Metabolism of Sugar

Appears in

Oxford Companion to Sugar and Sweets

Oxford Companion to Sugar and Sweets

By Darra Goldstein

Published 2015

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The metabolism of sugars is a complex process that eventually produces energy to run the body—carbon dioxide and water, the same products that result from incinerating sugar at high temperatures. Sucrose is essentially a biological fuel.
When humans consume sugars, the sugars are rapidly broken down into monosaccharides—simple sugars such as glucose, fructose, and galactose. Acid secreted in the stomach can catalyze the breaking of the covalent bond between sugar units; sucrose is exceptionally susceptible to this cracking reaction. The resulting glucose and fructose, along with any remaining undigested sucrose, are passed to the small intestine, where enzymes complete the digestion. Each enzyme is targeted to break down a specific complex sugar. Lactase is the enzyme specific for the digestion of lactose, found in milk, into the simple sugars galactose and glucose, while sucrase plays the same role for table sugar. Molecular chaperones then actively escort individual glucose molecules across the intestinal membranes into the bloodstream, while fructose is left to diffuse through the membranes on its own, through a slower process.