(If you exercise moderately, this should take approximately an hour.) For heat, your body's main source is muscle tissue, so if you're not using it, it will be lost as radiation or convection. The rest of the body's heat loss mechanisms are through sweating and breathing. Air at room temperature contains about 20 percent oxygen; this is enough to supply all of the blood in your body for one minute of intense activity. After that, you will need more oxygen. Carbohydrates are the body's primary fuel source. They contain carbon, hydrogen, and oxygen. When you eat carbohydrates, they are broken down into glucose which is used by cells for energy. Most are converted directly into blood sugar which is then transported to different parts of your body where it is needed. Some remains in the blood stream for several hours and is removed by the kidneys. Proper nutrition is key to building up your muscles. Carbohydrates are the most important factor because they give your body the raw material it needs to build muscle mass. A high-carb diet can help prevent muscle loss during weight loss efforts. Healthy sources of carbohydrates include whole grains, potatoes, corn, peas, beans, fruit, and vegetables.