The Biological Basis of Lactose Intolerance
The most prominent reason why some people are 'not supposed' to drink milk is a natural, genetically programmed decline in lactase production. Lactase is the enzyme necessary to break down lactose, the main sugar in milk, into simpler, digestible forms (glucose and galactose).
Lactase Persistence vs. Lactase Non-persistence
Historically, humans were only meant to consume milk during infancy. After the weaning period, the body would naturally stop producing large amounts of lactase, a state known as lactase non-persistence. However, a genetic mutation occurred in some populations, primarily in Northern Europe, that allowed individuals to continue producing lactase into adulthood. This adaptation, called lactase persistence, became advantageous in societies that domesticated cattle and relied on dairy for nutrition.
Today, lactase non-persistence remains the global biological default, affecting a majority of adults, especially those of Asian, African, and Indigenous American descent.
What happens with lactose malabsorption?
For people with low lactase levels, undigested lactose travels to the large intestine. There, colonic bacteria ferment it, leading to a cascade of unpleasant gastrointestinal symptoms.
These symptoms can include:
- Bloating and abdominal cramps
- Intestinal gas and flatulence
- Diarrhea
- Nausea
The severity of symptoms depends on an individual's level of lactase deficiency, the amount of lactose consumed, and other gut health factors.
Health Concerns and Controversies
Beyond simple intolerance, a number of health issues and controversies surround dairy consumption, contributing to the idea that milk may not be ideal for all humans.
Dairy Allergies vs. Intolerance
It is critical to distinguish between lactose intolerance and a milk allergy. An allergy is an immune system reaction to milk proteins, most often casein or whey. Symptoms can range from hives and digestive upset to severe, life-threatening anaphylaxis. This is distinct from lactose intolerance, which is a digestive issue rather than an immune response.
Acne and Inflammation
Studies have linked milk consumption, particularly skim and low-fat varieties, with an increased risk of acne. The theory suggests that milk's influence on certain hormones, such as insulin and insulin-like growth factor-1 (IGF-1), can exacerbate acne issues. Additionally, some individuals may have a sensitivity to the milk protein casein, which can trigger inflammation throughout the body.
Potential Cancer Risks
Research on the link between dairy and cancer is mixed, but some findings have raised concerns. High dairy intake has been associated with a potentially increased risk of prostate cancer in men. Conversely, other studies have shown a link between dairy and a lower risk of colorectal cancer. The reasons for these inconsistencies are not fully understood and more research is needed.
Saturated Fat and Hormones
Dairy products are a major source of saturated fat in many diets, which can increase the risk of heart disease. For this reason, low-fat or skim milk is often recommended for adults. The presence of hormones in milk is another point of contention, with some studies exploring potential effects, though the evidence is not conclusive.
The Nutritional Debate: Cow's Milk vs. Alternatives
For those who choose to or must avoid dairy, finding alternative sources of key nutrients is essential. The table below compares the nutritional profiles of cow's milk with some popular plant-based alternatives.
| Feature | Cow's Milk (Full-Fat) | Soy Milk (Fortified) | Almond Milk (Fortified) | Oat Milk (Fortified) |
|---|---|---|---|---|
| Calories | 149 kcal | 80-100 kcal | 40-90 kcal | 120-150 kcal |
| Protein | 8 g | 7-9 g | 1 g | 3-4 g |
| Calcium | ~300 mg (25% DV) | ~300 mg (fortified) | ~450 mg (fortified) | ~450 mg (fortified) |
| Saturated Fat | 5 g | 0.5 g | 0 g | 0.5 g |
| Vitamin D | Fortified (~25% DV) | Fortified (~25% DV) | Fortified (~25% DV) | Fortified (~25% DV) |
Note: Nutritional information can vary significantly between brands and formulations. Always check the product label for specific values.
Calcium from non-dairy sources
Calcium, often cited as a key benefit of dairy, is readily available from many other sources. These include leafy greens (kale, collard greens), beans, fortified juices, tofu, and other vegetables. A well-planned diet can provide sufficient calcium without any dairy products.
Protein from non-dairy sources
High-quality protein is abundant in plant-based sources like soy milk, legumes, nuts, and seeds. As shown in the table, soy milk contains a comparable amount of protein to cow's milk, and other alternatives can contribute to daily protein goals as part of a balanced diet.
