Human energy expenditure can be measured in meaningful units, is one possible message of this May 1938 article in Popular Science:
Meter Gauges Work in Bread-Slice Units
How rapidly exercise uses up the energy in the food you eat is graphically demonstrated by a device called the “bread-o-meter” at the Franklin Institute in Philadelphia, Pa. When a visitor mounts a bicycle frame and pedals vigorously, a generator produces electricity in proportion to his effort, and figures on a board show how many slices or loaves of bread would be needed to furnish this energy.
The article does not specify the type of bread, or give specific clues as to its composition and approximate energy content.