This strategy of breaking food into its simplest parts and reconverting it into complex forms for storage is precisely what our body should do when we have eaten plenty. And this is precisely what the body should not do in the face of an immediate physical emergency (stressor). First, the body turns down the parasympathetic nervous system and down goes insulin secretion. Second, the body makes sure that energy storage is stopped. With the onset of stress, the body releases glucocorticoids, which block the transport of nutrients into fat cells. This counteracts the effect of any insulin still floating around.
In addition to halting the storage of energy, you want your body to gain access to the energy already stored. You want to dip into your bank account, turn stored nutrients into your body’s equivalent of cash to get you through this crisis. Our body reverses all the storage steps, through the release of stress-response hormones. These cause triglycerides to be broken down in the fat cells and, as a result, free fatty acids and glycerol pour into the bloodstream.
The same hormones trigger the degradation of glycogen to glucose in cells throughout the body and the glucose is then flushed into the bloodstream. These hormones also cause proteins in non-exercising muscles to be converted back into individual amino acids.
The stored nutrients have now been converted into simpler forms. Our body makes another simplifying move. Amino acids are not a very good source of energy, but glucose is. Our body shunts the amino acids to the liver, where they are converted into glucose. The liver can also generate new glucose; a process called ‘gluconeogenesis’ and this glucose is now available for energy during the emergency.
As a result of all these processes, a lot of energy is available to the exercising muscles. There is a burst of activity, the emergency is averted and the extra energy is consumed. Say, you are running away from some danger, it does not make sense to provide extra energy to the arm muscles.
The stress hormones (glucocorticoids and the rest of the gang) block energy uptake into the non-exercising muscles. Somehow the exercising muscles override this blockade and grab all the nutrients floating around in circulation. No one knows how this signalling takes place.
As will be clear, this response to stress is great in the face of physical stress. Not such a great idea when the stress is purely psychological—uncomfortable social settings, public speaking, or fights (verbal) with your wife. All the free energy floating in the blood and no muscles to use it!