This article deals with lactose intolerance, which is the result of an enzyme deficiency, rather than lactose allergy, which is an IgE-mediated reaction. For information about allergy, see the separate Food Allergy and Food Intolerance article.
- Lactose is a disaccharide sugar found exclusively in milk. Absorption of lactose is dependent upon the enzyme lactase.
- Lactase is the enzyme that hydrolyses lactose to the monosaccharides, glucose and galactose and is present in the tips of the villi of the small intestine.
- This enzyme is essential in babies but tends to decrease in amount after the age of 2 years, although symptoms of lactose intolerance rarely occur before the age of 6 years.
- It is argued that it is only because we have the unusual habit of ingesting milk from other species, usually the cow, that the enzyme persists beyond the age of weaning.
Most milk intolerance in young children is due to allergy to cow's milk protein and not deficiency of lactase. The immature gut of the premature baby may be deficient in lactase but it soon increases as it is presented with milk.
Lactose enhances the absorption of calcium, magnesium and zinc. It also promotes the growth of lactobacilli and provides galactose, which is essential for the formation of cerebral galactolipids and hence development of the brain.
Types of lactase deficiency
- Primary lactase deficiency: autosomal recessive and also known as adult-type hypolactasia, lactase nonpersistence, or hereditary lactase deficiency. Deficiency of lactase develops at various ages.
- Secondary lactase deficiency: follows damage to the intestinal mucosa - eg, acute viral or bacterial gastroenteritis, uncontrolled coeliac disease or inflammatory bowel disease or chemotherapy. This resolves when the disease process is over and the intestinal mucosa heals. It is more common in children and especially in the developing world.
- Congenital lactase deficiency: an extremely rare autosomal recessive disorder and associated with minimal, or complete absence of, lactase activity. It becomes apparent once milk is introduced, usually with intractable diarrhoea once milk or lactose formula has been introduced.
- Developmental lactase deficiency: occurs in premature babies (<34 weeks of gestation) and improves once the intestine matures.
- Primary lactase deficiency - affects up to 75% of the world's population (although not all develop symptoms of lactose intolerance)[3, 4]. Ability to digest lactose is gradually lost from the time of weaning onwards to some degree in most populations. Prevalence of lactase deficiency varies enormously geographically - for example, 5-17% in Northern Europe and 70-95% in Africa and Asia[1, 5]. This relates to the level of use of dairy products in the diet.
- Age of presentation also varies geographically, with Northern Europeans presenting later, whilst those from Japan or China tend to lose lactase ability more quickly after weaning and present earlier.
- Lactose intolerance in adults is very common and lactose may be found in many unexpected sources. These include saccharine processed meats, bread, cake mixes, soft drinks and lagers. This may account for unexplained symptoms, including some cases of irritable bowel disease.
Symptoms are caused only by the ingestion of lactose, found in milk and other dairy products. They are affected by the amount of lactose ingested and how fast the small intestine is presented with the lactose load. They will be more marked if the lactose reaches the intestinal mucosa fast but less so if gastric emptying is delayed as by being ingested with a large meal. Degree of symptoms is also affected by individual sensitivity and colonic flora. Individuals in countries where diet tends to include more dairy products (such as the UK) are more likely to inherit a retained ability to digest lactose and not experience symptoms.
Symptoms result from reduced absorption of lactose which is then broken down by intestinal bacteria, leading to gas and short-chain fatty acids.
Gas build up causes:
- Abdominal discomfort.
The acidic and osmotic effects of undigested lactose may cause:
- Loose watery stool - with a degree of urgency an hour or two after ingestion of milk.
- Perianal itching due to acidic stools.
Symptoms occur from one to several hours after ingestion of milk or dairy products. These symptoms are very nonspecific and occur with other disorders such as milk-protein sensitivity, allergic-type reactions to other substances in the meal, or intolerance of other saccharides.
Secondary lactase deficiency may produce more severe symptoms and dehydration may occur.
- In children there may be malnutrition and failure to thrive but this is uncommon.
- In adults there is usually nothing to find or perhaps a little bloating and discomfort during an attack.
Diagnosis can be made on clinical features alone - eg, re-introduction of lactose leads to symptoms. Specialised tests are rarely required.
A trial of a two-week period of a strict lactose-free diet, with careful attention to food labels, should be tried. If symptoms resolve but recur on re-introduction of lactose-containing foods, the diagnosis can be made.
