Sometimes looking at things from a distance can help to see the big picture. Such is the case with many aspects of human physiology, including the body’s use of carbohydrates for fuel.
This article is intended as an overview of carbohydrates and how they are processed by the body. The 10,000 foot view that follow can be serve as a reference for addressing client questions as well as a starting point for exploring the following subject in greater detail.
- All carbohydrates are not created equal. Some may be complex and contain a high amount of fiber, while others may be simple monosaccharides (sugars) and require little to no additional processing in order to be digested.
- Carbohydrates are ingested: Carbohydrates enter into the stomach, and then into the small intestine, where most absorption occurs. The principle enzyme that breaks down carbohydrates in the mouth is alpha amylase. Carbohydrates eventually get broken down into simple sugars such as the disaccharides lactose, sucrose, maltose, or monosaccharides fructose, galactose, glucose, as well as smaller chains of glucose molecules known as glucose polymers.
- These simple sugars are then absorbed through the lining of the small intestine and are transported to the liver through the portal vein. The liver, the body’s largest glandular organ, in turn converts all these simple sugars into the “common” sugar known as glucose 6 phosphate.
- The liver releases this new glucose into the bloodstream to raise blood sugar appropriate levels. If the supply of this newly ingested glucose is too high, it raises the blood sugar level.
- The pancreas stores the hormones insulin and glucagon which are the primary controls of the blood glucose levels in the body. When the blood sugar reaches increasing levels, insulin is released to transport the glucose into the cell. When blood glucose is low, glucagon triggers the liver to release stored glycogen as glucose to replenish blood-sugar levels. This intermittent release of stored glycogen from the liver to regulate blood sugar primarily for brain function is known as glycogenolysis.
- The muscle tissue is then the first destination for “insulin-carried” glucose, only after exercise when muscle energy stores are low. Insulin opens up receptor sites on muscles allowing for the uptake of glucose to replenish depleted muscle energy stores. Once glucose is absorbed into the cell it is used for energy, the unused portion of this new glucose is then converted and stored in the muscle or liver as glycogen.
- Among the over 60 known functions the liver serves, the organ has the potential to store limited amounts of glycogen. This liver glycogen, unlike muscle glycogen, can be transported into the central circulation and used for brain function as well as for physical activities. When liver and muscle stores are full, and there is still a considerable amount of excess insulin-carried glucose in the bloodstream, it will be converted into the adipose tissue and stored as fat.
- It is important to note that if the carbohydrates ingested are already simple sugars, or are low in soluble dietary fiber, they will be taken up into the blood quickly. Insulin over-reacts in this situation, and since its function is to eliminate sugar from the blood, within 20 to 30 minutes, blood sugar levels may fall below resting levels in a condition known as hypoglycemia. This reduces the glucose supply to the central nervous system and exhibits a noticeable effect on higher brain function, making the person feel sluggish, tired, and run down. One way to minimize this effect is to eat smaller meals with less simple sugars.
- If glucagon reaches the liver and for reasons such as extreme dieting, or overexertion, the it cannot provide glucose for the blood, there is a sequence of events that result in the eventual breakdown of blood proteins, and the undesirable cannibalism of organ and muscle tissue for the needed glucose energy. This process of tissue breakdown for energy is known as gluconeogenesis.
Jeukendrup, A.E. and Gleeson M. (2010). Sports nutrition: An introduction to energy production and performance, 2nd ed. Human Kinetics, Champaign, IL
Mead, J.R., Irvine, S.A., Ramii, D.P. (2002). Lipoprotein lipase: structure, function, regulation and role in disease. Journal of Molecular Medicine, Dec; 80(12):753-69. Epub 2002 Oct 24.