One interesting thing I learnt from the Zero fasting app are the different zones or stages of intermittent fasting. These are my notes on the Zero Fasting Zones.


0-4 hours

  • Breakdown of food into glucose, amino and fatty acids.
  • During this phase your primary source of energy will be from your last meal.
  • Leptin increases and Ghrelin decreases.
  • Insulin then moves the nutrients from the above breakdown to be stored or used as energy.

By the end of the zone, your glucose and insulin levels will return to the levels before your meal. The fewer carbs consumed in your in your last meal, the sooner you will exit this zone and into the next.


4-16 hours

  • Your primary fuel source will still be glucose.
  • Glucose levels will continue to drop.
  • Insulin release should be low.
  • Glucagon starts to rise and triggers liver glycogen breakdown.
  • mTOR suppression allows the start of autophagy.

Once your glycogen stores get low, your body will start to use stored fat and ketones to make up the difference. The less glycogen you have stored, the faster you will switch over to using stored fat. As your body exits the anabolic state and into the catabolic state you open yourself up to a much higher rate of autophagy.

Fat Burning

16-24 hours

  • As the glycogen stores near depletion, glucose from gluconeogenesis begins.
  • Lipolysis breaks down your fat stores.
  • Fatty acids are then used for energy.
  • Increase in autophagy
  • The live makes ketones from excess energy produced from fatty acids.

Your brain still needs glucose to function, so your body converts non-carbohydrate sources into glucose. The rest of your body switches to fat for your energy needs. If weight loss is your primary goal, you want to get into this zone as soon as possible. The longer you are in this zone, the more fat you will burn.


24-72 hours

  • This is a multiday fast.
  • Liver glycogen is no longer a fuel source for your body.
  • Gluconeogenesis has reached its peak and drops off.
  • Ketones increase.