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.

This is a collection of notes and questions/answers on time restricted feeding, also popularly known as intermittent fasting, from Dr. Peter Attia. Most of these questions and answers are from Peter’s videos, his podcast (The Drive), and the Zero fasting app by Kevin Rose.

How to use a 16:8 fast (or other length of fast) for weight loss?

  • It all comes down to what is eaten during the eating phase. It is a trick question. In some cases you can actually gain weight. It all boils down to what you do, and what you eat, during the feeding window.
  • The biggest mistake is that people think that because they are fasting, is that they can eat whatever they want. Which is not true for most people. Some people will get away with it, based on their activity and exercise, but the vast majority of people will not.
  • Stick to Peter’s three framework of CR, DR, and TR. Pull one of those levers all the time, two often, and sometimes all three. You don’t really need fasting to lose weight if you pull CR and DR.

Which fasting zone is best for exercise?

  • You can and should exercise during all zones of the fast. The performance will very depending on which zone. Some workouts will not be appropriate for some zones.
  • Trying to put on muscle mass will be tough in a catabolic state. You should be doing it in a anabolic zone.
  • Can you use exercise to accelerate fasting states? with high intensity workouts you can quickly deplete glycogen and take you to the next zone faster.
  • In a longer fast, high intensity training will suffer. Workouts that are glycolytic will suffer after depletion.
  • In a shorter fast, If the goal is to preserve or gain lean mass, workout before the refeed. The drawback is that you will feel crappy the longer the fast.
  • If you want to preserve lean mass and want to lose weight, you do the workout in the morning during the fast, probably in the morning. You accelerate the catabolic state. You will generate enough net leucine to preserve muscle tissue but you wont gain muscle.

When does your body burn fat?

  • Fat oxidation is what we call fat burning. It is the extraction of energy from fat.
  • If you want fat loss, fat oxidation is essential. You need net fat loss. You need to have more fat leaving the cell than there is coming in.
  • Can you lose fat before hitting hour 16 of a fast? absolutely.
  • You can accelerate the fasting window by keeping your insulin level is low during the feeding window by keeping carbs low. Managing stress. Sleeping well. Exercising to manage that. This way when you are eating you are not undoing what you did during the fast by changing the metabolic environment.

Would a daily 18 hour fast slow down the metabolism?

  • This describes the basal metabolic rate, which is supposed to support the body at rest. Which takes more energy than you realize.
  • There are studies that show that there is no reduction in the BMR as a result of TRF. Longer fasts show a transient reduction in the BMR but it returns after refeed.