Lactose intolerance

Definition

Lactose intolerance is the most common form of carbohydrate intolerance, a malabsorption syndrome: because of a deficiency of the intestinal enzyme lactase, milk sugar (lactose) cannot be split sufficiently. Normally, disaccharidases in the brush border of small-bowel enterocytes (including lactase, maltase, isomaltase and sucrase) break disaccharides down into monosaccharides.

Classification

• Acquired (primary) lactase deficiency (primary adult hypolactasia): the most common form; lactase activity declines after weaning
• Secondary lactase deficiency: with damage to the small-bowel mucosa, e.g. celiac disease, tropical sprue or acute intestinal infections; in infants also transiently after intestinal infections or postoperatively after abdominal operations; enzyme activity rises again after recovery from the underlying disease
• Congenital enzyme deficiency: rare, e.g. congenital lactase or sucrase-isomaltase deficiency

Occurrence & epidemiology

In neonates, lactase activity is high, allowing milk to be digested. In the majority of the world's population it decreases after weaning, so that older children and adults can no longer digest significant amounts of lactose; lactase deficiency is therefore the usual state for most adults worldwide. Certain populations of Northwest European, East African and Middle Eastern descent, who historically raised animals for milk, produce lactase throughout life (lactase persistence); this is genetically mediated and occurs in a minority of the world's population.

Aetiopathogenesis

Undigested disaccharides create an osmotic load that draws water and electrolytes into the bowel – causing watery diarrhea. In the colon, bacteria ferment the carbohydrates to gases (hydrogen, carbon dioxide, methane); the consequences are excessive flatus, bloating, abdominal distension and abdominal pain.

Clinical features

Symptoms are similar in all disaccharidase deficiencies. Adults have watery diarrhea, bloating, excessive flatus, nausea, borborygmi and abdominal cramps after lactose intake. A child with lactose intolerance develops diarrhea after larger amounts of milk and may fail to gain weight. Many affected people notice early on that dairy products cause symptoms.

Symptoms typically require more than about 250–375 ml of milk; symptoms after much smaller amounts suggest another cause. Diarrhea may be severe enough that other nutrients are no longer absorbed. The picture can resemble irritable bowel syndrome.

Diagnosis

The diagnosis is usually made clinically from a careful history, supported by a dairy challenge or a lactose-free period. Test results are always correlated with symptoms.

• Stool pH: acidic stool (pH below 6) in chronic or intermittent diarrhea suggests lactose intolerance
• Hydrogen breath test (lactose breath test): after drinking 50 g of lactose, hydrogen produced by bacteria is measured at 2, 3 and 4 hours; a rise of more than 20 ppm above baseline indicates malabsorption; sensitivity 78%, specificity above 95%
• Lactose tolerance test: blood glucose before and 60 and 120 minutes after an oral lactose load; in intolerance it rises by less than 1.1 mmol/l (20 mg/dl), and diarrhea, bloating and discomfort occur within 20–30 minutes; sensitivity about 90%, specificity 90–95%

Keep learning in the app

Further reading (open access)

1. MSD Manual Professional: Carbohydrate Intolerance
2. StatPearls: Lactose Intolerance

Cross-references