Food Intolerances: Understanding Their Effects on Your Digestion

Food intolerances: not an allergy, but a real digestive impact

You finish a meal and, a few hours later, your stomach swells, cramps set in, and you find yourself rushing to the bathroom. These symptoms — familiar to millions of people — may point to a food intolerance. It's important, however, not to confuse them with an allergy: unlike IgE-mediated allergic reactions — which are immediate and sometimes severe — intolerances generally result from an enzymatic deficiency or a non-immunological hypersensitivity. The mechanism is different, but the discomfort is very real.

The three most common food intolerances

Lactose: when milk sugar becomes a problem

Lactose intolerance is probably the best known. It occurs when the small intestine does not produce enough lactase, the enzyme responsible for breaking down milk sugar. Undigested lactose then travels to the colon, where gut bacteria ferment it, producing gases (hydrogen, methane). The result: bloating, flatulence, diarrhoea, and abdominal pain, typically arising 30 minutes to two hours after consuming dairy products.

Globally, this affects the majority of adults: in some populations, prevalence reaches 65 to 70%.

Gluten: sensitivity without autoimmunity

Non-coeliac gluten sensitivity is distinct from coeliac disease (an autoimmune condition). It involves inflammation of the intestinal lining and malabsorption, without the immune system producing the antibodies characteristic of coeliac disease. Symptoms — diarrhoea, fatigue, brain fog, bloating — can closely mimic those of irritable bowel syndrome (IBS). Wheat, barley, and rye are the main triggers.

FODMAPs: the fermentable sugars that irritate the gut

FODMAPs (fermentable sugars such as fructans, galacto-oligosaccharides, and excess fructose) are found in everyday foods: onions, garlic, pulses, apples, and pears. In sensitive individuals — particularly those with IBS — these short-chain carbohydrates trigger excessive fermentation in the colon and an osmotic influx of water, leading to bloating, gas, and alternating diarrhoea and constipation.

How intolerances disrupt your gut microbiota

Beyond the immediate symptoms, unmanaged food intolerances have deeper effects on the gut's bacterial ecosystem. Chronically imbalanced fermentation alters the composition of the microbiota: it promotes pathobionts (potentially harmful bacteria) and reduces overall microbial diversity. This dysbiosis in turn worsens digestive symptoms, creating a vicious cycle that is difficult to break.

The intestinal barrier itself can be weakened, increasing its permeability and fuelling a state of low-grade inflammation — with potential repercussions well beyond the gut (fatigue, mood, the gut-brain axis).

Diagnosis: an essential step before any elimination

One of the most common mistakes is to self-diagnose and eliminate entire food groups without medical confirmation. This can lead to significant nutritional deficiencies — calcium in the case of lactose, fibre and complex carbohydrates with FODMAPs — without actually resolving the underlying problem.

Validated diagnostic tools include:

• The hydrogen breath test for lactose intolerance
• Duodenal biopsy (with serology) for coeliac disease
• A low-FODMAP elimination diet over four to six weeks, followed by a gradual and supervised reintroduction

Always consult a gastroenterologist or allergist before making lasting changes to your diet.

Appropriate dietary strategies

Once a diagnosis has been established, several approaches can help restore digestive comfort:

• Lactose intolerance: reduce dairy intake or replace it with calcium-fortified plant-based alternatives; certain aged cheeses and yoghurts are often better tolerated due to their lower lactose content
• Gluten sensitivity: follow a strict gluten-free diet (essential in confirmed coeliac disease) or reduce ultra-processed wheat-based foods
• FODMAP hypersensitivity: follow the low-FODMAP protocol during the elimination phase, then methodically reintroduce foods to identify your personal tolerance threshold — as not everyone reacts to the same foods or the same quantities

A rising prevalence: why now?

Recent data paint a clear picture. In France, the prevalence of food allergies and intolerances in children has doubled over a decade, reaching 8% in those under 15 (ANSES data). In adults, it is estimated at 3.5%. In the United States, 11% of adults report suffering from a confirmed food allergy.

The reasons for this rise are multifactorial: ultra-processed diets, a reduction in microbial diversity from childhood, decreased exposure to environmental micro-organisms, and changes in agricultural and industrial practices. France's National Food-Nutrition-Climate Strategy 2025–2030 also highlights the emergence of new allergens linked to the rise of alternative proteins (pulses, insects, tree nuts).

Looking after your gut means looking after yourself

Food intolerances are not an inevitability, but they deserve to be taken seriously and properly diagnosed. Identifying your triggers, adapting your diet in a targeted way, and supporting your microbiota with gut-friendly habits (dietary variety, well-tolerated fibre, stress management) are the cornerstones of better digestive wellbeing.

Your gut is sending you signals — learn to listen to them with the right tools.