Five Primary Functions of Carbohydrates

There are five primary functions of carbohydrates in the human body. They are involved in energy production, energy storage, building macromolecules, synthesizing protein, and assisting in lipid metabolism.

Energy Production

The primary role of carbohydrates is to supply energy to all cells in the body. Many cells prefer glucose as a source of energy versus other compounds like fatty acids. Some cells, such as red blood cells, can only produce cellular energy from glucose. The brain is also highly sensitive to low blood-glucose levels because it uses only glucose to produce energy and function (unless under extreme starvation conditions). About 70 percent of the glucose entering the body from digestion is redistributed (by the liver) back into the blood for use by other tissues. Cells that require energy remove the glucose from the blood with a transport protein in their membranes. The energy from glucose comes from the chemical bonds between the carbon atoms. Sunlight energy was required to produce these high-energy bonds in the process of photosynthesis. Cells in our bodies break these bonds and capture the energy to perform cellular respiration, a series of chemical reactions that break down glucose to produce ATP, the energy currency of cells.

Cellular respiration occurs in three main stages. The first stage, glycolysis, is the breakdown of glucose glycolysis. Glycolysis, or the splitting of glucose, occurs in a series of ten enzymatic reaction steps. The second stage of glucose breakdown is the citric acid cycle, which occurs in the energy factory organelles, called mitochondria. One carbon atom and two oxygen atoms are removed, yielding more energy. The energy from these carbon bonds is carried to another area of the mitochondria for the final stage of cellular respiration, the electron transport chain. The energy yield from a single glucose molecule is around 30 – 32 molecules of ATP, with the majority of ATP produced in the electron transport chain.

Energy Storage

If the body already has enough energy to support its functions, the excess glucose is stored as glycogen (the majority of which is stored in the muscles and liver). A glycogen molecule may contain in excess of fifty thousand single glucose units and is highly branched, allowing for the rapid dissemination of glucose when needed to make cellular energy.

The amount of glycogen in the body at any one time is equivalent to about 4,000 kilocalories—3,000 in muscle tissue and 1,000 in the liver. Prolonged muscle use (such as exercise for longer than a few hours) can deplete the glycogen energy reserve. Remember that this is referred to as “hitting the wall” or “bonking” and is characterized by fatigue and decreased exercise performance. The weakening of muscles sets in because it takes longer to transform the chemical energy in fatty acids and proteins to usable energy than glucose. After prolonged exercise, glycogen is gone, and muscles must rely more on lipids and proteins as an energy source. Athletes can modestly increase their glycogen reserve by reducing training intensity and increasing their carbohydrate intake to between 60 and 70 percent of total calories three to five days before an event. People who are not hardcore training and choose to run a 5-kilometer race for fun do not need to consume a big plate of pasta prior to a race, since, without long-term intense training, the adaptation of increased muscle glycogen will not happen.

The liver, like muscle, can store glucose energy as glycogen, but in contrast to muscle tissue, it will sacrifice its stored glucose energy to other tissues in the body when blood glucose is low. Approximately one-quarter of total body glycogen content is in the liver (which is equivalent to about a four-hour supply of glucose), but this is highly dependent on activity level. The liver uses this glycogen reserve as a way to keep blood-glucose levels within a narrow range between meal times. When the liver’s glycogen supply is exhausted, glucose is made from amino acids obtained from the destruction of proteins in order to maintain metabolic homeostasis.

Building Macromolecules

Although most absorbed glucose is used to make energy, some glucose is converted to ribose and deoxyribose, which are essential building blocks of important macromolecules, such as RNA, DNA, and ATP. Glucose is additionally utilized to make the molecule NADPH (nicotinamide adenine dinucleotide phosphate), which is important for protection against oxidative stress and is used in many other chemical reactions in the body. If all of the body’s energy, glycogen-storing capacity, and building needs are met, excess glucose will be stored as fat.

Sparing Protein

When there is insufficient glucose to meet the body’s needs, glucose is synthesized from amino acids. Because there is no storage molecule of amino acids, this process requires the destruction of proteins, primarily from muscle tissue. The presence of adequate glucose basically spares the breakdown of proteins from being used to make glucose needed by the body.

Lipid Metabolism

As blood-glucose levels rise, the use of lipids as an energy source is inhibited. Thus, glucose additionally has a “fat-sparing” effect. This is because an increase in blood glucose stimulates the release of the hormone insulin, which tells cells to use glucose (instead of lipids) to make energy. Adequate glucose levels in the blood also prevent the development of ketosis. Ketosis is a metabolic condition resulting from elevated ketone bodies in the blood. Ketone bodies are an alternative energy source that cells can use when the glucose supply is insufficient, such as during fasting. Ketone bodies are acidic, and high elevations in the blood can cause it to become too acidic. This is rare in healthy adults, but can occur in alcoholics, people who are malnourished, and individuals who have Type 1 diabetes. The minimum amount of carbohydrate in the diet required to inhibit ketosis in adults is 50 grams per day.

Carbohydrates are critical to support life’s most basic function—energy production. Without energy, none of the other life processes are performed. Although our bodies can synthesize glucose, it comes at the cost of protein destruction. As with all nutrients, though, carbohydrates are to be consumed in moderation, as having too much or too little in the diet may lead to health problems.