Once dietary carbohydrates are broken down into monosaccharides, they are absorbed by the cells of the small intestine. Glucose and galactose are absorbed via active transport, while fructose is absorbed via facilitated diffusion. These monosaccharides then enter the capillaries and travel to the liver via the hepatic portal vein where hepatocytes metabolize fructose and galactose. Glucose molecules continue on through the liver and re-enter vascular circulation via the hepatic vein, contributing to blood sugar levels and nourish the body’s cells.
Glucose Stored as Glycogen:
After someone eats a meal composed of carbohydrates, blood sugar rises and β-cells in the pancreas release the hormone insulin, inducing the body’s cells to take up glucose for their energy needs. Any excess glucose, however, is converted into glycogen (an animal polysaccharide characterized by long, branching chains of glucose) in the liver and muscle cells. The liver contains about one-third of the body’s glycogen, whereas the muscles can store two-thirds (which is monopolized mostly by the muscle cells for their own energy needs, especially during physical activity); the brain also stores a minor reserve of glycogen in case of emergencies. When blood sugar is below normal, the α-cells of the pancreas secrete the hormone glucagon, signaling the liver to break down glycogen and release glucose into the bloodstream. The adrenal hormone epinephrine also acts on the liver to liberate glucose from glycogen storage during times of stress when the cells of the brain, muscles, etc. need an energy boost.
Glucose Stored as Fat:
Glycogen, however, is only a short-term storage of glucose because it retains water and takes up too much space for the body to maintain reserves for more than a few days (and only a few hours during exercise). Therefore, to accommodate excessive glucose, the liver metabolizes glucose and reassembles its components into fat, sending it to the body’s adipose tissue for more long-term (and practically unlimited) storage. Since fats provide almost twice the energy (nine kilocalories per gram) as carbohydrates, this fatty tissue is able to store more energy into more compact units compared to glycogen. Once glucose has been converted to fat, however, it cannot be converted back into glucose like glycogen can. During times of carbohydrate deficiency, the body can modify fat metabolism and create ketone bodies which can enter the citric acid cycle after being converted to acetyl-CoA; the metabolic pathway proceeds from there to produce ATP, the main energy currency of the cell. If ketone bodies accumulate, though, a condition called ketosis can develop: the pH of the blood and body fluids becomes acidic, denaturing body proteins and possibly resulting in coma and death. To avoid ketosis, a minimum of 50-100 grams of carbohydrates should be consumed each day.
Berg, J. (2002). Chapter 21 Glycogen Metabolism. Retrieved May 23, 2015, from http://www.ncbi.nlm.nih.gov/books/NBK21190/
Flatt, J. (1970). Conversion of carbohydrate to fat in adipose tissue: An energy-yielding and, therefore, self-limiting process. Retrieved May 23, 2015, from http://www.jlr.org/content/11/2/131.full.pdf
Whitney, E., & Rolfes, S. (2013). The Carbohydrates: Sugars, Starches, and Fibers. In Understanding Nutrition (14th ed., pp. 109-111). Belmont, CA: Thomson/Wadsworth.
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