4.4: Functions of Carbohydrates

Learning Objectives

• List and describe functions of carbohydrates in the human body.

There are four primary functions of carbohydrates in the human body. They are energy production, energy storage, sparing protein, and preventing ketosis.

Energy Production

The primary role of carbohydrates is to supply energy to all cells in the body; each gram of carbohydrate supplies 4 kilocalories. Many cells prefer glucose as a source of energy versus other compounds like fatty acids. Some cells, such as red blood cells, are only able to 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. Cellular respiration is basically a controlled burning of glucose versus an uncontrolled burning. A cell uses many chemical reactions in multiple enzymatic steps to slow the release of energy (no explosion) and more efficiently capture the energy held within the chemical bonds in glucose.

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 muscle and liver). A molecule of glycogen may contain over 50,000 single glucose units and is highly branched, allowing for the rapid dissemination of glucose when it is needed to make cellular energy (Figure \(\PageIndex{1}\)).

Figure \(\PageIndex{1}\):The structure of glycogen enables its rapid mobilization into free glucose to power cells.

The amount of glycogen in the body at any one time is equivalent to about 4,000 calories—3,000 in muscle tissue and 1,000 in the liver. During high intensity exercise, proportionally more carbohydrate is used as fuel. During low intensity exercise, proportionally more fat is used as fuel. Prolonged muscle use (such as exercise for longer than a few hours) can deplete the glycogen energy reserve. This is sometimes referred to as “hitting the wall” and is characterized by fatigue and a decrease in 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 increase their glycogen reserve modestly by reducing training intensity and increasing their carbohydrate intake to between 60 and 70 percent of total calories three to five days prior to an event. People who are not participating in long-term intense 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, the liver 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 (through a process known as gluconeogenesis).

Sparing Protein

In a situation where there is not enough glucose to meet the body’s needs, glucose is synthesized from amino acids (through gluconeogenesis). Because there is no storage molecule of amino acids, this process requires the destruction of proteins, primarily from muscle tissue. Therefore, the body cannot use proteins to make new cells, repair tissue damage, support our immune system, and perform many other functions. The presence of adequate glucose basically spares the breakdown of proteins from being used to make glucose needed by the body.

Preventing Ketosis

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 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 an elevation of ketone bodies in the blood. Ketone bodies are an alternative energy source that cells can use when glucose supply is insufficient, such as during fasting. Ketone bodies are acidic and high levels of ketones in the blood can cause the blood to become too acidic (a condition known as ketoacidosis) and cause damage to body tissues. Ketoacidosis is rare in healthy adults, but can occur in alcoholics, people who are malnourished, and in 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.^1,2 Symptoms of ketosis include dehydration, bad breath (sometimes called "keto breath"), and high blood acidity.

Key Takeaways

• The four primary functions of carbohydrates in the body are to provide energy, store energy, spare protein, and prevent ketosis.
• Glucose energy is stored as glycogen, with the majority of it in the muscle and liver. The liver uses its glycogen reserve as a way to keep blood-glucose levels within a narrow range between meal times.
• The presence of adequate glucose in the body spares the breakdown of proteins from being used to make glucose needed by the body.
• Adequate glucose also prevents ketosis which can make the blood too acidic.

References

1. Batch JT, Lamsal SP, Adkins M, Sultan S, Ramirez MN. Advantages and Disadvantages of the Ketogenic Diet: A Review Article. Cureus. 2020;12(8):e9639. doi:10.7759/cureus.9639. Accessed June 16, 2021.
2. Food and Nutrition Board of the Institute of Medicine. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. Washington, DC: National Academies Press; 2002.