METABOLISM OF CARBOHYDRATES

Metabolism is the term used to describe the interconversion of chemical compounds in the body, the pathways taken by individual molecules, their interrelationships, and the mechanisms that regulate the flow of metabolites through the pathways. Metabolic pathways fall into three categories.
(1) Anabolic pathways, which are those involved in the synthesis of larger and more complex compounds from smaller precursors—for example, the synthesis of protein from amino acids and the synthesis of reserves of triacylglycerol and glycogen. Anabolic pathways are endothermic.
(2) Catabolic pathways, which are involved in the breakdown of larger molecules, commonly involving oxidative reactions; they are exothermic, producing reducing equivalents, and, mainly via the respiratory chain , ATP.
(3) Amphibolic pathways, which occur at the “crossroads” of metabolism, acting as links between the anabolic and catabolic pathways, for example, the citric acid cycle.

      Knowledge of normal metabolism is essential for an understanding of abnormalities that underlie disease. Normal metabolism includes adaptation to periods of fasting, starvation, and exercise, as well as pregnancy and lactation. Abnormal metabolism may result from nutritional deficiency, enzyme deficiency, abnormal secretion of hormones, or the actions of drugs and toxins.

A 70-kg adult human being requires about 8 to 12 MJ (1920-2900 kcal) from metabolic fuels each day, depending on physical activity. Larger animals require less per kilogram body weight, and smaller animals more. Growing children and animals have a proportionally higher requirement to allow for the energy cost of growth. For human beings, this energy requirement is met from carbohydrates (40%-60%), lipids (mainly triacylglycerol, 30%-40%), and protein (10%-15%), as well as alcohol. The mix of carbohydrate, lipid, and protein being oxidized varies, depending on whether the subject is in the fed or fasting state, and on the duration and intensity of physical work.

There is a constant requirement for metabolic fuels throughout the day; average physical activity increases metabolic rate only by about 40% to 50% over the basal or resting metabolic rate. However, most people consume their daily intake of metabolic fuels in two or three meals, so there is a need to form reserves of carbohydrate (glycogen in liver and muscle), lipid (triacylglycerol in adipose tissue), and labile protein stores during the period following a meal, for use during the intervening time when there is no intake of food.

If the intake of metabolic fuels is consistently greater than energy expenditure, the surplus is stored, largely as triacylglycerol in adipose tissue, leading to the development of obesity and its associated health hazards. By contrast, if the intake of metabolic fuels is consistently lower than energy expenditure, there are negligible reserves of fat and carbohydrate, and amino acids arising from protein turnover are used for energy-yielding metabolism rather than replacement protein synthesis, leading to emaciation, wasting, and, eventually, death.

In the fed state, after a meal, there is an ample supply of carbohydrate, and the metabolic fuel for most tissues is glucose. In the fasting state, glucose must be spared for use by the central nervous system (which is largely dependent on glucose) and the red blood cells (which are wholly reliant on glucose). Therefore, tissues that can use fuels other than glucose do so; muscle and liver oxidize fatty acids and the liver synthesizes ketone bodies from fatty acids to export to muscle and other tissues. As glycogen reserves become depleted, amino acids arising from protein turnover are used for gluconeogenesis.

The formation and utilization of reserves of triacylglycerol and glycogen, and the extent to which tissues take up and oxidize glucose, are largely controlled by the hormones insulin and glucagon. In diabetes mellitus, there is either impaired synthesis and secretion of insulin (type I diabetes, sometimes called juvenile onset, or insulin-dependent diabetes) or impaired sensitivity of tissues to insulin action (type II diabetes, sometimes called adult onset or noninsulin-dependent diabetes), leading to severe metabolic derangement. In cattle, the demands of heavy lactation can lead to ketosis, as can the demands of twin pregnancy in sheep.