- Stage 1 Fasting (8-12 Hours): Stable Blood Sugar
- Stage 2 Fasting (12-18 Hours): Ketosis, Fat Burning, and Mental Clarity
- Stage 3 Fasting (24 hours): Autophagy and Anti-Aging
- Stage 4 Fasting (36-48 hours): Growth Hormone and Recovery
- Stage 5 Fasting (72+ hours): Stem Cells and Immune Function
- A Brief Guide to Fasting
- The Takeaway
Intermittent fasting is more than just a weight-loss strategy.
While skipping meals is a surefire great way to burn through your fat stores, the health benefits of intermittent fasting extend far beyond simple weight loss. Fasting promotes mental clarity and mood[*], improves immune function[*], increases muscle growth[*], and more.
However, not all fasts provide the same benefits. There are multiple stages of fasting, and the benefits you get from fasting depend on the length of your fast.
Here’s a look at the five main stages of fasting, the specific benefits that come with each one, and a brief look at how to incorporate fasting into your life.
Fasting begins about eight hours after your last meal. This stage is characterized by changes in your blood sugar levels.
Stable Blood Sugar
After about eight hours without food, your blood glucose begins to dip. You may experience hunger, fatigue, food cravings, and trouble concentrating.
However, if you can make it over this initial hurdle, these symptoms pass pretty quickly. By 12 hours, your body will begin to tap into your glycogen stores (sugar stores), and your blood glucose levels will stabilize[*].
At 12 hours, you’ll also start switching into the early stages of ketosis — your body stops relying on carbohydrates for fuel and begins burning your body fat stores instead[*].
Short-term fasting may also lower blood pressure and increase insulin sensitivity, making this type of fasting useful for people with type 2 diabetes or other blood sugar control issues[*].
After 16 to 18 hours of fasting, you should be in full ketosis[*]. Your liver begins converting your fat stores into ketone bodies — bundles of fuel that power your muscles, heart, and brain.
This stage of fasting has a few benefits:
Ketones suppress appetite, which means your hunger should actually start to subside during this fasting stage[*].
Paradoxically, fasting begins to get easier as you fast for a longer time. After the first stage of fasting, appetite generally declines.
Stage 2 fasting is ideal if you want to lose weight. Your body switches fully into fat-burning mode, and because you don’t have any food in your system, you begin to burn through body fat quickly.
In addition, this stage of fasting decreases ghrelin, the hormone that makes you want to eat a lot of food, and also stabilizes your insulin levels, which prevents food cravings[*].
These factors work together to prevent overeating. If you can do intermittent fasting for 16-18 hours a day, you’ll burn through body fat and fill up quickly when you break your fast, which makes it easy to stay in a calorie deficit and lose weight.
At this stage of fasting, you may also enjoy a boost in brain-derived neurotrophic factor (BDNF).
After a full-day fast, your body goes into repair mode. It begins recycling old or damaged cells and reducing inflammation. If you’re looking for anti-aging or anti-inflammatory benefits, a 24-hour fast may help.
When your body is under mild stress (such as exercise or an extended fast), your cells respond by becoming more efficient.
One thing they do is turn on autophagy. The term is Greek for “self-eating,” and that’s exactly what happens. During autophagy, your cells check all their internal parts, find anything that’s old, damaged, or functioning poorly, and replace them with shiny new versions. The old parts are either recycled into new materials or destroyed.
Studies show that fasting-induced autophagy comes with a variety of benefits.
- Fasting triggered autophagy in the brain, clearing out misfolded proteins linked to Alzheimer’s disease[*].
- Autophagy declines naturally as you age, which is connected to a variety of diseases. Fasting may be able to combat the age-related decline, keeping you biologically younger and protecting your cells from oxidative stress[*].
- Early research suggests that fasting-related autophagy may help kill cancer cells[*].
- Fasting reduces bodywide inflammation via autophagy[*].
At stage 4, you’re leaving intermittent fasting territory and entering a longer fast.
As you undertake multiple days of fasting, your growth hormone levels begin to shift, which provides you with a new set of benefits.
Muscle Growth and Repair
A study of healthy adults found that 48-hour fasting increased human growth hormone (HGH) secretion by up to 400%. It also increased the frequency of growth hormone bursts throughout the day[*].
HGH increases muscle mass[*] and stimulates faster muscle repair[*]. It may also speed up the healing process for wounds and more serious injuries[*]. HGH is so effective that taking it externally is banned in professional sports and is considered doping.
It seems counterintuitive, but occasionally going without food for two days may actually help you build muscle, not lose it.
The final stage of fasting begins after a full three days without food.
Before you read about the benefits, note that a 72-hour fast is a serious undertaking. If you’re going to try it, make sure you drink plenty of water, get plenty of electrolytes like sodium, magnesium, and potassium, and stop fasting if you feel lightheaded or otherwise unwell.
Immune System Regeneration
A 2014 study found that 72-hour fasting led to a near-complete rejuvenation of the immune system. Fasting triggered stem cell production, creating brand new immune cells to replace old ones[*].
The same study also had cancer patients fast throughout chemotherapy treatment. Normally, chemotherapy devastates the immune system, increasing the risk of infection and illness for cancer patients.
But when patients fasted during chemo, their immune system stayed strong throughout the process.
Fasting is fairly simple. You just have to follow three rules:
- Choose a fasting window. As you read, the stages of intermittent fasting each come with different benefits. Pick a window of time fast and stick to it. for example, if you’re doing an 18-hour fast, maybe you stop eating at 8 PM today and don’t break your fast until 2 PM tomorrow.
- No calories. A true fast (also called a water fast) includes zero calories during the fasting period. Water, coffee, tea, and other zero-calorie beverages are fine during water fasting, but anything with calories will break your fast.
- Stay hydrated. Drink plenty of water during your fast. make sure you also get your electrolytes by adding sea salt to the water and/or taking an electrolyte supplement.
For more details about different fasting schedules and how to use fasting as part of a healthy lifestyle, you may want to take a more in-depth look at fasting.
Fasting and the Keto Diet
You can also combine fasting with a ketogenic diet.
Several of the benefits of intermittent fasting come from your body being in ketosis — a state in which you burn fat as your main energy source.
By eating a ketogenic diet, you can get your body into ketosis before you start fasting. A keto diet suppresses hunger[*] and improves your health on its own. Pairing it with fasting provides even more health benefits.
You can start a keto diet today with a quick trip to the grocery store.
Intermittent fasting is a valuable tool to improve your health. It’s free, simple to do, and is generally safe for most people (although you still may want to consult your doctor about it before starting).
Now that you understand the different stages of fasting, you can choose the length of fast that best meets your health goals. And for even more health benefits, you may want to pair fasting with a ketogenic diet.