Elsevier

Surgery (Oxford)

Volume 27, Issue 1, January 2009, Pages 6-10
Surgery (Oxford)

Hepatopancreatobiliary I
Carbohydrate metabolism

https://doi.org/10.1016/j.mpsur.2008.12.002Get rights and content

Abstract

Carbohydrate normally accounts for about 50% of total dietary energy intake, but the general recommendation is for an increased consumption of complex carbohydrates. After digestion and absorption, carbohydrate is metabolized to provide energy (4 kcal/g) or is stored in muscle and liver as glycogen. The body’s carbohydrate stores are normally about 400–500 g in the fed state. Six-carbon glucose molecules are degraded by a series of chemical reactions to three-carbon pyruvate by the reactions of glycolysis; pyruvate can be further metabolized to lactate. These reactions occur in the cell cytoplasm without the involvement of molecular oxygen, so are described as anaerobic. Pyruvate (and lactate) can be further oxidized to CO2 and water by the reactions of the Krebs’ (tricarboxylic acid) cycle that occur in the mitochondria. Glucose is an essential fuel for the brain and for some other cells, notably red blood cells. Because body carbohydrate reserves are limited, and also because stored fatty acids cannot be converted to carbohydrate, the metabolism of carbohydrates in different tissues is tightly regulated. Some de novo synthesis of glucose is possible from non-carbohydrate sources, including glycerol and the carbon skeletons of some amino acids. Excess dietary carbohydrate is generally oxidized rather than stored.

Section snippets

The reactions of anaerobic glycolysis and glycogenolysis

The initial steps in the degradation of carbohydrate occur without the involvement of oxygen, and are therefore termed anaerobic processes. Degradation of glucose is referred to as glycolysis, whereas glycogenolysis begins with glycogen. Glycolysis effectively converts a six-carbon glucose molecule to two three-carbon pyruvate molecules (Figure 2). Glycolysis allows some of the chemical energy liberated by the breaking of chemical bonds in the glucose molecule to be conserved in the form of

Regeneration of NAD+

When glycolysis proceeds rapidly, the problem for the cell is that the availability of NAD+, which is necessary as a co-factor in the glyceraldehyde 3-phosphate dehydrogenase reaction, becomes limiting. The amount of NAD+ in the cell is very small – only about 0.8 μmol per g of muscle – relative to the rate at which glycolysis can proceed. In brief bursts of activity induced by electrical stimulation, muscle ATP turnover can reach 150 μmol/g/minute. If the NADH formed by glycolysis is not

Regulation of glycolysis

The rate of glycolysis must be regulated to ensure that ATP supply is matched to the rate of ATP hydrolysis and with the availability of other energy sources. As well as this local regulation within each cell, there is a need for a coherent response in the different tissues involved in carbohydrate metabolism, and for a coordination of the pathways of carbohydrate metabolism with those of fat and protein. Key steps in the metabolism of carbohydrate include the entry of glucose into the cell,

Carbohydrate utilization in different tissues

Some tissues, including red blood cells, the renal medulla and the retina, have no mitochondria and therefore no capacity for oxidative metabolism. These tissues therefore rely solely on glycolysis for energy production. Skeletal muscle can derive most of its energy requirement from oxidative metabolism or from anaerobic metabolism, and the choice of fuel depends on the energy demand, the metabolic capacity of the tissue, the availability of substrate and the availability of oxygen. Although

Gluconeogenesis: the formation of glucose from non-carbohydrate sources

Carbohydrate metabolism continues even in the absence of dietary intake, and some resynthesis of carbohydrate can take place using non-carbohydrate sources. Gluconeogenesis – the synthesis of glucose from non-glucose sources – becomes important during starvation by making glucose available to those tissues, including the brain, that cannot use other fuels. Gluconeogenesis also allows recycling of the lactate produced by tissues such as red blood cells.

A variety of substrates can contribute to

Hormonal control of carbohydrate metabolism

Several hormones play crucial roles in the regulation of carbohydrate utilization as well as in integrating the supply and utilization of carbohydrate fuels in different tissues. The blood glucose concentration must be maintained within narrow limits, and cannot be allowed to rise too far after a high-carbohydrate meal or to fall too far during fasting or muscular exercise.

Insulin is central to the regulation of carbohydrate metabolism. It also has a regulatory function in lipid and protein

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