There are a number of different disorders that can cause exercise intolerance. Exercise intolerance occurs when an individual is unable to exercise an average amount. It's an important phenomenon for both medical professionals and other individuals to be aware of, as exercising more is an impossible suggestion for an individual with exercise intolerance. To exercise, an individual needs to have nutrients and oxygen delivered adequately to their muscles. The muscles must be able to utilize the oxygen and nutrients. In addition, the muscles must be able to generate an appropriate amount of energy. When someone has exercise intolerance, they generally have a disorder that keeps one or more of these factors from occurring.
Cystic fibrosis is a genetic disease that progresses over time. It causes persistent infections in the lungs, and as time passes, the ability of the individual to breathe is severely limited. Cystic fibrosis patients have a defective gene that makes a thick buildup of mucus within the pancreas, lungs, and some other organs. When found in the lungs, this mucus clogs airways and entraps bacteria, which leads to infections, damage to the lungs, and eventual respiratory failure. Within the pancreas, this mucus buildup prevents digestive enzymes from being released, so the person doesn't absorb the nutrients they need from food. Cystic fibrosis causes exercise intolerance because it severely limits the body's ability to take in oxygen, while also preventing the muscles from metabolizing nutrients the way they should. Individuals with cystic fibrosis tend to experience persistent coughing, often with phlegm. They also have frequent severe lung infections that include bronchitis or pneumonia. Wheezing, shortness of breath, and poor growth are also common symptoms.