Fasting Timeline

0 – 4 Hours
From 0 to 4 hours after you eat a meal, you’re in Phase 1: the anabolic phase. 

This is a growth phase, because you’re using the energy you just ate for energy and for cellular and tissue growth. Your pancreas secretes insulin and starts using up the glucose in your bloodstream and storing the excess in your cells.

4 – 16 Hours 
From 4 to 16 hours, you’re in Phase 2: the catabolic phase. 

This is the breakdown state, where you’re putting all those stored nutrients to use. This is the point where your glycogen stores are being broken down and used for energy. 

Once those stores run out, your body switches over to using stored fat and ketone bodies for energy — this usually happens towards the end of the 16 hours. 

So, if you’re doing intermittent fasting with an 8-hour eating window and a 16-hour fast, you are basically staying within these two phases for the entire fast. 

And if you’re wondering how long it takes to get to that fat-burning stage at the end of phase 2, it really depends on what you’ve eaten. The higher your diet is in carbohydrates and starch, the longer it’s going to take to burn through those stores and reach that fat-burning stage. 

That’s why I always recommend that if you’re about to start a fasting protocol, you reduce the amount of carbohydrates and starches from what you normally eat. You don’t have to cut them out completely, but reducing your intake will help you reach fat-burning faster. 

Another really cool thing that happens during Phase 2 is that the growth regulator called mTOR goes down, which opens the door to a process called autophagy. 

Autophagy is one of the main benefits of fasting: it’s a cellular clean-up, which helps the old and malfunctioning cells to be mopped up and removed from the body. This phase is so powerful because autophagy removes cellular material that can be contributing to aging, cancer, and chronic disease.

16 – 24 Hours
Between 16 and 24 hours, you enter Phase 3: the primary fat-burning stage. 

At this point, your body is burning your fat stores to keep up with your energy demands, because you don’t have any glucose left. 

There’s also an energy sensor that gets triggered during this phase called AMPK. 

AMPK presses the gas pedal on autophagy and really dials up the amount of autophagy going on in the body.

24 – 72 Hours
Stage 4 kicks off between 24 and 72 hours: this is the ketosis stage. 

This is where your body has switched over completely to burning fat for energy. Ketosis is the process of your body splitting fat cells to release energy, which produces ketone bodies and ramps up all the benefits of the previous stages. 

Ketone bodies act as fuel for the brain when glucose is scarce. 

It’s important to note that nutritional ketosis, from fasting or low carb diets, and diabetic ketoacidosis, a potentially life-threatening condition, are not the same thing!

We have this built-in mechanism that during times of food scarcity, we are still able to supply energy to the brain in order to function and make good decisions, which is pretty amazing. 

Many people, myself included, have experienced significant improvement to their energy and mental clarity during this phase of a fast or when following a ketogenic diet, and it’s these ketone bodies that are putting you in that state. 

Another really cool thing that happens around the 24-hour mark is that your brain produces a chemical called brain-derived nootropic factor (BDNF). 

BDNF supports the growth of brain neurons and is really important for long-term memory, coordination, and learning. 

Researchers think that it’s probably a key part of why fasting is so effective for reducing our risk of Alzheimer’s disease as we get older. 

72+ Hours
During fasts that last more than 72 hours, you go into a deep state of ketosis, which amplifies all the previous stages. 

You’re getting all the benefits of weight loss, metabolic health and longevity really compounded as time goes on. 

With multi-day fasts, insulin levels and glucose remain low and you stay in a steady state of ketosis. 

Due to the lack of nutrients, your liver reduces the production of IGF-1 which is a hormone involved in growth and development. 

Short-term decreases in IGF-1 are associated with low oxidative stress and are likely anti-cancer and anti-aging. 

Fasting of 72+ hours is also shown to improve the body’s response to toxin exposure and stress hormones.

As long as you have more than 4% body fat and do not use prescribed medications you should be fine. Prescribed medications are often stored in fat tissues and during ketosis they may be released and overwhelm the system.

When your body goes in to a state of ketosis you may experience dizziness ,discomfort and a little bit of shaking, but that usually goes away within 30 to 60 minutes.

Fasting Myths

If you fast, or intend to, there are a few myths that you’re going to run into. I hear these 5 myths all the time, and it’s important to go into this lifestyle change with the knowledge of what’s true and what’s not. 

