When was the last time you “bonked”?

Posted May 17, 2007 at 11:45 AM by Bridget Sullivan

Section: Her Fitness, Her Health, Her Nutrition, Workout Fuel

Definition of a ”bonk”: “commonly used as a jargon term by endurance athletes, primarily cyclists and long-distance runners, to describe a condition when the athlete suddenly loses energy and fatigue sets in, usually caused when glycogen stores in the liver and muscles are depleted, resulting in a major performance drop. The term can be used both as a noun (’hitting the bonk’) and a verb (’to bonk halfway through the race’). This condition is also known to long-distance (marathon) runners, who usually refer to it as ‘hitting the wall.’” Below, Dr. Gabe Mirkin explains how you can prevent “bonking” during your next endurance event.

Question: Dear Dr. Mirkin: How can I know if I need to eat while I am exercising?

Answer: Fatigue during a workout or sporting event is usually caused by lack of water, salt or sugar.  Most athletes in sports that last more than a couple of hours know that they should drink and take in some salt, but they also need a source of sugar. When you exercise, you get your energy from sugar and fat stored in your muscles and sugar and fat in your bloodstream, and, to a lesser extent, from protein.  At first you get more than 80 percent of your energy from fat and sugar stored in muscles. Usually, at the start of exercise, almost 45 percent of the energy comes from stored muscle sugar.  As you continue to exercise, you use up fat and sugar stored in muscles and get far less from these stores.  After two hours of exercise, you have used up most of your stored muscle sugar (glycogen) and [you] get less than 15 percent of your energy from that source. At four hours, your muscles have almost no stored sugar at all.

When your muscles are depleted of their stored sugar, they become difficult to coordinate and feel heavy and hurt.  Your muscles can get some sugar from gluconeogenesis, a process in which your liver makes sugar from protein (branched chain amino acids), but that is not enough for all-out exercise.  During intense exercise, you need a source of sugar.  It can come from sugared drinks or any carbohydrate-rich foods.  You can use special sugared exercise drinks, sugar gels, carbonated soft drinks, exercise bars or any food that contains sugar or flour.

Photo: ©istockphoto.com/Joey Nelson

This post is written by Dr. Gabe Mirkin, M.D. and used with permission.  Dr. Mirkin is board certified in Sports Medicine and has practiced for over 40 years. He has completed more than 40 marathons and was a talk show host of a nationally-syndicated radio program for about 25 years. For more articles by Dr. Mirkin, please check out: www.DrMirkin.com