We need carbohydrates in the human body for health purposes (discussed in the next chapter) as well as energy production, energy storage, building macromolecules, sparing 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, 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 via 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 the process by which energy is captured from glucose. It 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.

The first stage in the breakdown of glucose is called glycolysis. Glycolysis, or the splitting of glucose, occurs in an intricate series of ten enzymatic-reaction steps. The second stage of glucose breakdown 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, making the cellular energy available in a form that cells can use.

Each gram of carbohydrates that we consume is equivalent to 4 kcal of energy. During exercise, carbohydrates are a major source of energy, and they become more predominantly used as exercise intensity increases.

Figure 4.11 illustrates the process of cellular respiration, where glucose and oxygen are utilized by cells to produce energy, releasing carbon dioxide and water as byproducts.

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 molecule of glycogen may contain fifty thousand single glucose units and is highly branched, allowing for the rapid dissemination of glucose when needed for cellular energy.

The amount of glycogen in the muscle is approximately 300 g and approximately 100 g in the liver.^[1] Prolonged muscle use (such as longer-duration exercise) can deplete the glycogen energy reserve, which can cause a decrease in exercise performance. After prolonged exercise, as glycogen stores deplete, 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, but this will only benefit athletes who exercise for durations longer than 90 minutes. If events are longer than 45 minutes, people can reduce stored glycogen use by ingesting carbohydrates.

The liver, like muscles, 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. 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 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, which is important for protection against oxidative stress and is used in many other chemical reactions in the body. If all of the energy, glycogen-storing capacity, and building needs of the body are met, excess glucose can be used to make fat. This is why a diet too high in carbohydrates and calories can add to the fat pounds—a topic that will be discussed shortly.

The sugar molecule deoxyribose is used to build the backbone of DNA.

Figure 5.13 represents a double-stranded DNA molecule, consisting of adenine, thymine, cytosine, and guanine nucleotides. The nucleotides are connected by a phosphate backbone, forming the characteristic helical structure of DNA.

Sparing Protein

In a situation where there is not enough 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 essentially spares the breakdown of proteins to make glucose.

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 the blood to become too acidic. This is rare in healthy adults but can occur in those with alcoholism, people who are malnourished, and individuals who have Type 1 diabetes.

Carbohydrates are critical to support life’s most basic function—the production of energy. 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. Unfortunately, access to affordable and nutritious food can be a challenge for individuals and families residing in lower-income communities. This limited access often leads to a greater reliance on processed and inexpensive foods that tend to be high in simple carbohydrates.