Metabolism
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Bloating, flatulence and diarrhoea after drinking milk - but not yogurt or cheese
Abdul is a 25 year old doctor from Sudan; he came to London to study for a PhD 6 months ago and has suffered from diarrhoea and flatulence since arriving, although in Sudan he rarely suffered from gastrointestinal upsets of any kind. His diet is not very different from what he ate in Sudan, but he has been introduced to taking milk in his tea and coffee. At first he thought this was strange, because he remembered his grandfather telling him that when he was constipated he drank a glass of the milk that his grandmother was about to use to make yogurt.
Since his
diarrhoea was worrying him, he went to see a gastroenterologist, who sent him
for a lactose tolerance test. This involved coming in to outpatients in the
morning, not having eaten or drunk anything since the night before, drinking
a solution containing 50 g of lactose, then measuring the increase in blood
glucose over 3 hours. Within about half an hour of drinking the lactose solution
he suffered painful abdominal cramps, and "explosive" watery diarrhoea
with abdominal bloating and severe flatulence.
The results of the lactose tolerance test are shown below, with Abdul's results in red and the mean and standard deviation for 10 control subjects in blue.
What conclusions can you draw from this graph?
It is obvious that there is no rise in blood glucose at all after Abdul has drunk the lactose solution. This suggests that he lacks the enzyme lactase, and so cannot hydrolyse lactose to glucose and galactose.
Why do you think Abdul's blood glucose does not fall as low as in the control subjects after 3 hours?
The marked fall to below the initial (fasting) glucose concentration in the control subjects is the result of the insulin secreted in response to the rise in blood glucose caused by the lactose. Since Abdul had no increase in blood glucose after drinking the lactose solution, there was no increase in insulin secretion, and hence no resultant drop in blood glucose.
Why do you think Abdul suffered from abdominal pain, bloating, diarrhoea and flatulence after the test dose of lactose?
The unabsorbed lactose remains in the intestinal lumen and provides a substrate for bacterial fermentation in the colon, resulting in the production of a variety of compounds, including short-chain fatty acids (e.g. the 4-carbon compound butyrate, the 3-carbon compounds pyruvate and lactate, and the 2-carbon compound acetate), as well as carbon dioxide and small amounts of hydrogen and methane.
The formation
of these short-chain fatty acids results in a considerable increase in the osmolality
of the intestinal contents, drawing water into the intestinal lumen - hence
the watery diarrhoea. The bloating, pain and flatulence are the result of the
carbon dioxide and other gases produced by the bacteria.
Abdul was unfortunate in that he went to a gastroenterologist in a hospital where the original lactose tolerance test was used. A considerably less unpleasant test involves taking only a few grams of lactose, then measuring hydrogen exhaled in the breath.
How do you think the hydrogen produced by bacteria in the colon can be exhaled on the breath?
Hydrogen has (H2) is sparingly soluble in water, and also lipid soluble, so it will cross the intestinal mucosa into the bloodstream down a concentration gradient. In the lungs it will again cross the epithelium from the bloodstream into the alveoli down a concentration gradient.
Abdul's interest in his condition leads him to do some literature research - he wonders whether he is abnormal in lacking lactase as an adult. Certainly his mother tells him that as an infant he was breast fed with no problems. The outcome of his research is summarised in the table below:
| population group or country of study | % of adults lacking lactase |
| UK white | 4.7 |
| *northern Germany | 7.5 |
| Tuareg (nomads of the central Sahara) | 12.7 |
| *western Austria | 15.0 |
| *southern Germany | 23.0 |
| *eastern Austria | 25.0 |
| US black | 26.2 |
| Turkey | 71.2 |
| Sri Lanka | 72.5 |
| Italy | 75.0 |
| Greece | 75.0 |
| South African black | 78.0 |
| Japan | 89.0 |
| Singapore born Chinese | 92.4 |
| Canadian born Chinese | 97.9 |
| Papua New Guinea | 98.0 |
*The populations of northern and southern Germany are of different origin, as are the populations of western and eastern Austria.
Comparative studies show that most adult mammals lack lactase, although the newborn of most species have lactase activity and ar well able to tolerate milk until they are weaned. (The exceptions are the marine mammals, whose milk has a very high fat content and no carbohydrate; suckling marine mammals do not have intestinal lactase)