Food intolerance can cause a range of gut and systemic symptoms . The possibility that these conditions might be caused by intolerance to lactose has often been missed because the addition of lactose to manufactured foods and drinks is not necessarily mentioned on the package label and it is not generally appreciated how widely lactose is used as a food additive. Lactose has about one sixth of the sweetness of sucrose and, unlike sucrose or glucose, is not metabolized effectively by yeast. Thus lactose can be added to foods and drinks without causing a sickly sweet taste and without generating carbon dioxide or ethanol as a result of yeast metabolism . Lactose is used as a browning agent in bread and cakes, is added to processed meats such as sausages and burgers, and is even injected into some chicken meat. It is also added to some soft drinks and lagers, breakfast drinks, and slimming products. The result is diagnostic confusion where advice is confined to the avoidance of dairy foods.
Lactose, a disaccharide, is not normally absorbed from the gut, but is hydrolysed by the action of brush border lactase into glucose and galactose, which are then absorbed by the enterocytes If the enzyme is absent (or deficient for the dietary load of lactose), then there will be the passage of non-absorbed lactose into the colon. This may result in an osmotic diarrhea of varying severity, along with cramps and flatulence, the symptoms of the condition known as lactose intolerance. All adult mammals, except for northern Europeans and certain other ethnic groups (for example, African tribes such as the Masai) are normally hypolactasic, with low intestinal lactase activities. Even among these groups, much of the activity of this enzyme is normally lost soon after weaning . There are thus three causes of hypolactasia (loss of activity of lactase) among such populations:
a) Inherited loss, after weaning (also known as 'acquired lactose intolerance of the adult', which is a normal physiological condition),
b) Congenital complete loss of lactase (which is very rare),
c) Loss of lactase due to secondary intestinal damage, for example cause by Giardia infections or rotavirus (in which the integrity of the intestinal mucosa is compromised).
In the absence of intestinal lactase, the passage of lactose into the large intestine results in hydrogen, methane and other metabolites being generated by colonic bacteria. These metabolites include toxins such as acetaldehyde, dimethylglyoxal, formic acid, propan 1,3 diol, indoles, and skatoles. Lactose itself can have a toxic effect if absorbed directly into the blood stream. These toxins act on ionic signaling pathways in the nervous system, heart, skeletal muscle and immune system  and it is now clear this can cause a range of debilitating systemic symptoms, in addition to the well known gut symptoms, such as those associated with irritable bowel syndrome , eczema, asthma and osteoarthritis.
Lactose intolerance was formerly assessed by giving an oral dose of 50 gm of lactose (equivalent to the lactose load in 1 litre of cow's milk) and then monitoring the plasma glucose for 3 hours. A normal response is a rather modest rise in the plasma glucose, of greater than 1.2 mmol/L. The 'lactose tolerance test' is now rarely carried out. Even some control subjects, with no known relevant history, may exhibit some symptoms and fail to produce a significant rise in the plasma glucose, showing how prevalent is this condition.
A trial lactose-free diet may also provide a useful initial assessment for lactose intolerance, but small bowel biopsy with the assay of brush border disaccharidases used to provide the definitive diagnosis. Genetic testing based on an EDTA blood sample is now possible, but this gives information as to susceptibility, rather than the presence of the condition, which, being polymorphic in its genetics, requires the investigation of a number of gene loci.
Our experience is that the comparison of breath levels of hydrogen and methane before and after the consumption of a 25 gm lactose load provides a convenient and practical way of detecting the condition of lactose intolerance; this is of considerable importance in the investigation of a number of chronic conditions and its increasing prevalence reflects the dietary load of lactose from manufactured foods, as well as the extent of loss of brush border lactase.
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