Chapter 11 Disorders of carbohydrate metabolism
Introduction
Glucagon is a 29 amino acid polypeptide secreted by the α-cells of the pancreatic islets; its secretion is decreased by a rise in the blood glucose concentration. In general, its actions oppose those of insulin: it stimulates hepatic (although not muscle) glycogenolysis through activation of glycogen phosphorylase, gluconeogenesis, lipolysis and ketogenesis. The control of ketogenesis is discussed on pp. 191–192. The combined effects of insulin and glucagon are shown diagrammatically in Figure 11.4.
Diabetes Mellitus
Diagnosis
The other indications and the protocol for the oral glucose tolerance test (OGTT) are given in Figure 11.6, and the interpretation of results in Figure 11.7. In the majority of patients suspected of having diabetes, the measurements indicated above will establish the diagnosis, and formal glucose tolerance testing is superfluous: it is only indicated when the diagnosis is in doubt. Note that the diagnostic values are the glucose concentrations fasting and 2 h after glucose: taking samples at 30-min intervals, as used to be recommended, is not required.
Monitoring treatment
Many patients (or their carers) monitor their own blood glucose concentrations at home using reagent strips and a glucose meter (see p. 184). This may be done more or less frequently as circumstances require: exercise, illness or a change of diet may alter insulin requirements, and more frequent testing will allow the patient to adjust the dosage accordingly. In type 1 diabetes, currently recommended targets for treatment in adults in the UK are preprandial blood glucose concentrations of 4.0–7.0 mmol/L and postprandial concentrations of <9.0 mmol/L. The corresponding figures in children are 4.0–8.0 mmol/L and <10 mmol/L. Urine testing for glucose is now little used; it should not be used to monitor type 1 diabetes. Such testing is only semi-quantitative and is of no value in the detection of hypoglycaemia: the urine is virtually free of glucose at normal blood glucose concentrations. Urine glucose excretion also depends on the renal threshold for glucose: if this is low (as, for example, in renal glycosuria, p. 198) glucose may be present in the urine at normal blood glucose concentrations. Urine testing for glucose should be used in type 2 diabetes only in patients unable or unwilling to do blood tests, and in older patients in whom control may not need to be so strict, particularly if they are treated with diet alone.