D-xylose and its use in the diagnosis of malabsorptive states

Abstract

MONG the numerous procedures employed in A the diagnosis of malabsorptive states [7], only a few have been technically simple, widely applicable, inexpensive, and yet informative of the absorptive ability of the small intestine; these include plasma turbidity [2-51, carotene C&71, and the urinary excretion of d-xylose after oral administration [ 7-3,8-751. The present study was undertaken to evaluate the oral d-xylose test and to compare it with other indices of absorption in patients with malabsorption of varying etiology. Physiologic Comiderations. Substances employed to measure absorptive functions are subject to the same physiologic processes of intestinal absorption as foodstuffs, i.e., digestion, translocation and transportation, In addition, various metabolic processes referable to the specific test substance itself and the type of measurement often require special consideration. To facilitate an understanding of the biologic significance of the xylose test and its validity in evaluating intestinal absorption (especially carbohydrate absorption), a review of the metabolism of xylose seems appropriate. Xylose is a five-carbon monosaccharide (pentose) with a molecular weight of 150. Foodstuffs such as fruits and grains are especially rich in pentoses and can produce an alimentary pentosuria [76]. As with most simple carbohydrates given alone or after digestion of a balanced meal, xylose is absorbed (translocated) primarily in the duodenum and proximal jejunum [77]. In healthy individuals an average of 65 per cent$ of xylose is absorbed. After feeding the pentose, Fourman [77] could not recover any $ This figure actually represents the quantity of xylose reaching the blood. It is exclusive of any amount metabolized during the process of translocation above that which is metabolized by the intestinal mucosa upon xylose by tube from the distal ileum. Benson and co-workers [75] state that in two well nourished patients with ileostomies (one year postoperative for ulcerative colitis) who were excreting normal amounts of xylose in the urine, “definite” quantities of xylose were recovered from the ileostomy drainage, Much of the unabsorbed xylose may be metabolized by the normal bacterial flora of the intestinal tract. As it is a simple sugar, i.e., monosaccharide, no “digestion” is necessary prior to the absorption of xylose by the intestinal mucosa. Although specific studies as to the role of pancreatic secretions, bile and intestinal juices are not available, the results in various diseases provide inferential evidence that they are not of quantitative significance. As would be expected with a watersoluble substance, an emulsifier such as Tween 80@ has no effect on xylose absorption [74]. Intraluminal conditions, such as near neutral pH, and possibly the presence of inorganic phosphate, may be important in the normal absorption of xylose. These two factors, together with intestinal motility, have been investigated principally in relation to glucose and other hexoses, and relatively infrequently with d-xylose. Ponz [78] demonstrated that the absorption of both xylose and glucose was decreased at either extreme of pH of the intestinal lumen (pH 3 or 10). The addition of phosphate buffer at pH 7 increased the rate of glucose absorption in rats [79]; although subsequent studies indicated that the action of phosphate was due to its pH [20], this work led to the phosphorylation theory (and subsequently including the hexokinase reaction) of carbohydrate absorption. The effect of intestinal motility upon xylose absorption ap-