Lactose intolerance (LI) is a common digestive condition affecting millions globally, resulting from the body’s reduced ability to process milk sugar. Individuals experiencing discomfort after consuming dairy often report gas and bloating, but they are uncertain whether their primary bowel issue is diarrhea or constipation. Symptoms of LI are highly variable, depending on the individual’s specific gut environment. This article clarifies the distinct biological mechanisms that result in these two opposing digestive outcomes.
Defining Lactose Intolerance
Lactose is a disaccharide, or double sugar, found in milk and dairy products. To be absorbed, lactose must be broken down into glucose and galactose in the small intestine by the enzyme lactase.
Lactose intolerance occurs due to a deficiency of the lactase enzyme, known as lactase non-persistence or hypolactasia. This deficiency is usually primary, meaning lactase production naturally decreases after childhood, but it can also be secondary, resulting from damage to the small intestine from illness or injury. LI is a digestive issue, distinct from a milk allergy, which is an immune response to milk proteins.
The Primary Symptom: Osmotic Diarrhea
The majority of people with lactose intolerance experience diarrhea as their primary bowel symptom after consuming dairy. This occurs because undigested lactose travels intact into the large intestine, or colon.
Lactose is osmotically active, meaning it pulls water into the intestinal tract to equalize concentrations. This influx of fluid significantly increases the water content of the stool, resulting in loose, watery bowel movements characteristic of osmotic diarrhea. The rapid transit of this fluid also contributes to abdominal distress and cramping.
In the colon, gut bacteria ferment the undigested lactose. This fermentation produces byproducts, including short-chain fatty acids and gases like hydrogen and carbon dioxide. The buildup of these gases causes common LI symptoms such as bloating, abdominal distention, and flatulence. The presence of excess hydrogen gas forms the basis for the hydrogen breath test used for diagnosis.
The Atypical Symptom: Constipation
A notable subset of individuals with lactose intolerance may experience constipation instead. This paradoxical response is linked to the activity of specific gut microorganisms. Constipation is reported in approximately 30% of cases, often presenting as hard, dry stools and infrequent bowel movements.
This atypical symptom is caused by the production of methane gas during lactose fermentation by gut microbes. Certain strains of bacteria and archaea generate higher levels of methane when acting on the undigested sugar. Methane gas has been demonstrated to slow down intestinal motility, or the movement of contents through the gut.
This slower transit time allows more water to be reabsorbed from the stool in the colon, leading to a firmer consistency and constipation. A person’s gut microbiota composition dictates whether they are primarily a hydrogen or methane producer, determining if lactose consumption results in diarrhea or constipation.
Managing Lactose Intolerance
Diagnosing lactose intolerance typically begins with an elimination diet, where all lactose-containing foods are temporarily removed to see if symptoms resolve. A definitive diagnosis is often confirmed using the hydrogen breath test, which measures the amount of hydrogen gas exhaled after a dose of lactose is consumed. A high level of hydrogen indicates that undigested lactose is being fermented by colonic bacteria.
Management strategies focus on reducing the amount of lactose consumed to a level the individual can tolerate without symptoms. Many people can consume small amounts of lactose, particularly when it is eaten alongside other foods. Dairy products that have naturally lower lactose content, such as hard cheeses and yogurt, are often tolerated well because of the fermentation process they undergo.
Over-the-counter lactase enzyme supplements are a widely used and practical option, as they provide the missing enzyme to break down the lactose in the small intestine before it can cause problems. These supplements must be taken immediately before consuming a food or drink that contains lactose. Utilizing lactose-free dairy alternatives also allows individuals to maintain adequate calcium and vitamin D intake without the digestive discomfort.