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Lactose intolerance: the signs and the treatment

Sarah Schenker
British Nutrition Foundation London

Lactose, or milk sugar, is a disaccharide consisting of glucose and galactose. It is found only in the milk of mammals and is the main carbohydrate found in milk and other dairy products. It cannot be absorbed by the body without prior digestion to its component sugars. The enzyme lactase is produced in the mucosa of the small intestine and is required for the breakdown process. Lactase activity is detectable in the fetal gut as early as eight weeks of gestation. In many people, lactase activity decreases after weaning, but, in some ethnic groups, such as western European Caucasians, lactase activity can persist into adult life, enabling total digestion of large quantities of dietary lactose. The ability to express lactase in adult life is inherited on a single gene, meaning that dual inheritance of the gene for absence of lactase leads to very low activity in those people. Intermediate lactase activity results when one gene for absence is inherited, and high lactase activity is present in those who inherit lactase-present genes.(2)
Lactase activity decreases with age and normally becomes apparent between the ages of 5 and 20 years. The loss of lactase activity is rarely total, but a decrease of 10-30% of the initial level of the enzyme activity can occur. Lactose intolerance is particularly common in Africans, Asians, black Americans, Greek Cypriots, Indians, Chinese and Australian aborigines.(3) It is estimated that 70% of adults worldwide experience a decrease in their ability to produce lactase after early childhood. In contrast, it is not common in northern Europeans, white North Americans and white Australians and New Zealanders, where prevalence is estimated to be 5%.
It has been speculated that the ability of northern Europeans to digest lactose is associated with the fact that climate in such countries is conducive to dairy farming, and consequently milk and dairy products have been part of the adult daily diet for centuries. In times of food shortage, this situation encouraged the spread in the population of a mutant gene that allowed lactase activity to persist after early childhood.

Congenital and secondary deficiency
Congenital lactase deficiency is a rare, autosomal recessive condition present from birth. Reference levels for lactase concentrations at birth are 0-2%.(4) Severe symptoms may lead to advanced dehydration and neurological damage and could be fatal. Treatment with lactose-free formula products and a completely lactose-free weaning diet is necessary.
Secondary lactose intolerance can be caused by injury to the intestinal mucosa or be secondary to various gastrointestinal diseases and conditions, such as:(5)

  • Rotavirus infection.
  • Parasitic gastroenteritis.
  • Coeliac disease.
  • Cows' milk allergy.
  • Immunodeficiency syndromes (eg, HIV).
  • Protein-energy malnutrition.
  • Small intestinal resection and other surgery.

Secondary lactose intolerance is usually transient, lasting from a few weeks to a few months.

The main symptoms of lactose intolerance include:

  • Colic.
  • Abdominal distension.
  • Nausea.
  • Flatulence and abdominal pain.
  • Watery, foamy, acid stools.
  • Failure to thrive.

 The symptoms vary widely among individuals and depend to some extent on the amount of residual lactase activity and the quantity of lactose ingested. However, it is thought that the interindividual variability in the colonic flora may account for differences in symptoms. When lactose has not been digested, it passes into the colon, where it is fermented by the resident bacteria, producing short-chain fatty acids and gases. Gas production may result in bloating and distension pain. Unabsorbed lactose also has an osmotic effect in the gastrointestinal tract, drawing fluid into the lumen and causing diarrhoea.
In principle, the symptoms of lactose intolerance are dose-dependent: the larger the amount of lactose administered, the more pronounced the symptoms are likely to be. However, symptoms can vary between individuals, and other factors can also affect the degree of intolerance, for example slow gastric emptying and long transit time, independent of lactase activity. Unabsorbed lactose increases the acidity of the colon contents, potentially causing changes in the composition of the colonic bacteria and their activity. Over time this adaptation of the bacterial flora can lead to improved tolerance of lactose.
A number of investigations can be used to diagnose lactose intolerance. These include testing liquid stools for reducing substances, the hydrogen breath test (there is an increase in expired breath hydrogen after lactose) and measurement of lactase activity in a jejunal biopsy specimen.

Foods to avoid
The treatment depends on the severity of the lactose intolerance, but requires either complete or partial removal of lactose from the diet. Foods that contain lactose include all mammalian milks and products made from them (ie, goats' and sheep's milk as well as cows' milk). In addition, use of lactose as an ingredient in the food industry is widespread because of its physiological properties, making its avoidance particularly difficult. It provides good texture, binds water and is a carrier for colour. It is also less sweet than most sugars - it is less than half as sweet as glucose and considerably less sweet than sucrose. It can be present in chocolate, ice cream, margarine and many manufactured foods such as sausages, breaded meats and fish, soups, crisps, puddings and sweets. Even tablet such as aspirin may contain lactose as a filler. Hard cheeses, such as Cheddar, Red Leicester, Double Gloucester, Stilton and Parmesan, and cheeses such as Edam, Brie and Camembert have a negligible lactose content and are generally well tolerated.
Yoghurt, despite its lactose content (about 6g per 100g of plain yoghurt), is better tolerated than ordinary milk (4.8g per 100g).(6) The increased tolerance of yoghurt may be linked to delays in gastric emptying and increased transit time or to changes in colonic fermentation.

Probiotics have been used in the treatment of lactose intolerance with varying degrees of success.(7) Lactobacilli produce beta-galactosidase, effectively lactase, which hydrolyses the lactose in dairy products. Studies have shown some improvement in intolerance symptoms when patients have been given different types of probiotocs, but more work is needed in this area before firm recommendations can be made.
The use of commercial enzyme preparations, based on microbial beta-galactosidase, can improve tolerance to lactose. These preparations are taken before meals or snacks, sprinkled on food or added to milk 24 hours before consumption. They impart a slightly sweet taste and tend to be less popular in the UK than in the US, where they are more widely used. Alternatively, pretreated lactose-reduced milk is available from supermarkets and health food stores. The hydrolysis of the milk is approximately 90%, but it is not suitable for infants or young children.

As milk and milk products are important sources of many nutrients, such as protein, calcium and riboflavin, avoidance of dairy products is not advised without good reason. Appropriate dietary modifications need to be made to ensure that nutrient deficiencies do not arise. For example, if alternative sources of calcium are not consumed, intake may be sufficiently low to compromise bone health.
Milk is a major source of nutrition in the diets of infants and young children, and a nutritionally adequate replacement for milk is essential. If dietary restriction is required from birth (congenital lactase deficiency) or early in life (secondary lactose intolerance caused by injury or disease of the gut), dietetic advice should be sought.
Lactose intolerance has been shown to be a risk factor for osteoporosis. A study of Italian women  demonstrated that both bone mineral density and calcium intake were significantly lower in those with lactose intolerance.(8) This finding supports other studies that have shown that women with osteoporosis have significantly higher prevalence of lactose intolerance and lower daily calcium intakes than age-matched controls of similar ethnic origin.(9)
The importance of achieving an adequate calcium intake must be emphasised in those with lactose intolerance. Among adults, the complete avoidance of dairy is rarely necessary, and hard cheese and yoghurt should be a regular part of the diet.



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