Lactose Intolerance: Why Some People Can't Digest Milk

Bloating after a glass of milk, a swollen belly after a cream sauce... Behind these discomforts lies a gene that is gradually shutting down and an enzyme that is pulling the curtain. Here’s what really happens in your intestines, and how to find the dose you can tolerate without eliminating everything.

The lactose gene: a programming present from birth.

Let's start with some good news: we are all born equipped. Each of us has a gene specifically dedicated to digesting milk sugar. This is not a privilege; it's the basics of mammalian life, as our first months are spent nursing.

But be careful about what we put behind the word "gene." A gene is not an order set in stone; it is more like a plan, information, a possibility. It produces strictly nothing until it is read and used by the organism. Some of these plans are essential, others can be postponed for a long time, and still others may never come to fruition.

The lactose digestion gene falls into the category of plans with a limited duration. It is active, very active even in infants, but then it can go dormant. And it is precisely this silent, gradual shift that explains why an adult may one day find themselves unable to tolerate what they drank without issue at five years old.

When intolerance strikes from the very first bottles.

There is a rare form, and it manifests immediately. In some babies, problems begin at birth: they cannot drink breast milk without immediately suffering from severe diarrhea. The genetic plan, in their case, does not function as it should from day one.

These situations remain exceptional, and they have nothing to do with the common intolerance that develops in adults. While an adult may simply feel bloated after a café au lait, the affected infant reacts dramatically at the slightest feeding.

In other words, we are talking about two very different realities under the same label. One is an anomaly present from the start, while the other is a nearly normal phenomenon that develops over the years. Confusing the two leads to absurd decisions, such as eliminating all dairy products from an adult's diet when tolerance has simply become partial.

Why the lactase gene turns off with age

The most common scenario is this: the gene gradually deactivates over the years. This affects about 75% of humans. From a natural perspective, it makes perfect sense: after a certain age, we are no longer supposed to nourish ourselves by sucking on a breast or a bottle nipple.

This phenomenon also concerns not just our own cells. Our gut bacteria, those countless tenants that help us digest, also include specialists in milk. In an infant, there are many more active bacterial genes for breaking down breast milk than in an adult. These little milk-digesting assistants gradually disappear after weaning, simply because no one needs skills they no longer use all the time.

A concrete consequence: the older we get, the higher the probability of no longer being able to break down lactose. The trap is that at sixty, one does not necessarily make the connection. We attribute a bloated belly or a little evening diarrhea to stress or fatigue, never to the glass of milk at four o'clock or the creamy sauce that covered the cutlet.

An uneven distribution according to regions of the world.

Another point that often surprises: continuing to digest milk in adulthood is more of an exception than the rule on the planet. Beyond the borders of Western Europe, Australia, and the United States, adults who can still tolerate milk consumption are rare.

And even in our latitudes, tolerance is not absolute. In Northern Europe, up to 15% of the population may be affected by this intolerance. This is not anecdotal, and it partly explains why "lactose-free" sections have multiplied in supermarkets in recent years.

This geographical discrepancy also helps to put things into perspective. Not being able to digest milk is not an exotic disease or a personal weakness; it is an extremely common way of functioning on a global scale. Those who retain the active enzyme throughout their lives are, in fact, the exceptions.

Lactose, this carbohydrate with a complex structure.

Let's now see what this famous lactose actually is. It is a carbohydrate, in other words, a sugar, found in milk. Its particularity: it is composed of two molecules linked together by a chemical bond. As long as this bond holds, the whole is too large to cross the intestinal wall and enter the bloodstream.

All digestion works on this principle of breaking down. Our digestive enzymes act like tiny pairs of scissors that reduce food into confetti, until they obtain the basic building blocks that our cells can recognize: sugar molecules, amino acids, fatty bodies. Powdered sugar, for instance, is already so small that it passes directly, while a chain carbohydrate requires prior work.

Nota Bene: an enzyme is simply a protein made by the body, whose role is to break or assemble a specific molecule. A given enzyme can only do one thing, but it does it very well. Without the right enzyme, there is no breakdown: the food remains whole and continues its journey.

Lactase, an enzyme produced by the small intestine itself.

The enzyme responsible for breaking down lactose into two parts is called lactase. Its origin is worth noting because it is unique. Most digestive juices are released into the intestine through a small opening in the intestinal wall, the duodenal papilla, after being produced elsewhere, particularly by the liver and pancreas.

Lactase, however, does not come from there. It is produced by the cells of the small intestine themselves, right at the tips of these cells, at the level of their smallest villi. These villi are microscopic fringes that line the inside of the intestine and give it a velvety appearance, a mechanism that greatly increases the surface area for exchange.

