Which of the following reflects the dehydration effect on work capacity for a 5% body weight loss?

• A. 5 percent
• B. 10 percent
• C. 20 percent
• D. 30 percent

Answer: (D)

As dehydration increases to five percent of body weight, the body’s ability to do work drops significantly because fluid loss reduces plasma volume, which lowers venous return and stroke volume. To maintain cardiac output, the heart rate rises, increasing perceived effort and accelerating fatigue. Heat dissipation also becomes less efficient, so core temperature climbs more quickly, further compromising performance and safety in demanding tasks. In firefighter rehab guidelines, a five percent fluid deficit is associated with roughly a thirty percent reduction in work capacity. This reflects the large combined impact on cardiovascular strain, heat tolerance, and fatigue. Smaller losses would not produce as large a drop in work capacity, while larger losses would imply even greater impairment, so the thirty percent figure best matches the level of dehydration described.