Calculation of whole body glucose metabolism M value

Provided that endogenous glucose output is zero or at least negligible, the glucose infused equals the glucose being translocated out of the glucose space (i.e. glucose metabolised, M value) during steady state conditions of hyperinsulinaemic euglycaemia (DeFronzo

Glucose flux into the glucose space ~2 mg-kg-1-min-1

Glucose space

Glucose space

Splanchnic bed (+liver)

Neural tissues

Blood cells Adipose tissue

Splanchnic bed (+liver)

Skeletal muscle (resting)

Glucose flux out of the glucose space ~2 mg-kg-1-min-1

Figure 4.3 Schematic diagram of postabsorptive glucose metabolism in healthy humans. Under postabsorptive conditions the glucose flux into the glucose space (extracellular fluid) equals the glucose flux out of the glucose space, resulting in constant glycaemia. The only source of glucose input is of hepatic and renal origin (endogenous glucose output). The approximate percentage contributions of various tissues to postabsorptive glucose utilisation is summarised and approximated from the following: Bjorntorp & Sjostrom (1978); Consoli(1992); Dinneen etal. (1992); Boyle etal. (1994); Ekberg etal. (1999); Zierler (1999); Gerich (2000).

etal. 1979). Because the plasma glucose concentration is almost never perfectly maintained (i.e. clamped) at constant, corrections must be done (Figure 4.5). DeFronzo etal. (1979) introduced a correction factor for this under the term 'space correction'. This space correction adjusts the glucose infusion rates from changes in glycaemia and not from real changes in glucose utilisation. For example, when the plasma glucose concentration during a clamp test is raised from 95 mg/dl at 80 minutes to 100 mg/dl at 100 minutes, the glucose

Glucose flux into the glucose space ~7 mg-kg-1-min-1

Exogenous glucose infusion

Neural tissues

Splanchnic bed ■.■[>■■ Other tissues Blood cells

Liver Kidneys

Liver Kidneys

Neural tissues

Splanchnic bed ■.■[>■■ Other tissues Blood cells

Glucose flux out of the glucose space ~7 mg-kg-1-min-1

Figure 4.4 Schematic diagram of insulin stimulated glucose metabolism during a euglycaemic (100 mg/dl) hyperinsulinaemic (40 mU-m-2-min-1) clamp test in healthy humans. Because endogenous glucose output is almost completely suppressed by hyperinsulinaemia, the exogenous glucose infusion accounts for whole body glucose requirements under these conditions. The diagram provides raw approximate percentage contributions of various tissues to insulin stimulated whole body glucose metabolism in healthy humans. The contribution of skeletal muscle may vary with individual insulin sensitivity.

infusion rate in the period between 80 and 100 minutes has actually exceeded the rate of glucose utilisation, otherwise plasma glucose concentration would not have increased. The reverse is true when the plasma glucose concentration declines, indicating that the glucose infusion rate could not completely account for the rate of glucose utilisation in this period.

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