Glucose tolerance is the body's reaction to an increased carbohydrate supply. It is measured by reactive changes in blood glucose levels. During a glucose loading test, 50-100 g of glucose are administered on an empty stomach in 0.5 l of water, and blood glucose levels measured at time 0 (start), and 30, 60, 90, and 120 minutes thereafter. Since glucose dissolved in water undergoes no digestive processes and passes an empty stomach without delay, the blood glucose level peaks 30 minutes after the meal (A). In healthy subjects, glucose is taken up by the cells even before peak levels are reached, due to simultaneous onset of insulin secretion, thereby preventing glucose levels from exceeding 120-150 mg/dl (6.68.25 mmol/l). The regulatory switch— reversal of the hormonal regulation with sinking glucose levels—occurs with some delay. Rapid flooding with glucose, causing rapidly rising insulin levels and subsequent rapid decline of glucose levels, frequently leads to blood glucose values below initial levels.
However, these statements, derived from experiments with watery glucose solutions, are not applicable to nutritive carbohydrate intakes. Postprandial glucose concentrations vary considerably, depending on food composition. For instance, high fat content prolongs gastric retention, whereas high fiber content leads to delayed intestinal absorption. Food preparation alone (raw, boiled, fried) affects glucose availability. Effectiveness of the processes involved in carbohydrate digestion is just as individually variable as is effectiveness of the response to the influx of glucose into the blood. A multitude of vitamins and trace elements are involved in carbohydrate metabolism. It makes sense, therefore, that the availability of those substances would affect glucose tolerance. Vitamin Bg and chromium deficiencies have been discussed in the context of reduced glucose tolerance for years.
The realization that identical amounts of carbohydrates—consumed in the form of different foods—may lead to different blood glucose profiles led to the development of the glycemic index (GI)
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