Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Where does the body draw from to get energy first?

  1. Jan 4, 2005 #1
    Once you run out of usable glucose from food you've eaten, where is the first place the body turns to to get energy? If there is insufficient food in your body, do your individual cells first turn to their vacuoles for nourishment? Does your body dip into the glycogen reserves in your liver? Is there some other place spare energy is stored for easy acess?

  2. jcsd
  3. Jan 4, 2005 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    AFAIK, the glycogen reserves are consumed first -- the liver begins converting the glycogen back to glucose and dumps it into the bloodstream as the blood glucose level falls. The liver does this automatically. Only if the liver failed would the cells have to turn to their own internal stores of energy.

    - Warren
  4. Jan 6, 2005 #3


    User Avatar
    Staff Emeritus
    Gold Member

    Glycogen reserves will be tapped into once you have run out of all usable energy sources from food, but not simply because you've run out of glucose. There are many other macromolecules in the food we eat that can be metabolized.
  5. Jan 6, 2005 #4
    as far as i remember, it goes in the following order at least for muscles under exercise:

    Glucose then Glycogen (as chroot said), then finally fats (from your fat stores)

    But under starvation conditions, your body uses protein derived mostly from your muscles as fuel before fats at first, but begins to use more fats (as % of energy) as the starvation continues. Glycogen reserves are never depleted entirely even under these conditions, as its the prefered energy for the brain (although they can use ketone bodies derived from fat) and sole source for red blood cells.
  6. Jan 6, 2005 #5
    Actually, the very first energy comes from creatine phosphate. But, only for a brief second or so.

  7. Jan 10, 2005 #6
    That sounds about right - lipids, amino acids (or more accurately keto acids after amino acids have be deaminated) can be respired too. Infact lipids yield more energy than carbohydrates, per mole.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook