Lactose intolerance: why milk becomes harder to digest with age

Milk that caused no issues in childhood may start to trigger bloating and rumbling by your twenties—and this can feel like a malfunction that needs fixing. In reality, the "malfunction" is actually the opposite: let's break down what happens to milk sugar along the way and at what point the scenarios diverge.

The molecule and the enzyme

Lactose is milk sugar, a disaccharide: two molecules, glucose and galactose, are linked together by a single bond. The intestinal wall can only absorb single sugars into the bloodstream, so lactose cannot be absorbed in its whole form. Until the bond is broken, it is not nutrition for the body, but simply a dissolved substance in the intestinal lumen.

It is broken down by lactase, an enzyme anchored to the brush border of the small intestine. The brush border consists of villi and microvilli that line the inside of the intestine; the enzymes for the final step of digestion are located directly on them, so breakdown and absorption occur at the same point. Lactase is most abundant in the middle part of the small intestine.

The broken-down glucose and galactose are absorbed, and galactose is converted into glucose in the liver. Milk sugar does not have a special metabolism: its only unique feature is who is capable of breaking the bond and in what quantity.

Why the enzyme fades — and why this is normal

In mammals, lactase is only needed by the offspring. After weaning, milk disappears from the diet, and enzyme activity decreases—in humans, this usually happens between the ages of two and twelve, gradually, down to a few percent of the childhood level. This is a programmed process, not the result of illness or lifestyle. Neither a "damaged" intestine nor a break in milk consumption triggers it: it proceeds on schedule for everyone who does not have an exception.

The exception is lactase persistence: a genetic trait where the enzyme continues to function throughout life. It is associated with variants in the regulatory region near the lactase gene and appeared in human populations relatively recently, along with dairy farming. It arose independently in several places—in Europe and, with different variants, among pastoralist peoples in Africa and the Middle East.

Hence the geography. In Northern Europe, persistence is present in the vast majority of adults, while in East and Southeast Asia it is rare, and reduced lactase activity is a trait of almost the entire adult population there. On a global scale, there are more people who lose lactase than those who retain it. In other words, "lactose intolerance" describes not a breakdown, but the original state of the species; what is unusual is the ability to drink milk as an adult without consequences.

Russia is heterogeneous in this regard, and there is no reliable nationwide figure: estimates vary between studies and regions. The qualitative picture is more stable—the proportion of people with reduced lactase activity is minimal in the northwest, increases to the east and southeast, and approaches East Asian levels among the indigenous peoples of Siberia and the Far East. The practical conclusion: the condition is common, and you should not consider it a rare finding in yourself.

What undigested lactose does

Then physics takes over. Lactose that remains intact is an osmotically active substance: it retains water in the intestinal lumen and draws it in from the tissues. The volume of contents increases, the intestine stretches, and transit accelerates. This leads to loose stools, which can be watery with a large dose.

Without being absorbed in the small intestine, lactose reaches the large intestine, where it is met by bacteria. For them, it is excellent food: they ferment milk sugar into short-chain fatty acids and gases—hydrogen, carbon dioxide, and in some people, methane. Gas causes bloating, rumbling, and distension, while acids change the nature of the stool. The large intestine handles any carbohydrate that reaches it this way—it does the same with fiber; the difference is that fiber is supposed to reach it, while lactose is not.

The main practical takeaway follows directly from this mechanism: symptoms depend on the dose. As long as the residual lactase activity is sufficient for what has been consumed, nothing happens. Everyone has their own threshold, but a guideline is known: most adults with reduced lactase tolerate about 12 g of lactose in one sitting—roughly a glass of milk—without noticeable complaints. The same volume, spread over two sittings or consumed with food, is tolerated even better: food slows gastric emptying, lactose enters the intestine more slowly, and the enzyme has enough time.

The diagnosis is usually confirmed by a hydrogen breath test: a person is given a dose of lactose and the hydrogen in their exhaled breath is measured—it should not be there, as it only appears from bacterial fermentation. A genetic test shows a predisposition to adult-type hypolactasia but does not detect acquired forms, which are discussed below.

Three conditions that are constantly confused

The words "I cannot have milk" stand for at least three different things, and the choice of product in the store depends on which one it is.

Primary hypolactasia, adult type. The age-related one. There is little enzyme, the intestine is healthy, the condition is permanent and does not progress to anything dangerous. The tolerance threshold is stable, and it can be determined through experience.

Secondary intolerance. Lactase sits on the very tips of the villi, so it is the first to suffer from any damage to the mucosa: after an intestinal infection, with giardiasis, with untreated celiac disease, or sometimes after a course of medication or surgery. Here, the enzyme is absent not because of genetics, but because the platform it attaches to is destroyed. The condition is temporary: after a rotavirus infection, tolerance usually returns within weeks; with celiac disease, it returns when the mucosa heals, which requires removing gluten, not milk. It is important not to confuse the adult type and the secondary form: in the first case, the restriction is permanent; in the second, it is temporary.

Cow's milk protein allergy. A different system and a different target: an immune reaction to casein and whey proteins. It has nothing to do with lactose, and the confusion here is costly—lactose-free milk does not help at all with an allergy, as the protein in it remains untouched. It occurs more often in children under one year old and usually passes with age; in addition to intestinal symptoms, there may be rashes, swelling, and respiratory reactions. Diagnosis is conducted by a doctor: self-testing with the product is dangerous here.

Congenital lactase deficiency is separate and very rare—the newborn develops severe watery diarrhea from the first feedings. This is an emergency situation, not a reason for dietary experiments.

How much lactose is in products and why cheese is easier than milk

Lactose is dissolved in the water part of milk, i.e., in the whey. Everything that separates the whey or gives bacteria time to ferment it reduces the lactose content—sometimes to trace amounts.

