Chapter 48 Malabsorption
Digestion involves enzymatic breakdown of proteins, carbohydrates and fats, and transport of nutrients across the intestinal epithelium into capillaries and lymphatics. Malabsorption implies a failure of these functions despite adequate nutritional intake. The most common causes in industrialized countries are pancreatic insufficiency, coeliac disease and Crohn disease (Fig. 3.48.1).
Osmotic diarrhoea results if there are undigested nutrients that cause large amounts of water to be retained in the stools by oncotic pressure. It can be found in any form of malabsorption but particularly in disaccharidase deficiency. Osmotic diarrhoea is watery, unless undigested fats turn it into steatorrhoea.
Deficiency of fat-soluble vitamins can occur where there is a significant failure of fat absorption. Vitamin D deficiency can lead to hypocalcaemia, osteomalacia and rickets, while lack of vitamin K can result in hypoprothrombinaemia and a bleeding tendency. Lack of vitamin A is a theoretical possibility but in practice it is rarely observed in malabsorption syndromes.
Potassium depletion causes thirst, polyuria, paraesthesiae and muscle weakness, while protein deficiency can cause muscle wasting, hypoproteinaemia and oedema. Anaemia is a common feature of malabsorption and may be caused by failure to absorb iron, folate and/or vitamin B12.