in items like bread, salad dressings, processed meats, and many baked goods. Reading ingredient labels is a crucial step for managing symptoms effectively.

Conclusion

The inability of some adults to drink milk is a natural, often genetically inherited, phenomenon rather than an illness. Primary lactase deficiency, where the body's lactase production wanes after childhood, is the most common reason, but other underlying health conditions can also play a role. While the symptoms of lactose intolerance can be uncomfortable, they are not dangerous and can be managed effectively with a combination of dietary control, lactase enzyme supplements, and probiotic use. By understanding the root cause, identifying personal tolerance levels, and being mindful of hidden lactose, adults can navigate this common digestive issue and maintain a balanced, healthy diet. For further information and resources, consult a healthcare provider or registered dietitian.

Note: While many people can self-manage, it's advisable to consult a healthcare professional for a proper diagnosis to rule out other digestive conditions like Irritable Bowel Syndrome or a milk protein allergy.

Outbound Markdown Link: For more detailed nutritional guidance on dairy-free living, consult the National Institutes of Health (NIH) resources on lactose intolerance.

Frequently Asked Questions

Lactose intolerance is a digestive problem caused by a lack of the lactase enzyme, resulting in discomfort from undigested milk sugar. A milk allergy is a potentially serious immune system reaction to milk protein, and symptoms can include hives, swelling, and breathing difficulties.

Yes, primary lactose intolerance often develops as individuals age, with lactase production naturally declining after childhood. Secondary lactose intolerance, caused by intestinal damage from illness, injury, or surgery, can also cause a sudden onset of symptoms.

A healthcare provider can offer a definitive diagnosis, but a good first step is a lactose elimination diet. If your symptoms improve after removing dairy, it is a strong indicator. For confirmation, a doctor may perform a hydrogen breath test.

Lactose is used as an additive in many processed foods beyond milk and cheese. Common hidden sources include certain baked goods, processed meats, salad dressings, coffee creamers, and even some medications.

Some individuals can build a tolerance by gradually introducing small amounts of dairy into their diet. Additionally, consuming dairy with other foods can help, as can the use of lactase enzyme supplements.

No, lactose content varies between dairy products. Hard cheeses and yogurt with live cultures often contain less lactose than milk and ice cream, making them better tolerated by many with lactose intolerance.

The most common form of lactose intolerance is genetically determined. A gene variant on Chromosome 2, which regulates lactase production, is responsible for the natural decline of the enzyme after weaning in most humans, with exceptions being populations that historically practiced dairy farming.