GLUCOSE AND INSULIN METABOLISM IN RELATION TO DIETARY CARBOHYDRATES

11.2 GLUCOSE AND INSULIN METABOLISM IN RELATION TO DIETARY CARBOHYDRATES

Carbohydrates are the major source of energy in the Western diet. According to the population nutrient intake goals for preventing diet-related chronic diseases, 55 to

75% of the daily energy intake should come from carbohydrates. 3 However, this is often not reached, and in many countries increased carbohydrate intake is recom- mended. Nutritionally, carbohydrates can be classified as digestible and nondigest-

ible, or available and nonavailable. 4 Mono- and disaccharides and starch, the most

The Role of Carbohydrates

common digestible carbohydrate, are rapidly digested and absorbed in the small intestine. Dietary fiber is a nutritional definition for the nondigestible part of plant

food and includes many oligosaccharides, cellulose, hemicellulose, pectins, gums, lignin, and associated plant substances. 5 For prevention of diet-related chronic dis- eases, an intake of at least 25 g/day of dietary fiber is recommended, 3 but in many countries this level of dietary fiber intake is not reached. Consumption of digestible carbohydrates leads to elevation of the blood sugar level. The rate of gastric emptying influences blood glucose and insulin values. 6 Gastric emptying, in turn, is affected by particle size after chewing 6,7 and by the

presence of viscous food polymers. 8 The current Western diet is often rich in rapidly digestible carbohydrates, which are absorbed quickly in the upper part of the small intestine and cause a rapid postprandial rise in blood glucose concentration. This normally reaches the peak value within 30 to 45 min, and then gradually decreases, returning to the fasting level within 90 to 180 min. The bloodstream carries the absorbed monosaccharides to the liver via the portal vein. As glucose is a key nutrient for many tissues, and the critical source of energy for some tissues (brain, kidney cortex, erythrocytes), the changes in plasma glucose are regulated within very narrow limits in healthy humans. The increase in plasma glucose after a meal is followed by a rapid discharge of insulin from pancreatic cells. This, in turn, suppresses the output of glucose from the liver and stimulates the uptake of glucose from plasma into the peripheral tissues. 9,10 Also, part of the glucose is taken up by the liver and converted to glycogen. The largest part of glucose is taken up by the muscle tissues. 11 The major part of absorbed fructose and galactose is taken up by liver and requires further metabolism before contributing to blood glucose. Their contribution to blood glucose level is small, but if excess fructose is consumed, hyperglycemia may result since fructose does not stimulate secretion of insulin. 12

Glucose uptake and further metabolism in different target cells are controlled by insulin. Basal insulin secretion occurs in pulses of 10 to 12 min. 13 This provides

a more efficient control of the glycemic response than a steady insulin infusion. Postprandial insulin secretion has two phases: a short acute first phase is seen within the first 10 min of the stimulus, followed by a second phase lasting until the stimulus

ceases. 14 The glycemic response is the main stimulus to insulin secretion, but other factors, such as nutrients, hormones, and neurotransmitters, may potentiate or inhibit the secretory response to glucose. 15–17 For example, when ingested with glucose, arginine attenuates and prolongs the rise of blood glucose. 18