Hard cheese—like Russian, cheddar, or parmesan—is tolerated by almost everyone, and this follows from the technology: the whey containing lactose is removed at the curd separation stage, and the remnants are fermented by starter cultures during maturation. In aged cheese, there is practically no milk sugar. Butter is almost pure fat; it has little water phase, and consequently, little lactose.

Yogurt and kefir are tolerated better than milk for another reason: the live bacteria in the starter culture carry their own beta-galactosidase and continue to break down lactose in the human intestine. Plus, the thick consistency stays in the stomach longer. Heat-treated yogurts lack this advantage—the bacteria in them are killed.

The flip side is lactose where you don't expect milk. Dried milk and milk whey are added to sausages, bread, sauces, chips, instant soups, milk chocolate, and protein mixes. All these components must be listed in the ingredients: according to TR TS 022/2011, milk and its derivatives, including lactose, are on the list of allergens that must be indicated regardless of quantity. How to identify them in the list is covered in the article on how to read labels. Lactose is also found in tablets as a filler, but it is present in tens of milligrams, and this usually does not affect well-being.

Lactose-free milk and lactase supplements

Lactose-free milk is regular milk to which lactase has been added in advance: the sugar in it has already been broken down into glucose and galactose. The protein, fat, calcium, and calorie content remain the same, and there is the same amount of carbohydrates—just in a different form. Hence the taste: lactose itself is almost not sweet, while glucose and galactose are noticeably sweeter on the tongue, so such milk seems sweetish, even though no sugar was added to it.

One caveat: in galactosemia—a rare hereditary disorder of galactose metabolism—lactose-free milk is not suitable precisely because the galactose in it is already free.

Lactase supplements are the same enzyme in capsules or drops, taken with dairy food. They work based on the dose: one tablet covers a limited amount of lactose, and with a heavy dairy meal, the effect may not be enough. They make sense when you need to eat dairy away from home or in an unpredictable amount; for daily use, it is cheaper and easier to choose products based on your tolerance.

Complete avoidance of dairy in adult-type hypolactasia is not required and is usually not beneficial: the threshold for most is above zero, and regular small portions of lactose change the composition of intestinal bacteria in a way that subjectively improves tolerance.

How to cover calcium if dairy is removed

Dairy products in the Russian diet are the main source of calcium, and removing them entirely without replacing them means creating a deficiency. There are things to replace them with: hard cheeses (they have no lactose but plenty of calcium), canned fish with bones, sesame seeds and tahini, legumes, tofu on calcium salt, and fortified plant-based drinks—the latter should be checked by their ingredients, as not all manufacturers fortify them.

The situation with vitamin D is often imported from abroad, and in vain. In Russia, milk is not massively fortified with vitamin D, so it was never a source of it: fatty fish, sunlight, and, as prescribed by a doctor, supplements remain. Giving up milk does not take away vitamin D—but it does take away calcium.

Calcium and vitamin D supplements should not be prescribed by yourself. During pregnancy, diabetes, kidney disease, and when taking medication regularly, the doses and the necessity of the supplement should be discussed with a doctor: excess calcium with impaired kidney function is more dangerous than a deficiency.

And the boundary where this topic ends. Blood in the stool, nocturnal symptoms, fever, persistent abdominal pain, unexplained weight loss—this is not lactose. Such signs require examination, and attributing them to milk means losing time. This is even more true for children: one should not remove dairy from a child's diet on their own, neither for a test nor for a long time—a growing skeleton and protein in the diet are worth more than an experiment.

Count it from a photo

Frequently asked questions

Why could I digest milk in childhood, but not now?

The activity of lactase, the enzyme that breaks down milk sugar, decreases in humans after early childhood. This is a normal biological program common to all mammals, not a consequence of a disease. Retaining the enzyme into adulthood is a genetic trait that not everyone possesses.

How much milk can I drink if I am lactose intolerant?

The threshold is individual, but most adults with reduced lactase can tolerate about 12 g of lactose in a single serving—this is approximately one glass of milk. When consumed with other food and spread out over several portions, this same amount is tolerated even better.

Why do cheese and butter not cause symptoms?

Lactose is dissolved in whey, and during the production of hard cheese, the whey is separated, and the remaining sugar is fermented by starter cultures during the aging process. In butter, the water phase is minimal, so there is less than a gram of lactose per 100 g.

How does lactose intolerance differ from a milk allergy?

These are different mechanisms. With intolerance, there is a lack of the enzyme for milk sugar; symptoms are intestinal and dose-dependent. An allergy is an immune reaction to milk proteins, such as casein and whey proteins; it can be triggered by trace amounts, and lactose-free products do not help in this case.

Lactose-free milk is sweeter than regular milk—is sugar added to it?

No. The lactose in it has been pre-broken down into glucose and galactose, which taste sweeter on the tongue than lactose itself. The amount of carbohydrates and the calorie content remain unchanged.

Read next

• Gluten in a healthy gut: what actually happens to it — Gluten has become the word people reach for to explain bloating, fatigue and stubborn weight, and also the word people say with a smirk.
• Collagen supplements: what digestion does to them — Two equally confident claims circulate about collagen.
• Food combining: what happens when protein and starch reach the stomach together — Almost everyone has heard some version of the rule: meat on one plate, potatoes on another.

This article is for general information. It is not medical advice, a diagnosis, or a prescription for treatment or a diet, and it does not replace a consultation with your doctor. If you have a health condition, are pregnant, take medication, or follow a diet prescribed to you, decisions about food belong with your doctor.

Figures from regulations, guidelines and studies are given as they stood when this article was prepared and may since have changed; check them against the primary sources. This article is not advertising, an offer, or individual advice, and neither the author nor the site owner is responsible for decisions taken on the basis of it.