Navigating the Dairy Decision: A Balanced Perspective
Ultimately, whether humans 'should' drink milk is a complex question with no single answer, depending on individual genetics, dietary needs, ethical considerations, and personal tolerance. For many, particularly those with lactase persistence, dairy can be a nutrient-dense part of a healthy diet. For the majority of the global population, and for those concerned about allergies or other health risks, abstaining from or moderating dairy intake is the natural, healthy choice.
The key is to be informed. Understanding the biological context of lactose intolerance and the potential health concerns associated with dairy allows individuals to make the best decision for their body. If dairy is limited or avoided, a thoughtful approach to nutrition is necessary to ensure adequate intake of calcium, vitamin D, and protein from other food sources.
Conclusion
The perception that humans 'shouldn't' drink milk is not a universal truth but is rooted in several scientific realities. Lactose intolerance, the most common factor, affects a large portion of the adult population and is a normal evolutionary outcome. While dairy provides valuable nutrients, concerns regarding allergies, hormonal impacts, and certain health conditions lead many to seek alternatives. As an adult, milk is not a dietary necessity; a balanced diet can provide all necessary nutrients from a wide variety of sources, both dairy and non-dairy. The decision is a personal one, best made with full awareness of one's own physiological response and nutritional requirements. For more information on dairy alternatives, see this guide from the American Society for Nutrition.
Frequently Asked Questions
Is lactose intolerance a disease?
No, lactose intolerance is not considered a disease but rather the normal biological state for most mammals, including humans, after infancy.
Can you develop lactose intolerance later in life?
Yes, it is common for people to develop primary lactose intolerance as they age, as the body's production of the lactase enzyme gradually decreases after childhood.
What is the difference between lactose intolerance and milk allergy?
Lactose intolerance is a digestive issue caused by the inability to digest lactose, while a milk allergy is an immune system response to milk proteins (casein or whey).
Can people with lactose intolerance consume any dairy?
Many individuals with lactose intolerance can tolerate small amounts of lactose, especially from fermented products like yogurt or hard cheeses, which have lower lactose content. Lactase supplements can also help with digestion.
How can I get enough calcium without drinking milk?
You can obtain plenty of calcium from non-dairy sources such as leafy green vegetables (kale, broccoli), fortified juices, almonds, tofu, beans, and certain fish.
Are plant-based milks a good substitute for cow's milk?
Plant-based milks can be a good substitute, but their nutritional content varies widely. For example, soy milk is high in protein, while almond milk is generally low. It's important to choose fortified options to ensure adequate calcium and vitamin D intake.
What about the hormones in milk?
Cow's milk contains naturally occurring hormones, and some cows are treated with artificial growth hormones. While some people express concern over these hormones, scientific evidence does not show a clear negative effect in humans from normal milk consumption.
Citations
- Healthline. 5 Proven Health Benefits of Milk. (2023). https://www.healthline.com/nutrition/milk-benefits
- The Daily Campus. Why You Shouldn't Drink Milk. (2022). https://dailycampus.com/2022/09/14/why-you-shouldnt-drink-milk/
- National Center for Biotechnology Information (NCBI). Lactose Intolerance - StatPearls. (2025). https://www.ncbi.nlm.nih.gov/books/NBK532285/
- Cleveland Clinic. Lactose Intolerance: Symptoms, Diagnostic Tests & Treatment. https://my.clevelandclinic.org/health/diseases/7317-lactose-intolerance
- The Conversation. Do humans need dairy? Here's the science. (2016). https://theconversation.com/do-humans-need-dairy-heres-the-science-70434
- NHS. Dairy and alternatives in your diet. https://www.nhs.uk/live-well/eat-well/food-types/milk-and-dairy-nutrition/
- Physicians Committee for Responsible Medicine (PCRM). Health Concerns About Dairy. https://www.pcrm.org/good-nutrition/nutrition-information/health-concerns-about-dairy
- American Society for Nutrition. Going nuts about milk? Here’s what you need to know about plant-based milk alternatives. (2019). https://nutrition.org/going-nuts-about-milk-heres-what-you-need-to-know-about-plant-based-milk-alternatives/
- Crystal Creamery. Nutrition Showdown: Dairy Milk vs. Plant-Based Milk. https://crystalcreamery.com/blog/nutrition-showdown-dairy-milk-vs-plant-based-milk
- Harvard T.H. Chan School of Public Health. Milk not necessary for adults, can be good option for kids. (2019). https://hsph.harvard.edu/news/milk-health-adults-kids/
