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-60 0 60 120 180 240 300 Time after glucose (min)

Fig. 6.5 Rates of glucose release from liver, from exogenous (dietary) and endogenous (gluconeogenesis + glycogenolysis) sources, in normal subjects before and after drinking 75 g glucose in water. The rate of glucose disappearance from the circulation (i.e. utilisation by all tissues) is also shown. All rates are in mg/min. The measurements were made by radioactive tracer techniques. Labelled [H3]-glucose was infused into the circulation at a constant rate; the extent to which it was 'diluted' with unla belled glucose was used to estimate the rate of entry of glucose into the circulation ('total glucose appearance'). In addition, the glucose drink was labelled with [14C]-glucose, so that the rate of entry of exogenous glucose into the circulation could be measured. The 'endogenous' glucose production was then calculated by difference. Total glucose entry into the circulation (the sum of exogenous and endogenous appearance) increased after the glucose drink, and hence the blood glucose concentration rose - top panel. Release of endogenous glucose from hepatocytes was markedly suppressed. The rate of disappearance of glucose from the circulation also increased, stimulated by the increased insulin concentration. Based on Fery, F.D., Attellis, N.P. & Balasse, E.O. (1990) Mechanisms of starvation diabetes: study with double tracer and indirect calorimetry. Am J Physiol 259: E770-E777. With permission of the American Physiological Society.

6.1.2.2 Carbohydrate metabolism in other tissues after breakfast

Other tissues respond to the increase in insulin concentration. In skeletal muscle and adipose tissue, glucose uptake will be stimulated by the rise in insulin through increased numbers of GLUT4 transporters at the cell membrane, and by increased disposal of glucose within the cell. At the same time, the plasma concentration of non-esterified fatty acids falls because fat mobilisation in adipose tissue is suppressed; this will be discussed in more detail below. Therefore tissues such as skeletal muscle, which can use either fatty acids or glucose as their energy source, switch to utilisation of glucose. Not all the glucose taken up by muscle is oxidised under these conditions; insulin also activates muscle glycogen synthase and glycogen storage will replenish muscle glycogen stores (Fig. 6.6). Thus, after a meal containing carbohydrate, there is a general switch in metabolism to the use of glucose rather than fatty acids, but there is also a major switch to the storage of glucose as glycogen. The pattern of postprandial glucose metabolism, and some important regulatory points, are illustrated in Fig. 6.7.

6.1.2.3 Disposal of glucose after a meal

As we have discussed, the amount of glucose in the meal (typically 80-100 g) would be enough to raise the concentration of glucose in the plasma about 8fold. In fact, in a normal healthy person, the peak glucose concentration after such a breakfast will be about 7-8 mmol/l (Fig. 6.4), a rise of only 60% at most from the postabsorptive value of 5 mmol/l. On the other hand, the insulin concentration may have gone from around 60 pmol/l to perhaps 400 -500 pmol/l (Fig. 6.4), a very much bigger percentage change; this illustrates the relationship between 'controlling' and 'controlled' variables discussed earlier (see Fig. 6.2). The glucagon concentration in 'systemic' (mixed) blood plasma may change less, but there will be a change in the insulin/glucagon ratio, probably

Time after meal (hours)

Fig. 6.6 Concentrations of muscle glycogen after a single meal in normal subjects, studied by the technique of nuclear magnetic resonance. The meal, shown by the arrow, contained 290 g carbohydrate and 45 g fat. Redrawn from Taylor, R., Price, T.B., Katz, L.D., Shulman, R.G. & Shulman, G.I. (1993) Direct measurement of change in muscle glycogen concentration after a mixed meal in normal subjects. Am J Physiol 265: E224-E229. With permission of the American Physiological Society.

Time after meal (hours)

Fig. 6.6 Concentrations of muscle glycogen after a single meal in normal subjects, studied by the technique of nuclear magnetic resonance. The meal, shown by the arrow, contained 290 g carbohydrate and 45 g fat. Redrawn from Taylor, R., Price, T.B., Katz, L.D., Shulman, R.G. & Shulman, G.I. (1993) Direct measurement of change in muscle glycogen concentration after a mixed meal in normal subjects. Am J Physiol 265: E224-E229. With permission of the American Physiological Society.

Brain -

Brain -

Fig. 6.7 The pattern of glucose metabolism after a carbohydrate breakfast. The direct pathway of glycogen storage is shown (glucose from small intestine going to liver glycogen), as is the indirect pathway (glucose forming lactate in the small intestine or in peripheral tissues, lactate then being used for liver glycogen synthesis): see Section 4.1.2.1.

Fig. 6.7 The pattern of glucose metabolism after a carbohydrate breakfast. The direct pathway of glycogen storage is shown (glucose from small intestine going to liver glycogen), as is the indirect pathway (glucose forming lactate in the small intestine or in peripheral tissues, lactate then being used for liver glycogen synthesis): see Section 4.1.2.1.

greater still in the hepatic portal vein - i.e. in the concentrations of hormones reaching the liver.

By the end of the absorptive period - about 5 hours after the meal - approximately 25 g of the 100 g of carbohydrate ingested will have been stored and 75 g oxidised. Thus, although glucose oxidation in tissues was increased after the meal, the 'drive' for glucose storage is such that around one-quarter of the glucose in such a meal is stored for later use.

What happens towards the end of the absorptive period depends, of course, on what the subject decides to do. Most likely, another meal will be taken, and glucose storage will increase further; but exercise and other factors (e.g. stress, illness) if they occur, will influence the disposition of the nutrients. These factors will be considered in later chapters.

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