Sugar Tolerance Tests. Clinical quantitative assessment of the efficiency of the digestion and absorption of carbohydrates in humans rests mainly on relatively simple tests in which carbohydrate loads (at least 50 g) are ingested and blood samples are taken to estimate the sugar levels attained at various time intervals after ingestion. The levels are then compared with those obtained in normal subjects. The most commonly used test is the oral glucose tolerance test (OGGT). Typically, nonpregnant adults take 75 g of glucose over 5 minutes and the glucose is estimated in serum at 0, 30, 60, 90, and 120 minutes. A pregnant woman takes 100 g of glucose and has another estimate at 180 minutes. A child takes 1.75 g/kg up to the maximum of 75 g (58). Values above normal indicate some form of inadequate handling of the ingested glucose. This test is often used to assess for diabetes mellitus. The reproducibility of the OGTT has been claimed to be poor, even when repeated in the same individual (59). An oral tolerance test also exists for galactose. As the liver is a major site of galactose metabolism, the test has been used to assess liver function. Similar oral tolerance tests exist for fructose and the disaccharides lactose (lactase deficiency) and sucrose (sucrase deficiency).
Breath Hydrogen Tests. Carbohydrates that have not been digested or absorbed reach the colon and become fermented by the resident bacteria. Hydrogen gas is produced and is excreted in the breath. Measuring breath hydrogen thus provides an estimate of whether malabsorption of a sugar or carbohydrate occurs (see also Chapter 57). It was first used to detect lactose intolerance and has since been used in numerous studies on carbohydrate intolerance ( 60). It has a number of weaknesses; for example, it gives no indication of the amount of carbohydrate absorbed before the sugar reached the colon, and the hydrogen in the breath is only a fraction of that formed.
Oral Tolerance Tests and the Glycemic Index. Nutritionists use a form of oral tolerance test to assess the so-called glycemic potential of different foods. A carbohydrate load is ingested and the level of blood sugar is measured over a period of time. The increments in blood glucose are then compared with equivalent increments from different foods by normalizing these values to a baseline obtained with glucose, usually by using the area under the 2-h glucose curve after feeding a 50-g carbohydrate portion, and expressing it as a percentage of the mean obtained after 50 g of glucose. This normalized figure, designated the glycemic index of the food (61), enjoyed considerable popularity in the dietetic management of diabetes and hypoglycemia. However, there is a large scatter in glycemic index for each group of foodstuffs, attributed to many factors such as its form when eaten, the way it is processed, how it is chewed, how it is emptied from the stomach, and the physiologic and metabolic responses of individuals. The glycemic index has been criticized as a crude index of questionable value ( 6.2), but it still has its adherents (63). What it does illustrate is that carbohydrate foods differ widely in their effects on blood glucose and hormonal responses after a meal. Diets with a low glycemic index, designated lente, have been claimed to be useful in diabetes, hyperlipidemia, and even healthy subjects. They appear to prolong satiety and hence lead to a better control of food intake (63).
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