Question

1. A condition known as lactose intolerance is characterized by painful abdominal cramping, gas, and diarrhea. The cause of the problem is an inability to digest the milk sugar, lactose. How would this cause the observed symptoms? (Hint: consider where carbohydrates are normally digested and what happens if it cannot be digested, where does it go and what happens because of it?)

Answer 1

1. Lactose intolerance is caused by inability to digest the lactose present in milk. Lactose is a disaccharide made up of glucose and galactose. Carbohydrates are initially digested by salivary amylase to form maltose. The partially digested carbohydrates will complete their digestion in the small intestine, where starch is digested to maltose. Maltase is the enzyme present in brush border epithelium that breaks down maltose to glucose.

Lactose present in the milk is broken down by lactase enzyme in the small intestine. Lactase is secreted by brush border epithelium of small intestine as well. In lactose intolerance, the intestine does not produce lactase enzyme. This will cause lactose to persist in the intestine. This leads to increased osmotic load. As a result, water content of the intestine increases, causing osmotic diarrhea. The digested food when it enters the large intestine is acted upon by colonic bacteria. The colonic microbes produce lactase which breaks down lactose to monosaccharides. This further increases the water influx into the intestine. The breakdown of lactose also leads to production of gases such as hydrogen, carbon dioxide and methane. All of these products and increased levels of water cause symptoms of lactose intolerance such as painful abdominal cramping, gas, and diarrhea.