Bonking usually begins with dehydration. Often times, an athlete becomes dehydrated for one of three reasons: 1) they weren't prepared, 2) they chose not to hydrate well because of race-pace competition, or 3) they were trying to avoid weigh-down and didn't carry enough fluid.
No matter the reason, dehydration, along with heat and lack of carbs and electrolytes, can result in a quick and nasty downward spiral that leads to the big bonk.
Here's how it spirals out of control, and how you can stop it before it happens.
Moderate Dehydration Begins
Usually, an athlete needs at least 18 to 24 ounces of fluid per hour to maintain good performance and only a low-to-moderate dehydration level. With moderate-to-high intensity activity, especially during the summer, some slight dehydration is expected and will occur. When these minimal fluid needs aren't met for an activity greater than 90 minutes, moderate dehydration ensues. Generally, this means a 2- to 3-percent body-weight loss.
You Perceive Increased Effort
With moderate dehydration, many athletes will experience a perception of increased effort. This means that even though your pace may be on target, you feel like you're expending extra effort. Your legs feel heavier. Your heart rate and breathing may be more labored. When this occurs, some athletes will stop or slow down, rehydrate, refuel, and regroup. Others will push harder, fight for their pace, and not stop or slow to hydrate. Dehydration and lack of fuel worsens.
Nausea Sets InWith more dehydration, athletes may begin to feeling nauseous. Once the stomach feels hollow and upset, it can be hard to convince yourself to drink or eat. This only leads to more dehydration, and lack of electrolytes and carbohydrates. In more critical cases, an athlete will no longer be able to keep fluids or solids down, even when they stop and attempt to refuel.
At a 3-percent body-weight loss, performance—not just the perception of effort—begins to suffer. If an athlete keeps pushing and doesn't hydrate, it will only get worse.
Severe Dehydration Occurs
This is much more dangerous and generally occurs at a 4- to 5-percent body-weight loss. Usually, the symptoms have slipped beyond increased perception of effort, nausea and performance decline. The athlete now has difficulty concentrating. At 6 percent loss of body weight due to fluid loss, the body can have difficulty regulating its blood volume and temperature, and medical help is usually needed.