"Lactose intolerance" is a group of symptoms (cramps, diarrhea, flatulence, "rumblings in the tummy", nausea, and combinations thereof) that are caused by the inability to digest the milk sugar lactose. The condition is widespread, but is less prevalent in people of Northern European ancestry.
Lactose is a disaccharide of glucose and galactose, and its digestion requires the actions of the enzyme lactase (a beta-galactosidase located in the surface membrane of small-intestinal epithelial cells). If the lactose is taken in as part of the diet, but not digested by lactase, this sugar is then metabolized by bacteria in the intestine, leading to the unfortunate symptoms listed above.
The activity of lactase is high in newborns, but declines after weaning; that is, the production of lactase drops off with age. Northern Europeans are exceptional in that lactase activity often remains high in adults. The exact mechanism leading to the decline in activity in most humans is still not known. Temporary lack of lactase activity may also be due to such causes as viral enteral infections, in which the epithelial tissues normally secreting the enzyme do not produce enough to digest incoming lactose.
2 month old male, "fussy", diarrhea and vomiting; flatulence. Breast-fed.
Viral gastroenteritis 10 days previously; watery diarrhea and flatulence began then and continued to present. Before the infection, mother's milk was well tolerated, without noticeable diarrhea or flatulence.
Body weight within normal limits. Moderately dehydrated.
Urine shows presence of reducing sugars. No reaction for glucose.
1. How much lactose is present in milk?
2. What does the presence of reducing sugars in the urine indicate?
3. What does the absence of glucose in the urine indicate?
4. What does the infant's tolerance of his mother's milk tell you about the possibility of primary lactose intolerance?
5. What is the significance of the viral gastroenteritis in this case? How would a viral infection affect the digestion of lactose?
6. What is causing the flatulence?
7. What sort of formula would you recommend for orally re-hydrating this infant, one using a standard mixture, or a special one in which milk sugar (lactose) is replaced by cane sugar (sucrose)? Explain.
Primary lactose intolerance is a genetic disorder resulting in deficiency in a particular disaccharidase, lactase, that cleaves the sugar into constituent monosaccharides glucose and galactose. Secondary lactose intolerance is a result of infections or other disturbances in the gastrointestinal tract, which result in insufficient amounts of the (otherwise fully active) enzyme for digestion of lactose.
Undigested lactose can exert an osmotic effect, drawing water into the small intestine. This may accelerate the passage of intestinal contents into the large intestine, and so contribute to the diarrhea associated with lactose intolerance. Colonic bacteria can then digest the lactose, forming first short-chain carboxylic acids (e.g., propionic, acetic, formic, and lactic acids). Formic acid is then cleaved by formic lyase to yield hydrogen gas and carbon dioxide. Furthermore, colonic bacteria may then form methane from the carbon dioxide and hydrogen gas. The result is copious amounts of gas in the large intestine, leading to the flatulence and "rumblings" associated with lactose intolerance.
The principal treatment is the removal of lactose from the diet, but it is usually not necessary to avoid lactose entirely. Low lactose milks are commercially available for lactose-intolerant infants and small children. Nondairy substitutes can now be found for a variety of dairy foods. There are also available commercial preparations of a beta-galactosidase that can be taken orally, with food, to promote digestion of the sugar. Many people can tolerate yogurt, even when they cannot tolerate milk; yogurt with "active cultures" contains live bacteria that produce lactase.
Lactose is often used in pharmaceutical preparations; patients with lactose intolerance should be made aware of this potential source of lactose, and a pharmacist consulted for alternative preparations that avoid lactose.
1. How much lactose is present in milk? An 8 ounce serving of whole milk contains 9 to 14 grams of lactose. Human milk contains about the same amount of lactose as dairy milk.
2. What does the presence of reducing sugars in the urine indicate? Reducing sugars include lactose and glucose, but not sucrose. One might suspect diabetes, if the reducing sugar were shown to be glucose.
3. What does the absence of glucose in the urine indicate? This indicates that the condition is not diabetes.
4. What does the infant's tolerance of his mother's milk tell you about the possibility of primary lactose intolerance? Since the infant tolerated his mother's milk, this is likely not a case of primary lactose intolerance, but of secondary lactose intolerance.
5. What is the significance of the viral gastroenteritis in this case? How would a viral infection affect the digestion of lactose? The viral infection has likely cause a shedding of a substantial fraction of the cells in the epithelial lining of the small intestine. These are the cells that produce lactase, and when they are lost so goes the ability to digest lactose. Fortunately, these cells are easily re-grown, so the condition is only temporary.
6. What is causing the flatulence? Colonic bacteria are fermenting the undigested lactose into hydrogen gas, carbon dioxide, and methane.
7. What sort of formula would you recommend for orally re-hydrating this infant, one using a standard mixture, or a special one in which mild sugar (lactose) is replaced by cane sugar (sucrose)? Explain. Sucrose is a disaccharide of glucose and fructose; the linkage is alpha-1 to beta-2. It is digested not by lactase (a beta-glucosidase) but by sucrase, which is an alpha-glucosidase. High levels of sucrase are produced naturally in the gut epithelia, and there should still be appreciable sucrase activity there despite the losses in lactase activity. Thus the infant should still be able to digest sucrose, and so one would prefer the oral rehydration formula that uses sucrose, not lactose.