Lactose: How Your Body Digests It (and Why It Sometimes Goes Wrong)

What exactly is lactose?

Lactose is the sugar naturally found in milk and most dairy products. Chemically, it is a disaccharide: a molecule made up of two simple sugars — glucose and galactose — bonded together. For the intestine to absorb it, this pair must first be split apart. That is where a key enzyme comes in: lactase.

Produced by cells lining the small intestine, lactase cleaves the lactose molecule in two, allowing the resulting monosaccharides to be absorbed into the bloodstream. When everything works as it should, this process is seamless and efficient. When lactase is in short supply, things get rather more complicated.

Why lactase can fall short

In the majority of mammals — and in many humans — lactase production naturally declines after weaning. This phenomenon, known as primary hypolactasia, is genetic in origin. It is estimated that 65 to 70% of adults worldwide experience this physiological drop in lactase activity, with considerable variation between populations.

There is also secondary hypolactasia, which is temporary and caused by damage to the intestinal lining. A severe bout of gastroenteritis, inflammatory bowel disease, or untreated coeliac disease can all transiently reduce lactase production.

What happens when lactose goes undigested

Without sufficient lactase, undigested lactose reaches the colon, where it encounters the gut microbiota. The bacteria there ferment it — and that is when symptoms begin:

• Gas (hydrogen, methane) produced by bacterial fermentation
• Bloating and abdominal cramps
• Flatulence, sometimes severe
• Osmotic diarrhoea: lactose draws water into the colon, speeding up transit

The severity of symptoms depends on the amount of lactose consumed, individual sensitivity, and the composition of the microbiota. This is why two people with the same degree of lactase deficiency can react in entirely different ways.

Lactose isn't all bad

Often cast as the villain, lactose actually has some lesser-known benefits. It acts as a prebiotic, encouraging the growth of bifidobacteria in the gut. These beneficial bacteria play a role in immunity and microbiota balance. Lactose also helps the body absorb essential minerals such as calcium, copper, and zinc.

What's more, regular, gradual exposure to lactose can lead to improved tolerance over time, thanks to adaptation of the gut microbiota. Complete elimination is therefore not always the best approach.

What the experts recommend

The World Gastroenterology Organisation (WGO) is clear on this point: cutting out dairy products entirely is not recommended, except when medically necessary. Doing so risks deficiencies in calcium, protein, and vitamins.

Practical guidance for people with lactose intolerance:

• Split your intake: up to 12 g of lactose per sitting (roughly a medium glass of milk) and 24 g per day in total are generally well tolerated by most intolerant individuals
• Have dairy with a meal to slow gastric emptying and reduce symptoms
• Opt for yoghurt: live cultures (Lactobacillus bulgaricus and Streptococcus thermophilus) help digest lactose within the gut
• Choose aged cheeses (such as cheddar, mozzarella, or provolone): the maturing process leaves them with very little residual lactose
• Use lactase supplements before lactose-rich meals to improve tolerance

Foods to watch out for (and alternatives)

Lactose can lurk in unexpected places: shop-bought sauces, processed meats, commercially produced baked goods, and milk chocolate. Here is a quick reference:

Food	Approximate lactose content
Whole milk (100 ml)	4–5 g
Whole milk yoghurt (100 g)	4 g
Ice cream (100 ml)	3–8 g
Powdered milk (100 g)	36–52 g

Plant-based alternatives (almond, soya, oat, or rice drinks) can replace milk in everyday use. For calcium, consider green vegetables (broccoli, cabbage), pulses (lentils), almonds, sesame seeds, and sardines.

If dairy is restricted over the long term, calcium and vitamin D supplementation may be worth considering — always on medical advice.

In summary

Lactose intolerance is common, often uncomfortable, but rarely a permanent dietary dead end. Understanding the mechanism — a lactase deficiency that allows lactose to ferment in the colon — makes it possible to adapt your diet sensibly, without unnecessary restriction. Testing your own tolerance, diversifying your calcium sources, and adopting a few simple habits is usually enough to restore lasting digestive comfort.