Lactose, fructose, gluten: making sense of intolerances

Healthy state

The word “intolerance” is used for very different things, which is exactly what makes it confusing. There are three levels. An allergy is a defence reaction of the immune system, usually through IgE antibodies IgE The antibody of classic allergy. It sits on mast cells and triggers their alarm on contact with the allergen. More in the chapter “Allergies and intolerances: how the gut learns to tolerate food”, and can occur even with small amounts. Coeliac disease Coeliac disease An autoimmune disease in which gluten damages the lining of the small intestine. It is diagnosed with a blood test and usually a biopsy while you are still eating gluten. is an autoimmune disease Autoimmune disease A disease in which the immune system attacks the body's own tissue, for example the thyroid in Hashimoto's or the pigment cells in vitiligo. More in the chapter “Autoimmune diseases and the gut: Hashimoto's, vitiligo, type 1 diabetes and more” in which gluten damages the lining of the small intestine. An intolerance in the narrow sense is purely a digestive matter: the body lacks an enzyme or a transporter, and the sugar stays in the gut. The treatment differs completely depending on the level, so it is worth keeping them apart.

Milk sugar (lactose) is a double sugar. The enzyme lactase sits on the surface of small intestinal cells, in the so-called brush border, and splits it into glucose and galactose, which then enter the blood. In most mammals and in a large share of people worldwide, lactase declines after weaning. Only people with a particular gene variant keep it into adulthood (“lactase persistence”), which is common mainly in northern Europe and some herding peoples. Worldwide, then, a fall in lactase is the normal case and persistence the exception.

Fruit sugar (fructose) is absorbed through the transporter GLUT5; a second route (GLUT2) kicks in when glucose is present at the same time. That is why fructose from fruit, where both sugars are roughly balanced, is tolerated better than fructose in excess, for example from juice, honey or sweetened drinks. Sugar alcohols Sugar alcohols (polyols) Sweeteners such as sorbitol, xylitol or maltitol, which also occur in fruit. They are barely absorbed and can cause bloating and diarrhoea. such as sorbitol are absorbed only hesitantly and can “add up” with fructose. The absorption capacity differs from person to person and from day to day.

Gluten is a mixture of proteins in wheat, rye and barley. The gut cannot break it down completely into single building blocks; fragments remain which, in people with the tissue types HLA-DQ2 or DQ8, can activate the immune system. This affects about one percent of people (coeliac disease). In everyone else, gluten is digested without an immune reaction. In addition there is wheat allergy, a separate and rare allergy to wheat proteins.

One point matters for understanding this: lactase sits on the tips of the small intestinal villi, the places that are easiest to injure. When the lining is damaged, lactase is usually the first thing to go. More on this under “Why it tips”.

Disrupted state: causes and effects

What is not absorbed in the small intestine moves into the large intestine. There, bacteria break the sugar down into gases (hydrogen, carbon dioxide, in some people methane) and short-chain fatty acids Short-chain fatty acids Acids such as acetate, propionate and butyrate that gut bacteria make from fibre. They nourish the gut wall and act throughout the body. More in the chapter “Butyrate and other short-chain fatty acids”. In addition, the undigested sugar draws water into the gut (an osmotic effect). This explains the typical mix of bloating, rumbling, loose stool and diarrhoea, and why symptoms usually start a few hours after eating, when the load reaches the large intestine. That the sugar feeds bacteria there is not bad in itself; it becomes unpleasant when the amount exceeds what can be processed or the gut reacts particularly sensitively.

The amount decides. Most adults with lactase deficiency tolerate about 12 grams of lactose, roughly a glass of milk, without marked symptoms; complete avoidance is rarely necessary. Tolerance also depends on whether lactose comes with a meal (slower stomach emptying) and on whether the large intestine bacteria “get used” to lactose. Hard cheese, butter and many aged cheeses contain only traces; yoghurt and kefir are usually better tolerated because the bacteria in them have already broken down some lactose. Symptoms and measurement do not always go together: someone who is “positive” on a breath test Breath test After drinking a sugar solution, hydrogen and methane in the breath are measured several times. This can reveal sugar intolerances and signs of bacterial overgrowth. More in the chapter “Gut tests: what they show and what they do not” does not have to have symptoms, and the other way round.

Behind a lactase deficiency there is often no inborn problem but damage to the lining. After a gut infection, with untreated coeliac disease, with inflammation or with small intestinal bacterial overgrowth Small intestinal bacterial overgrowth (SIBO) Too many bacteria in the small intestine, where normally few live. Bloating and diarrhoea are typical; it is usually diagnosed with a breath test. More in the chapter “When the small intestine is too densely colonised” (SIBO, see its own chapter), lactase is missing temporarily. Lactose intolerance is then a pointer to the cause. When the lining heals, lactase often returns. So if you suddenly cannot tolerate milk, do not only cut it out, but ask about the cause.

With fructose, the picture is less clear. “Fructose malabsorption” can be measured by a breath test, but the test is poorly reproducible and the test dose is far above what people usually eat. A targeted dietary trial is more helpful. Hereditary fructose intolerance (a missing enzyme in the liver) is something entirely different; it is rare and starts in infancy. Also rare is an inborn lack of the enzyme sucrase-isomaltase, which affects table sugar and starch products and sometimes only shows up in adulthood as “IBS Irritable bowel syndrome Recurring abdominal pain linked to bowel movements, without tests finding another cause. Today it is understood as a disorder of gut–brain interaction. More in the chapter “The gut-brain axis””.

Many who feel sensitive to gluten actually react to something else: in a small blinded study, fructans, sugar chains from wheat, caused more symptoms than gluten itself. Expectation also counts: people who believe they are getting gluten have symptoms even when it is a placebo Placebo A dummy treatment without an active ingredient. Comparing with it shows how much a remedy achieves beyond expectation and chance. (“nocebo”). This does not mean the symptoms are imagined; they are real, but their cause often does not lie in gluten. Coeliac disease should still be ruled out before cutting out gluten, because the tests only work under a normal gluten intake.

Be careful with IgG blood tests for foods: they only show what your body has come into contact with, not what you do not tolerate. Allergy societies explicitly advise against them. Anyone who drops many foods after a positive result risks deficiencies and fear of food without the cause of the symptoms changing.