The Obesity Code: Unlocking the Secrets of Weight Loss (Why Intermittent Fasting Is the Key to Controlling Your Weight)

by Jason Fung

CORTISOL IS THE so-called stress hormone, which mediates the flight-or-fight response, a set of physiological responses to perceived threats. Cortisol, part of a class of steroid hormones called glucocorticoids (glucose + cortex + steroid), is produced in the adrenal cortex. In Paleolithic times, the stress that led to a release of cortisol was often physical: for instance, being chased by a predator. Cortisol is essential in preparing our bodies for action—to fight or flee. Once released, cortisol substantially enhances glucose availability,1 which provides energy for muscles—very necessary in helping us to run and avoid being eaten. All available energy is directed toward surviving the stressful event. Growth, digestion and other long-term metabolic activities are temporarily restricted. Proteins are broken down and converted to glucose (gluconeogenesis). Vigorous physical exertion (fight or flight) soon often followed, burning up these newly available stores of glucose. Shortly thereafter, we were either dead, or the danger was past and our cortisol decreased back to its normal low levels. And that’s the point: the body is well adapted to a short-term increase in cortisol and glucose levels. Over the long term, however, a problem arises.