Human body!!. Charles Hugh Smith says so:
We use the word “oil” for both petroleum and vegetable oil. As you might expect, they are both chains of hydrocarbons. As a result, a gallon of vegetable oil contains about the same calories as a gallon of gasoline (31,000 calories). This is why biodiesel fuel works as a replacement for petroleum diesel.
Since a human can metabolize vegetable oil as fuel, then by simple extrapolation we can estimate that a human being could ride a bike 900 miles on a gallon of vegetable oil–or lose about nine pounds of human fat on the ride. That means that a mere 27 pounds of human fat would theoretically enable a rider to bike across the entire continental U.S. Alternatively, the rider could sip 3 gallons of vegetable oil to provide the same calories. Not a terribly enticing diet for a cross-country ride, but the take-away is simple: the human body is an extremely efficient engine, and it’s difficult to burn the enormous quanitity of energy stored in 10 pounds of human fat. No wonder it’s hard to lose weight, and no wonder our nation (and other developed nations) are suffering an epidemic of obesity.