The work therefore takes place in direct contact with the wall. When lactose brushes against this border, it breaks down, the chemical bond loosens, and the released sugar molecules can then be absorbed and enter the bloodstream. It is also in the small intestine, where the largest digestive surface and the smallest food particles are found, that the question "can I tolerate lactose?" is played out.

What happens in the absence of lactase

Let's now remove the enzyme, or say that it hardly works anymore. Lactose comes into contact with the wall, no one cuts it up, so it remains too large to be absorbed. Nothing dramatic happens: it simply slides along the small intestine and continues its journey.

Except that at the end of the road, there is the large intestine, and there it's rush hour for bacteria. This intact sugar becomes a feast for a whole population of gas-producing microbes. The bloating, rumbling, and tense stomach that follow are, to put it kindly, the thanks of a well-fed colony.

The classic symptoms are therefore stomach pains, diarrhea, and bloating, with a delay of a few hours after the meal that makes the culprit difficult to identify. Unpleasant, yes. Serious, no: lactose intolerance remains a much milder disorder than other intestinal problems that go unnoticed for years.

Symptoms similar to gluten intolerance, but a different mechanism.

On paper, the list of discomforts resembles that of a sensitivity or intolerance to gluten: painful stomach, irregular transit, bloating. Hence the frequent confusions and elimination diets launched somewhat randomly.

However, the mechanism is quite different. In disorders related to gluten, fragments of wheat proteins manage to pass through the intestinal wall, ending up in areas where they don't belong and angering the immune system. It is this immune reaction that causes the damage, and in its most serious form, it can even destroy the intestinal villi.

Nothing like that with lactose. Poorly absorbed particles do not cross the barrier; they do not provoke any attack from the immune system; they simply nourish the flora of the large intestine. It is neither an allergy nor a true incompatibility: it is simply a problem of breakdown. The practical consequence is significant, as there is no lesion to fear, only discomfort proportional to the amount consumed.

A reduced rather than absent enzymatic activity

Here is the point that almost everyone ignores, and it is the most useful of all: in the vast majority of cases, lactase is still present. It has not disappeared from the intestine; it has simply slowed down. It is estimated that the enzymes operate at about 10 to 15% of their original capacity.

Ten to fifteen percent is not insignificant. This means that a small amount of lactose will indeed be broken down and absorbed normally, and that one must exceed a certain threshold for the surplus to move into the large intestine and feed gas-producing bacteria.

Hence the classic mistake. Many people, as soon as a doctor utters the word intolerance, eliminate all dairy products as if they were poisons. However, most of the people affected do not have a complete genetic incapacity; they react to an excess. Stopping the consumption of milk and all its derivatives overnight would therefore be largely exaggerated.

Temporary factors that may worsen lactose sensitivity

Another important thing: your tolerance is not a fixed value. It can collapse for a few weeks and then rebound. Three circumstances regularly occur in these kinds of episodes.

The first is a course of antibiotics. These medications do not discriminate: they also decimate a good portion of our beneficial bacteria, and the gut flora takes time to get back in order, several weeks for adults, even longer for young children and the elderly. The second is an excess of stress. The third is gastroenteritis, which shakes the intestines from top to bottom.

In these three cases, one may suddenly find themselves unable to tolerate a yogurt that was perfectly fine the previous month. But as soon as things return to normal, even a sensitive gut can bounce back. The right reaction is therefore not to eliminate the food from your life forever, but to reintroduce it later, in quantities that suit your stomach.

Test your own limits rather than erase everything.

The practical question remains: how do you know where your threshold is? There is no universal rule, so it’s best to become your own observer. If you notice that your digestion significantly improves by giving up a large glass of milk, nothing prevents you from testing for yourself the amount you can tolerate and the precise moment when things go wrong.

Proceed gradually, one food at a time, and note the reactions over two or three days. Often, a piece of cheese, a drop of milk in tea, or a bit of custard with chocolate cake pose no problem. A good reflex as well: yogurt, which is nothing more than milk predigested by bacteria. The lactose has already been largely broken down into lactic acid and smaller sugar molecules, which lightens the work you have left to do.

So remember: lactose is a two-molecule sugar, lactase produced by the villi of your small intestine is the only one that knows how to separate them, this enzyme slows down in three out of four people with age, and the unbroken surplus will party in the large intestine. Nothing more, nothing less.

So rather than banning an entire aisle of the supermarket, take stock of what really bothers you, reduce the large doses, keep the small ones, and give another chance to foods that were excluded after a course of antibiotics or a stomach bug. Your stomach has more leeway than you think, as long as you don’t ask too much of it all at once.