Fundamentals of Human Nutrition/Storage
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.
Carbohydrates are the body’s preferred source of energy since they get digested quickly compared to proteins and fats. Important dietary carbohydrates consist of monosaccharides, disaccharides, and polysaccharides. Some polysaccharides, such as cellulose, are resistant to chemical breakdown so they pass through the intestinal tract undigested. On the other hand, when other carbohydrates are consumed they get broken down into their most elementary form called monosaccharides, which are smaller units of sugar like glucose, fructose, and galactose. About five percent of this process occurs in the mouth and stomach with the help of mastication and salivary α-amylase. The rest of the process takes place in the upper part of the small intestine where pancreatic juice that contains the enzyme pancreatic-amylase can further assist in breaking down dextrins into shorter carbohydrate chains (“Introduction to Nutrition”, 2012). As soon as the carbohydrates are chemically broken down into single sugar units, they are quickly absorbed by the small intestine where they then enter the bloodstream and eventually ends up in the liver. The liver converts fructose and galactose to glucose. Glucose gets transferred back into the bloodstream so it can be transported to the various tissues and organs that need it for energy (Goodman, 2010).
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.
Carbohydrates don't increase body weight by direct conversion and storage to fat. Instead, an increase in body weight only occurs if energy input is significantly greater than energy output. If a person is consuming more carbohydrates than they are burning for energy, the body converts the extra carbohydrates in the form of glycogen and stores some of it in the liver and a majority of it in muscle; however, the human body is only capable of storing a limited amount of glycogen. Therefore, the carbohydrates that can’t be stored are either oxidized in response to immediate energy demands or converted to fat by the process of de novo lipogenesis. Even though most cells can perform de novo lipogenesis, the process mostly occurs in the liver and adipose tissue because liver cells and adipocytes are particularly well adapted. The first step in de novo lipogenesis is creating a compound called acetyl CoA from glucose and adipose or fat tissue (Herman & Kahn, 2012). The molecules of this compound are then linked together to form triglyceride molecules. When energy stores run low during exercise or because of inadequate calorie intake, the body starts breaking down and converting triglycerides back into glucose to use for energy. Only having a small supply of triglycerides won’t negatively impact ones health, but as the number of triglycerides increase the risk of getting heart disease also increases (“Carbohydrates to Triglycerides”).
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.
Converting Carbohydrates to Triglycerides. (2015). Retrieved from https://www.ncsf.org/enew/articles/articles-convertingcarbs.aspx#
Goodman, B. (2010). Insights into digestion and absorption of major nutrients in humans. Advances is Physiology Education, 34, 44-53. Retrieved from http://advan.physiology.org/content/34/2/44 (Links to an external site.)
Herman, M., Kahn, B. (2012). Adipose tissue de novo lipogenesis. Retrieved from http://www.asbmb.org/asbmbtoday/asbmbtoday_article.aspx?id=15872 (Links to an external site.)
Digestion and Absorption of Carbohydrates (2012). An Introduction to Nutrition. Retrieved from http://2012books.lardbucket.org/books/an-introduction-to-nutrition/s08-02-digestion-and-absorption-of-ca.html
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