Major Organs’ Needs Limit Change in Metabolism, Biologists Find

Athletes are limited in how far they can run, cycle or swim, not by the restrictions of bone or muscle but because major organs use too much energy. The liver, kidneys and the stomach are the key to controlling metabolism, biologists Jared Diamond and Kimberly Hammond of UCLA report in the April 3 issue of the journal Nature.

They said the most any person can increase metabolism is about fivefold. To see why they could not go beyond that, the scientists checked the metabolic rates of 50 animals. None could do much better. Mice forced to run on a treadmill collapsed with exhaustion after six hours, no matter how much food they received or how much they trained, which suggests that there is a limit on how much fuel a living creature can convert to energy. That limit, they found, is controlled by the internal organs.