"Bonk" is a jargon term used by endurance athletes, primarily cyclists, to describe a condition when athlete's 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".

Whenever an athlete is engaged in exercise over a long period of time their body produces energy via two mechanisms:

• via fat metabolism and
• via breakdown of glycogen into glucose, followed by glycolysis.

How much energy comes from either source depends on the intensity of the exercise. During intense exercise which approaches one's VO2max, most of the energy required comes from glycogen.

The average human body usually stores enough glycogen to generate 1500 to 2000 calories of energy. Intense cycling or running can easily consume 600-800 or more calories per hour. Unless glycogen stores are replenished during exercise, after 2 hours of continuous cycling or 15-20 miles of running, glycogen stores will be depleted. Symptoms of depletion include general weakness, fatigue, and manifestations of hypoglycemia, such as dizziness and even hallucinations. Symptoms will not be relieved by short periods of rest. This condition is potentially dangerous and should be avoided.

There are several approaches to prevent glycogen depletion:

1. Carbohydrate loading is used to ensure that the initial glycogen levels are maximized, thus prolonging the exercise. This technique amounts to increasing complex carbohydrate intake during the last few days before the event.
2. Consuming food or drinks containing carbohydrates during the exercise. This is an absolute must for very long distances; it's estimated that Tour de France competitors receive up to 50% of their daily caloric intake from on-the-bike supplements.
3. Lowering the intensity of the exercise to the so-called 'fat loss' level ( heart rate of 130 bpm for a 30-year-old athlete ) will lower both the energy requirements per unit of distance and the fraction of the energy that comes from glycogen.

Bonk training

"Bonk training" is an exercise program designed for weight loss. It suggests that running or cycling every morning for 45-60 minutes before breakfast, when glycogen store levels are low, will force the body to "bonk" shortly into the exercise, and subsequently burn more fat to generate the energy. It is not clear how medically sound this idea is.

Sports terminology | Endurance sports

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