Glucose (a type of sugar) is the primary energy source for the average modern human. When carbohydrates are digested, glucose is released into the bloodstream and disperses into the body’s tissues, eventually entering the individual cells where it is used to power their processes. Other macronutrients (types of food molecules) capable of providing energy include fat and exogenous (consumed rather than internal) ketones. Protein can also be used for energy but first must be broken into amino acids, which the liver can convert into glucose if needed.
During long-duration, low-intensity effort (including normal daily life), energy is produced from glucose and fat via aerobic respiration – a slow but very efficient process that requires oxygen for completion. For high-intensity effort, energy can be more rapidly produced from glucose via anaerobic respiration – a much less efficient yet extremely fast process that does not require oxygen and releases lactic acid as a byproduct. When a cell’s energy needs are met, a limited amount of additional glucose is converted into glycogen and stored in the muscle and liver tissue. If glucose from food is not available when energy needs increase, muscle glycogen can be broken back down into glucose for use in the muscles themselves, while liver glycogen can be broken back into glucose and released into the blood to fuel other tissues including the brain.
There are about 4 grams (about a teaspoon) of glucose in the blood of an average 150lb person and about 100 grams and 400 grams of glucose stored as glycogen in the liver and muscles respectively.