Fasting Myth #1: Fasting puts you into starvation mode and decreases your metabolism.

Metabolism is measured by BMR — your basal metabolic rate. This measures the amount of energy your body burns to keep all your organs functioning optimally. 

When your BMR goes up, your metabolism goes up. This supports healthy weight maintenance and weight loss when necessary. When your BMR goes down, your metabolism goes down, and this can lead to difficulties with weight loss or a weight loss plateau. 

Your BMR goes up naturally when you’re cold — your body has to shiver to maintain heat — and your BMR goes down naturally when you reduce your daily intake of calories. This doesn’t happen after one meal; it happens when you’re running at a caloric deficit for a long period of time. 

This is why in some cases, you may have heard from a doctor, nutritionist or health coach that in order to lose weight, you actually need to eat more food, but of course not high calorie dense foods.

It’s counterintuitive, but it’s true. 

When you experience reduced metabolism, you feel cold, tired, hungry, and have low energy. If you are already experiencing those things, you may need to first spend some time repairing your metabolism and getting your food intake back on track before jumping into fasting.

Fasting Myth 2: Fasting makes you lose muscle mass.

This is the first concern I hear from clients who want to build or maintain their lean muscle mass. 

They’re worried when I suggest adding fasting to their routine, because they’ve heard so many times that when you’re fasting, your body breaks down muscle to use for energy. 

In just about all cases, this is not going to happen. The hierarchy of how your body converts fuel to energy is that first you burn glucose, then you burn fat stores, then you burn muscle. 

Your body will not start breaking down muscle tissue until your fat stores account for less than 4% of your total weight. 

To put that into context, elite male marathon runners — who are extremely lean — have a body fat percentage of around 8%. 

So for most of us, we’re never going to get to a place where our body is going to start breaking down muscle tissue for energy. 

When you’re fasting, your body actually conserves muscle. Fasting triggers the release of growth hormone, which helps you to build your lean muscle mass. 

During the fast, your muscles might feel a little deflated or look a little smaller, but this is due to water loss, and it will come back once you get more hydrated. 

If you’re trying to continue building muscle while fasting, you have to exercise. There’s no other way to do it. 

Regardless, exercise and daily movement are paramount: things like weight training, bodyweight training, yoga, walking, biking — all these things will help your body build and maintain muscle mass when you’re fasting. 

Fasting Myth #3: Fasting causes low blood sugar.

While you might experience some symptoms of low blood sugar — like shakiness or feeling ‘hangry’ — those usually only happen when you first start incorporating fasting into your lifestyle. 

If you are prone to hypoglycemia or if your diet is very high in sugar or starch going into the fast, that can cause some blood sugar issues. 

But once you become fat-adapted — when your diet is a good fit before your fast and you start to burn fat for energy easily — your body will have all the nutrients it needs to keep a steady balance while burning fat and running on ketone bodies. 

Fasting Myth #4: Fasting results in overeating.

This is something that many people are afraid of, and studies do show that when you first start fasting, there is an increased risk of overeating, particularly if you’ve ever had any kind of disordered eating in the past.

Fasting can trigger your old habits so if that’s your situation, it is important that you’re working with a therapist to make sure that this change doesn’t lead you down a bad path. For many people with a history of eating disorders, fasting is not going to be a good fit. 

However, if you have not experienced disordered eating in the past, it’s normal that your hunger hormones won’t have fully regulated in the early stages of a fast. 

This can make you want to scarf down everything in sight after your fasting window ends. 

I’ve been there before too, but I would argue that it’s usually a sign you’re not ready for that length of fast or that you need to go into the next one with your diet a little more dialed in. 

Fasting over the long run actually helps to reset the hunger hormones — the leptin and ghrelin that we talked about earlier — and your appetite becomes more stable over time. 

Fasting Myth #5: Fasting leads to nutrient deficiencies

Actually, the opposite tends to happen. 

Your body reduces the excretion of vitamins, minerals and the by-products of broken-down fat and protein during a fast. And because you have less bowel movements (particularly with those longer multi-day fasts), you’re losing less through your stool as well. 

Electrolytes, magnesium, potassium, calcium, phosphorus and so on all remain stable during longer-term fasts. 

The only time that micronutrients — vitamins and minerals — become a major problem is when you’re chronically deficient in those nutrients already and you’re doing a longer multi-day fast. 

In that case, you may want to take a multivitamin.