5. Carbohydrates

Carbohydrates are the main source of energy in nutrition and usually provide 40–60% of the energy supply in western diets. The majority of the carbohydrate derived from a normal diet reaches the body’s peripheral tissues as glucose. Glucose is utilised by all cells and serves as metabolic fuel for muscle, liver, heart, kidneys and gut and as the obligate energy source for brain, renal medulla and erythrocytes. Glucose is the main carbohydrate utilised during foetal life; about 7 g/kg per day (approximately 5 mg/kg per minute) of glucose crosses the placenta in the last trimester of pregnancy. The major source of non-protein calories in parenteral nutrition (PN) is D-Glucose (dextrose), which is provided in the monohydrate form for intravenous use. Dextrose usually contributes most of the osmolality of the PN solution. By considering the consequences of excessive glucose intake during PN, and by taking into account the rate of glucose production and oxidation, it is possible to establish recommendations. Energy provision during PN includes the use of intravenous fat emulsions (IVFE) (see Lipids chapter). IVFE provide a concentrated source of calories with a low osmotic load (2.0 kcal/ml for a 20% emulsion, compared to about 0.8 kcal/ml for a 20% dextrose solution). The optimal glucose/lipid ratio remains to be defined. Glucose tolerance may be influenced by cyclical PN, metabolic status, acute illness and always requires careful monitoring.