Which percentage of body weight loss is associated with performance loss and fatigue?

• A. 2% of body weight
• B. 5-10% of body weight
• C. 10-15% of body weight
• D. 15% or more of body weight

Answer: (B)

Dehydration lowers performance and increases fatigue because losing body water reduces plasma volume, makes the heart work harder to maintain blood flow, and impairs the body’s ability to regulate temperature. When fluid loss reaches a moderate level, these effects compound: cardiac output can’t keep up with demand, core temperature rises more quickly, muscles don’t receive oxygen and nutrients as efficiently, and mental sharpness can drop. That combination shows up as slower work capacity, reduced power and speed, and quicker onset of fatigue during demanding, high‑intensity efforts like CrossFit workouts.

Smaller losses may begin to affect some aspects of performance, but the noticeable decline that athletes commonly notice across workouts tends to appear at this mid-range dehydration level. More extreme fluid losses are dangerous and lead to serious performance collapse and health risks, while minimal losses are often within normal fluctuations.