Why is Yogurt Ok but Not Milk? Understanding Lactose Digestion

November 22, 2025 •

3 min read

An estimated 65% of the world's population has some form of lactose maldigestion. For these individuals, the puzzling question of why is yogurt ok but not milk is answered by a fascinating biological process involving fermentation and friendly bacteria.

Quick Summary

The live cultures in fermented yogurt break down much of the lactose, significantly lowering the sugar content compared to milk, which makes it easier to digest for those with sensitivity.

Key Points

Lactase Deficiency: Lactose intolerance is caused by the body's reduced ability to produce the enzyme lactase, which is needed to digest milk sugar.
Fermentation Process: During fermentation, live bacterial cultures in yogurt consume much of the lactose, converting it into lactic acid.
Live Cultures Aid Digestion: The surviving live bacteria carry their own lactase enzyme, which continues to break down lactose in the small intestine.
Lower Lactose Content: Yogurt contains significantly less lactose than milk, especially strained varieties like Greek yogurt.
Product Choice Matters: To reap the benefits, choose yogurts labeled 'live and active cultures,' as pasteurized products kill the helpful bacteria.

The Science of Lactose Intolerance

Lactose intolerance is a digestive issue caused by a deficiency of the enzyme lactase in the small intestine. Lactase is required to break down lactose, the sugar found in milk, into two simpler sugars: glucose and galactose. When undigested lactose reaches the large intestine, it is fermented by gut bacteria, which produces gasses and fluids. This process leads to common and uncomfortable symptoms such as bloating, gas, cramps, and diarrhea.

Many humans naturally produce less lactase after infancy, with the level of lactase deficiency varying from person to person. This means some people can tolerate small amounts of lactose, while others must avoid it almost entirely. While milk contains a high amount of lactose, the processing of other dairy products like yogurt and cheese alters their lactose levels, changing how the body reacts to them.

Fermentation: Yogurt's Digestive Advantage

Yogurt is made by fermenting milk with specific live cultures, primarily the bacteria Lactobacillus bulgaricus and Streptococcus thermophilus. These beneficial bacteria consume lactose as their food source, converting it into lactic acid. This process is what gives yogurt its characteristic thick texture and tangy taste. By the time the fermentation is complete, much of the original lactose from the milk has already been consumed, resulting in a lower lactose product.

The Role of Live and Active Cultures

What truly makes live, probiotic yogurt digestible is that the bacteria survive passage through the stomach and continue their work in the small intestine. The bacterial cells carry lactase, which helps break down any remaining lactose present in the yogurt as it moves through the digestive tract. This provides an internal digestive aid that milk, which is unfermented, does not offer. This is why fresh, live yogurt is recommended over heat-treated versions where the cultures have been killed. The live cultures essentially perform part of the body's digestive job, preventing the unpleasant symptoms that result from undigested lactose reaching the colon.

Comparing Dairy: Milk, Yogurt, and Greek Yogurt

Different dairy products have varying levels of lactose, depending on their processing. Here is a comparison to illustrate the difference:


Feature	Milk	Regular Yogurt	Greek Yogurt
Processing	Minimally processed	Fermented with live cultures	Fermented and strained to remove whey
Lactose Content	Approx. 12g per 250ml cup	Approx. 5g per 125g portion	Approx. 4g per 6oz (170g) portion
Presence of Live Cultures	No	Yes, in varieties labeled 'live and active'	Yes, in varieties labeled 'live and active'
Digestibility for Intolerant	Difficult	Much easier, due to fermentation and live cultures	Easiest of the three, due to straining and live cultures

Additional Factors Affecting Digestibility

Greek Yogurt: The straining process used to create Greek yogurt removes a significant portion of the lactose-containing whey, making it even lower in lactose than regular yogurt.
Fermentation Time: A longer fermentation period allows the bacteria more time to consume lactose, further reducing the final lactose content. This is common in homemade yogurts.
Meal Context: Consuming dairy products as part of a meal can also slow digestion and improve tolerance for individuals with a mild intolerance.

The Benefits of Yogurt Beyond Digestion

Beyond being a more digestible option for many with lactose intolerance, yogurt offers numerous health benefits. It is a rich source of calcium, protein, and B vitamins. The probiotics it contains can also contribute to a healthy gut microbiome, which is linked to a stronger immune system and improved overall well-being. For those managing lactose intolerance, choosing yogurt with live and active cultures allows them to gain these nutritional benefits without the typical digestive discomfort. Research has confirmed that the live cultures in yogurt effectively improve lactose digestion in lactose-maldigesting individuals.

Conclusion

The difference between milk and yogurt for those with lactose sensitivity boils down to one critical factor: fermentation. While milk contains a high concentration of lactose that the body cannot properly digest without sufficient lactase, yogurt's live bacterial cultures actively break down this sugar. This, coupled with the potential for further straining in products like Greek yogurt, results in a final product that is significantly lower in lactose and easier on the digestive system. By choosing fresh yogurt with live cultures, individuals can enjoy a delicious and nutritious dairy product, free from the discomfort caused by milk. For authoritative insights into this topic, consider research published by the National Institutes of Health.

Frequently Asked Questions

Not all yogurt is the same. For people with lactose intolerance, it is crucial to choose yogurt labeled 'live and active cultures.' These contain the beneficial bacteria that help break down lactose. Heat-treated or long-life yogurts have killed cultures and offer no digestive assistance.

Yes, Greek yogurt is strained to remove the whey, which contains a large portion of the lactose. This process results in a product with significantly lower lactose levels than regular yogurt, making it even easier for many lactose-sensitive individuals to digest.

The live and active cultures in yogurt contain the lactase enzyme, which they produce naturally. When you consume the yogurt, these bacteria pass through the digestive system and help break down the lactose within the yogurt, easing the burden on your body's own reduced lactase production.

While yogurt is easier to digest for most with lactose intolerance, some with more severe lactase deficiency or other dairy sensitivities may still experience issues. It's important to understand your personal tolerance level, and it's always best to consult a healthcare provider for personalized advice.

The key difference is the bacteria. In milk, the body's own lactase enzyme is solely responsible for digestion. In yogurt, the live bacteria aid this process by predigesting the lactose and then providing additional enzymatic support within the gut.

Some sources suggest that the fat content in yogurt might slow down gastric emptying, allowing more time for the bacterial lactase to work on the lactose. Full-fat and Greek yogurt varieties may be better tolerated for this reason, though individual responses vary.

Yes, beyond just aiding lactose digestion, the probiotics in live yogurt can promote a healthier gut microbiome. A balanced gut is linked to numerous health benefits, including a stronger immune system and better digestive function overall.