Question
Endurance athletes consume a lot of food. For example, during a multiple day professional bicycle race, a rider might take in food that, if burned, would correspond to 6000 kcal/day. (Recall that 1 kcal is the same as 1 �food calorie�, where �food calories� are the value reported on nutritional labels) Over the course of the race, the rider�s weight change is negligible, so his energy input and output must balance.
Some fraction of the energy used is spent on mechanical work. The work done is mostly expended against air resistance associated with moving the bicycle and rider through the atmosphere. An estimate of this value turns out to be about 4 MJ (megaJoules) per day of riding.
Compare the 6000 kcal input to the 4 MJ of work done. What fraction of the energy in the food is used for mechanical work?
The fraction of energy in the food used for mechanical work would be 16 percent of the daily intake.
We next consider that the rider is giving off heat. The greatest share of that goes to evaporate water, both through respiration (where water vapor is exhaled) and perspiration (where water evaporates from the skin). At physiological temperature, the enthalpy of evaporation of liquid water is 44 kJ/mol (i.e., the enthalpy of the reaction H2O (liquid) �> H2O (gas) is ?H = 44 kJ/mol).
How much liquid water would the rider have to evaporate for the energy budget to balance? If there�s no weight change, then this is the amount of water the rider would drink during the course of the day. Does the number seem reasonable? Explain your answer.
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