Carbohydrates
Energy storage
If the body already has enough energy to support its functions -- if you’ve already consumed an adequate amount of cabohydrates to go through your day -- then the excess is stored as glycogen in the muscles and liver. The amount of glycogen in the body at a given time is about 4,000 kilocalories (about 3,000 in muscle tissues and 1,000 in the liver). One molecule of glycogen may obtain over 50,000 single glucose units and is highly branched, which allows for rapid and convenient conversion of glucose to ATP [5].
Prolonged muscle use can deplete this reserve, commonly know as “hitting the wall”. Your muscles will feel weaker because it takes longer to transform the chemical energy in fatty acids and proteins to usable energy than glucose, which the body can use much more efficiently.
Unlike muscle, the liver sends its stored glucose to other tissues in the body when blood glucose is low, as glucose is only used (converted into ATP) directly from muscle. Between meal times, the liver keeps blood-glucose levels within a narrow range between meal times -- homeostasis in action!
Energy production
Glucose is the primary source of energy for the body’s cells. The cells use it more efficiently than any other energy source, in the process of cellular respiration.
Some cells, such as red blood cells, can ONLY produce energy from glucose, as opposed to lipids or amino acids. The brain is also highly sensitive to low blood-glucose because it only uses glucose to function, unless under extreme starvation [5].
Building macromolecules
Some glucose is converted to ribose and deoxyribose, which are the sugars that make up RNA and DNA, respectively.
While most of the products of glycolysis will continue through cellular respiration, there is an alternative path that the products can take, known as the pentose phosphate pathway (PPP). It is this pathway that enables the formation of ribose and deoxyribose, both of which are pentoses, or 5-carbon sugars [6]. ("Pent" refers to five, "ose" refers to sugar!)
The reduced (has an extra electron) form of nicotinamide adenine dinucleotide phosphate, or NADPH, is another product of glucose through the PPP. This compound protects against oxidative stress and is used in many chemical reactions in the body.
Excess glucose can be also used to make fat, with the help of the hormone insulin.
Sparing fat and protein
When there’s not enough glucose to meet the body’s needs, glucose is synthesized from amino acids. There’s no storage molecule for amino acids (like glycogen is for glucose), so the process just destroys proteins, which will primarily come from muscle tissue. This process is known as gluconeogenesis: the production of glucose from non-carbohydrate sources [7].
As blood-glucose levels rise, the use of lipids as an energy source is also inhibited. An increase in blood glucose stimulates the release of insulin, which is what tells cells to use glucose, instead of lipids, to make energy [8].
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Adequate blood-glucose levels prevent ketosis, a metabolic condition resulting from elevated levels of ketone bodies in the blood (the body continually makes a small amount of ketones to be used for energy). Ketones are an alternative energy source that cells can use when glucose levels are insufficient; unlike fatty acids, ketones can be used as energy for the brain, as they can cross the blood-brain barrier. They are acidic, and high elevations can be dangerous. In a typical adult, the minimum amount of carbohydrares to inhibit ketosis is about 50 g/day, but for optimal brain function, one's carb intake should be higher [9].