By Gabe Mirkin, MD
Sled dogs can run more than 100 miles a day, at sub-8-minute-mile pace for weeks on end. Humans couldn’t possibly run as long or as fast and recover from such abuse of their muscles. The explanation for the dogs’ incredible endurance is their mitochondria.
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 cell contains from 2 to 2,500 mitochondria.
Muscles have two major sources of converting food to energy. They have 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, which converts sugar to energy. Since dogs have over 70 percent more mitochondria per cell than humans have, their cells can convert fat far more efficiently into energy.
If humans had as many mitochondria as dogs do, the world records for all endurance events would improve incredibly. Increasing mitochondria could also help to slow aging and to prevent diseases such as dementia, cancer, diabetes, heart attacks, and strokes. However, at this time the only way that we know 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.
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 dogs’ 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).
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.
However, 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.
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.
Humans could have more endurance if they had as many mitochondria in their muscle cells as sled dogs do. All human endurance records would topple by a huge margin if a new drug or training method that increases mitochondria appears on the market.
At this time, the only safe way we have to increase the number of mitochondria in muscles is by exercising intensely enough to create an oxygen debt. This means that you must exercise so intensely that you become short of breath and your muscles burn. However, every time you exercise intensely, you damage your muscles and feel sore the next day. If you try to exercise intensely when your muscles are still sore from a previous workout, you are at high risk for injuring yourself.
You need to plan to take intense workouts that damage your muscles on one day, feel sore on the next day and go easier. When the soreness lessens, take your next hard workout. Realize that how often you can exercise intensely depends completely on how long it takes for muscles to recover.
Top cyclists go hard on one day, and recover by going fairly hard on the next day. If their muscles feel very sore, they go very easy or take the day off. The schedule for other sports will vary depending on how muscles are used; for example, swimmers can usually take one hard workout and one easy workout every day.
If you have never ridden very hard, on one day you could do controlled 50- or 100-pedal stroke intervals with recoveries until your leg muscles start to feel heavy and hurt. Try to do 20 100-pedal stroke intervals with one- to three-minute recoveries, depending on how you feel. The next day, ride easy for an hour or two depending on how you feel.
If you are a trained athlete and regularly ride hard, on one day, do controlled racing with your friends for 20 to 80 miles. The next day, do controlled short (less than 30 seconds) or long (greater than two minutes) intervals. Always reduce the intensity of your workouts or take the day off when your legs feel heavy or stiff before a workout. Then repeat the cycle of “race” days and interval days, with days off as needed.
This is just a rough recommendation for training. You have your own training methods and should follow what you think is best for you. Remember, though, that if muscles do not burn on your hard days, you will not significantly increase the number of mitochondria in your muscle cells.
Caution: Intense exercise can cause a heart attack in a person with blocked arteries. Check with your doctor before making a major change in your exercise or training program.
Reprinted with permission from Dr. Gabe Mirkin’s Fitness and Health e-Zine. Gabe Mirkin, M.D., is a sports medicine doctor and fitness guru. A practicing physician for more than 50 years and a radio talk show host for 25 years, Dr. Mirkin has run more than 40 marathons and is now a serious tandem bike rider with his wife, Diana, often doing 30-60 miles in an outing. His website is http://drmirkin.com/.