But these chemicals are a by-product of the metabolic activity that powers the body.

ECONOMIST: A way to counteract part of the process of growing old

They combined conventional PET imaging, which requires a radioactive tracer to highlight the brain's metabolic activity, with other recent advances in the genetic engineering of brain cells that allows them to target specific brain circuits in lab animals.

WSJ: Brain Science: Mysterious Neural Circuitry Becomes Viewable

So, while activity and calories are one part of the equation, metabolic parameters are another essential part of the human equation that provides a more complete perspective.

FORBES: Calories�Cthe ubiquitous term in health and fitness.