The huge number of mitochondria in dogs’ muscles explains why sled dogs can run more than 100 miles a day, at sub-8-minute-mile pace for weeks on end, while humans could not possibly run as long or as fast and recover from such abuse of their muscles. Mitochondria are small areas in all the cells of your body except mature red blood cells, that turn food into energy more efficiently than any other means in your body. Each human muscle cell contains up to 2500 mitochondria.
Muscles have two major sources of converting food to energy. They have small chambers in cells called mitochondria that use the Krebs Cycle chemical reaction to convert all foods to energy. They also have glycolysis, inside the muscle cell and outside the mitochondria, that converts sugar to energy.
Since dogs have more than 70 percent more mitochondria per cell than humans have, their cells can convert fat far more efficiently into energy while they are exercising.
If humans had as many mitochondria as dogs do, the world records for all endurance events would improve incredibly. However, at this time the only way that we know how to increase mitochondrial number safely is to exercise repeatedly to exhaustion or to severely restrict calories. Studies have shown that exhaustive exercise even increases the number and size of mitochondria in the brain to increase memory and learning in mice (The Journal of Applied Physiology. August, 2014). Taking thyroid hormone increases the number of mitochondria, but excess thyroid hormone will turn your bones to chalk and damage every cell in your body.
Oxygen: the endurance advantage of being a dog
The limiting factor to how fast a human or dog can run over distance is the time it takes to process oxygen in exercising muscles. Once a muscle does not meet its needs for oxygen, the muscle becomes acidic which burns, hurts, and slows the athlete down. Sled dogs can use far more oxygen than humans can. The maximum amount of oxygen a person or dog can take in and use over time is called the VO2max. Top marathon runners and cyclists rarely have a VO2max of greater than 90 ml/kg/min. Untrained sled dogs have twice the oxygen capacity or 175 ml/kg/min VO2max. Trained racing sled dog’s VO2max is more than 3 times as much as that of the best-trained humans: 300 ml/kg/min.
Muscles burn carbohydrates, fats and, to a lesser degree, proteins for energy. The sugar in carbohydrates requires less oxygen than fat does to power your muscles. The problem is that humans have only a very limited amount of sugar stored in their muscles and liver. Humans start to run out of stored sugar after only 70 minutes of intense exercise. When a muscle runs out of its stored sugar, it hurts, becomes more difficult to coordinate and requires more oxygen than usual. A limiting factor in how long you can exercise a human muscle is how much sugar you can store in a muscle, how quickly you use it up, and how quickly you can restore the sugar in your muscles (Medicine & Science in Sports & Exercise, August 2005).
Can humans gain greater endurance?
Humans could attain greater endurance by storing more carbohydrates in their bodies, by taking carbohydrates continuously during exercise or by bringing oxygen to their muscles faster so they could burn fat more efficiently. Humans cannot store more carbohydrates than they already do because all extra carbohydrates are immediately turned to fat, so improvement in endurance will have to come from bringing oxygen into muscles faster or figuring out a way to burn fat with less oxygen.
If a human could teach his muscles to burn fat with less oxygen, he would be the best long distance runner, cyclist, or long-distance cross country skier ever.
Recovery from hard exercise
Since human muscles depend on sugar for energy during all-out exercise, and humans store only a small amount of sugar, humans cannot recover from hard exercise as fast as dogs do. Humans take a long time to restore muscle sugar called glycogen. Top marathon runners restore muscle glycogen in anywhere from a day to several days. Sled dogs can restore muscle glycogen almost as quickly as they are fed. They are able to restore more than 50 percent of their resting muscle glycogen after two consecutive 100-mile runs even when fed a low-carbohydrate, high-fat diet. Humans could never replace muscle glycogen that fast.