The inability to digest lactose (or “milk sugar”) is far more common than a food allergy. Such a reaction may be due to the lack of a particular enzyme needed to digest the lactose. People who are intolerant towards lactose have difficulties in digesting milk and milk products.
If the lactase enzyme is not produced at all, or is not present in sufficient amounts (called “hypolactasia”), the unhydrolysed lactose reaches the large intestine (see picture 3) and comes into contact with the colonic bacteria. These bacteria ferment lactose which results in the production of the gases hydrogens and methane and small organic molecules. These can give rise to a number of disorders, such as flatulence, rumbling and bloating, noises and diarrhoea. Non-specific abdominal disorders also often indicate lactose intolerance.1
Symptoms differ from person to person and many of them are not always typical of lactose intolerance. A correct diagnosis is therefore very important in order to provide the most appropriate treatment.
Primary lactase deficiency
In the majority of humans, intestinal lactase activity decreases during childhood and adolescence to about 5% to 10% of the activity at birth. This is called primary 'late-onset' or 'adult-type' hypolactasia. It occurs in almost 75% of the world's population. 2
The prevalence of hypolactasia varies greatly between different populations. In China, Asia and African and Native American Indian populations, 85 to 97% are lactose intolerant. Southern India’s population is predominantly lactose intolerant while those in the North tend to tolerate lactose well. Only those from North-Western Europe have lower incidence of lactose intolerance, although t5% in Belgium, the Netherlands and UK are lactose intolerant.
A large part of the South European population are also lactose intolerant: 44% in the South of France, 50% in Northern Italy and 70% in Southern Italy.
In the Unated States, tolerance differs among etnic origin. 1,3
A recessive autosomal gene causes this primary adult-type hypolactasia. It is thought that hypolactasia originally occurred in almost the entire human race, and that lactase persistence is increased by a selective advantage in certain cultures.4 Hypolactasia can begin to manifest itself from the age of about 5 years, but the fall in lactase activity can last up to the age of about 20 before problems may occur.
Infants need lactase in order to digest breast milk, which contains 7% lactose. Much of the world’s population then shows a decrease of lactase activity after the age of five to less than 10% of its original quantity.
Secondary lactose intolerance
In addition to this primary form of lactose intolerance, secondary lactose intolerance also often occurs.
Because lactase is a sensitive enzyme, it will also be the enzyme that is most often affected in intestinal problems.2 This is usually a temporary disorder affected by serious, acute or chronical intestinal infections, gluten sensitivity (coeliac disease), Crohn’s disease, allergy or use of certain medication (eg. antibiotics), which causes damage to the intestinal mucosa, disabling the lactase production temporarely.
Consequently, lactose intolerance can also occur in gastro-enteritis or diarrhoea. In certain cases lactose then has to be excluded from the diet for some time. 4
References :
- Lee. Human adult-onset lactase decline: an update. Nutrition Reviews 1998; 56(1): 1-8.
- BŸller H. Clinical aspects of lactose intolerance in children and adults. Scand J Gastroenterol Suppl 1991; 188: 73-80.
- Vonk RJ et al. Lactose intolerance: analysis of underlying factors. Eur J Clin Invest 2003; 33: 70-5
- Sahi T. Genetics and epidemiology of adult-type hypolactasia. Scand J Gastroenterol Suppl 1994; 202: 7-20.
- Sahi T. Hypolactasia and lactase persistence. Historical review and the terminology Scand J Gastroenterol Suppl.1994; 202: 1-6.
- Vesa TH et al. Lactose intolerance. J Am Coll Nutrition 2000; 19(2): 165S-75S
- Van den Driessche M. Lactose-intolerantie, verwarring en misverstanden. Nutrinews 1999; 2