- A lactose tolerance test involves a test dose of 2 g of lactose per kg body weight up to a maximum of 50 g, given after a fast, and noting the rise in blood glucose, rather like a glucose tolerance test. A positive test is reproduction of symptoms and rise in serum glucose by <1.11 mmol/L, 60-120 minutes after ingestion. However, this rather higher dose than is presented in a normal meal has been criticised as representing an atypical situation. This has now been superseded by breath hydrogen tests.
- Breath hydrogen test - if carbohydrate is unabsorbed in the gut, it is fermented by bacteria in the large intestine and hydrogen gas is produced, absorbed into the blood and excreted by the lungs. Thus, carbohydrate malabsorption can be determined by measuring the exhaled hydrogen concentration after a carbohydrate load. Normally, the fermenting bacteria are confined to the large intestine but, when bacterial overgrowth in the small intestine occurs, upper small bowel fermentation of ingested lactose occurs and causes an early rise in the exhaled hydrogen concentration. There will still be a later rise in exhaled hydrogen during large bowel fermentation. Antibiotics may produce false negative results. For diagnosis of lactose intolerance, 0.5-1.0 g/kg to a maximum of 12-25 g of lactose is given and an increase greater than 20 ppm of hydrogen is diagnostic.
- Genetic tests are available but so far cannot cover all the genetic mutations that exist within different populations.
- If difficulty remains, a small intestinal mucosal biopsy can be obtained by endoscopy for direct assay of lactase activity as well as that of other brush border disaccharidases. The gold standard test is analysis of enzyme and carbohydrate levels and ratios from biopsy; however, it is usually too invasive a test for a mild condition.
- Recurrent abdominal pain of childhood.
- Irritable bowel syndrome.
- Allergy to milk proteins or other constituents of milk.
- Deficiency of other disaccharidases.
- Infantile colic.
- Diverticular disease.
- Ulcerative colitis.
- Coeliac disease.
- Cystic fibrosis.
Avoiding milk and dairy products will relieve symptoms in most. However, lack of milk and dairy products can result in the loss of a vital source of calcium, especially if the person is on a vegetarian diet. Furthermore, a 2010 systematic review concluded that most people with the condition could tolerate a glass of milk per day.
Primary lactase deficiency
- Varying amounts of lactose can be tolerated - this needs to be determined. Things can be further improved by taking the lactose in divided portions throughout the day and with meals.
- Yoghurt and curds may be tolerated due to thicker consistency, slower gastric emptying and because their preparation means that the lactose is partially hydrolysed. Live yoghurts contain bacteria which partly hydrolyse their own lactose.
- Dairy products with a higher fat content, such as ice cream, chocolate milk, cheese, and full-fat rather than skimmed milk, are better tolerated. The fat content slows gastric emptying.
- Hard cheeses, such as Cheddar, Edam, Parmesan and Emmental, contain very little lactose and may be well tolerated.
- Milk substitutes can also be used but they contain fewer nutrients compared with cow's milk.
- Lactase enzyme preparations available commercially from health food shops can be combined with lactose foods but these are expensive and the evidence of efficacy is variable.
Secondary lactase deficiency
- Resuscitation with intravenous rehydration may occasionally be required in secondary lactase deficiency.
- Antibiotics should be avoided unless there is strong evidence for a bacterial cause.
- Parents/carers should be advised to continue giving formula or breast milk or regular milk during an acute diarrhoeal illness in most cases. Alternative preparations may be considered in higher-risk cases, such as infants younger than 3 months, or malnourished children.
Developmental lactase deficiency
Tube feedings with milk containing lactose in premature infants usually contain breast milk, or if not possible, reduced-lactose milk. Full-strength lactose formula is more likely to induce intolerance. Evidence for adding lactase to feeds in this time period remains weak. Breast milk contains components which aid lactose absorption.
Congenital lactase deficiency
Babies with severe deficiency require a diet full of essential nutrients but excluding lactose.They cannot be breast-fed; they need lactose-free formula milk and must be weaned on lactose-free foods.
Most people with lactase deficiency suffer very little. Transient lactase deficiency affects a significant number of infants following severe gastroenteritis. Improper early feeding with lactose-based products without the recognition of lactose malabsorption can lead to chronic diarrhoea and malnutrition.
Lactose enhances the absorption of several minerals, including calcium, magnesium and zinc. In addition, milk products are high in calcium that is extremely important in bone growth. Children can quickly become deficient and so calcium supplements are required if there is restriction of eating dairy products.
The following may contain unexpected lactose and patients and carers need to be advised to monitor food labels:
- Various drugs, whether prescribed or over-the-counter.
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