Why can lactose cause diarrhea in some people?

This is one of the hallmark symptoms of lactose intolerance, and it happens because of osmosis and bacterial fermentation working together.

Step 1: You drink milk

Milk contains lactose.

Normally, the enzyme lactase in the small intestine breaks lactose into:

• glucose

• galactose

These sugars are then absorbed into the bloodstream.

Step 2: There isn't enough lactase

If someone has lactose intolerance, there isn't enough lactase to digest all the lactose.

As a result:

Lactose remains intact.

Since it hasn't been broken down, it cannot be absorbed.

Instead, it continues into the large intestine (colon).

Step 3: Lactose attracts water

Here's where diarrhea begins.

Lactose is osmotically active, meaning it attracts water.

Imagine lactose as a sponge.

As it moves through the intestine, it pulls water into the bowel.

Intestine

Water → 💧💧💧
          ↓↓↓

      Lactose

Normally, the large intestine reabsorbs much of the water from digested food.

But when a lot of lactose is present, extra water stays inside the intestine instead of being absorbed.

The stool becomes softer and more watery.

Step 4: Bacteria ferment the lactose

The bacteria living in the large intestine begin feeding on the lactose.

During fermentation they produce:

• hydrogen gas

• methane (in some people)

• carbon dioxide

• short-chain fatty acids

These byproducts:

• increase gas,

• stimulate the intestine,

• and can speed up the movement of its contents.

Step 5: The intestine moves faster

The extra water and the fermentation products encourage the colon to move its contents along more quickly.

This is called increased intestinal motility.

Because food and water move through the colon faster than usual, there is less time for water to be reabsorbed.

As a result, the stool remains loose.

The result

The combination of:

undigested lactose

+

extra water in the intestine

+

bacterial fermentation

+

faster intestinal movement

can lead to diarrhea.

Why doesn't everyone get diarrhea?

Everyone is different.

Symptoms depend on several factors, including:

• how much lactase a person produces,

• how much lactose they consume,

• how quickly food moves through their gut,

• and the types of bacteria living in their colon.

Some people mainly experience:

• bloating,

• gas,

• or abdominal cramps,

while others develop diarrhea.

A simple analogy

Imagine pouring sugar onto a dry sponge.

The sponge immediately draws in water.

Now imagine adding yeast, which starts producing bubbles.

The sponge becomes wetter, fills with gas, and expands.

Something similar happens in the colon:

• lactose draws in water,

• bacteria ferment it and produce gas,

• and the intestine moves the contents along more quickly.

A note about young children

In babies and young children with significant lactose malabsorption, diarrhea can sometimes be severe enough to cause dehydration if it persists. This is one reason persistent diarrhea in infants should always be assessed by a healthcare professional.

Key takeaway

Diarrhea in lactose intolerance occurs because undigested lactose pulls water into the intestine through osmosis while gut bacteria ferment the lactose, producing substances that increase gas and speed up intestinal movement. Together, these processes leave less time for water to be absorbed, resulting in loose, watery stools.