Humans get lousy mileage compared to apes
Metabolic energy use by humans is far higher than for other great apes
Dr. Herman Pontzer, a biological anthropologist at Hunter College in New York, measured the amount of energy used by humans, gorillas, chimps and orangutans, and found that, when corrected for body size, humans' metabolic rate is considerably higher than that of all the apes, by several hundred calories a day. Put another way, we get considerably worse mileage than our nearest relatives.
Dr. Pontzer suggests that this is the evolutionary price we pay for big brains, a long lifespan, and a reproductive rate that is much higher than the other Great Apes.