5 Stages of Fasting
Fasting can be broken down into 5 stages. These 5 stages are based on where the body gets its glucose from. The body has two main sources of fuel: glucose (sugar) and fat. Most cells can use either fuel source with the main exception being the brain which cannot use fat (triglycerides) directly for energy, also the retina, parts of the kidney, red blood cells, etc. But most of the body can use glucose and fat for energy, kind of like a hybrid car that can run on gasoline and electricity.
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The body also stores energy in two main ways: glucose and fat. Glucose is stored mostly in the liver in the form of glycogen (~100 grams) which are long chains of glucose. When the body needs energy, it breaks down the glycogen back into glucose molecules and sends it out into the bloodstream.
When glycogen levels in the liver become low, the body relies on fat from the fat cells (adipocytes). This fat from the fat cells travels to the liver where it is broken down into fatty acids and a glycerol. The body can then use these fatty acids for energy and the glycerols can be converted into glucose for cells that can only use glucose for energy as mentioned above.
5 Stages of Fasting
Stage 1: Feeding (0-4 hours)
This stage lasts from 0-4 hours after a meal. At this time there is a lot of glucose around as people are eating a mix of protein, carbs and fats. When there is glucose present from food, all the cells of the body are using glucose preferentially as their main source of energy.
There are two main nutrient sensors in the body: insulin and mTOR. When the body sensing carbs and protein, these sensors go up and the body goes into storage mode to store away this energy you just consumed for later on when you need it. The liver will store the glucose as glycogen and the fat cells will store the fat as triglycerides.
Stage 2: Post Absorptive (4-16 hours)
This stage lasts from 4-16 hours after a meal. During this time the nutrient sensors, insulin and mTOR as mentioned above, start to drop and the body is getting its energy now from glycogen in the liver, not so much from the food you consumed as that's being stored away as mentioned in stage 1. The liver can store about 20-24 hours' worth of glucose for the whole body when no food is available. Of course this is dependent on what diet you follow. For instance, if you are on a ketogenic diet, the liver doesn't store glycogen so there isn't any glucose available so your body runs primarily off fatty acids and ketones.
This stage is key to allow insulin levels to drop to give the body a break from being in storage mode to being in tissue repair and burn mode. Additionally, allowing insulin levels to come down will also benefit certain health conditions that are influenced by high insulin levels such as heart disease, diabetes, obesity, hypertension, PCOS, gout, etc. Also during this stage you will release water retention and salt as the kidneys excrete excess water and salt when insulin is low.
During this stage as insulin is dropping, the counterregulatory hormones, as they are collectively called, counter insulins actions. These hormones are cortisol, sympathetic tone, norepinephrine, growth hormone, etc. These hormones tell the body to get energy from its storage sites such as the liver and body fat to be burned.
Stage 3: Gluconeogenesis (16-30 hours)
This stage lasts from 16-30 hours after a meal. At this stage the glycogen (stored glucose) in the liver is running out and we haven't completely switched over to burning body fat so there is this intermediatory stage where the body is going to generate glucose from protein and glycerol from fat, as mentioned above. This process is called gluconeogenesis which means, the creation of new glucose.
Even though you are breaking down protein in this stage, which sounds bad, your body is going through what's called autophagy where its breaking down old and damaged cells and organelles that aren't working that well and recycles them. Autophagy is referred to by Dr. Yoshnori Ohsumi, who got the Nobel Prize for his work in this area, as the body's intracellular recycling system. So the body takes these old and damaged proteins and breaks them down into their amino acids and uses them to rebuild what's necessary. This is a great process and is like cellular rejuvenation! The body will decide what is necessary and what is not necessary to be built. As a bonus for those wanting to lose weight, during this process of autophagy, the body will reabsorb the excess skin and connective tissue.
The main regulator for autophagy is mTOR, the nutrient sensor I mentioned above. So the more mTOR, the less autophagy. Since mTOR is triggered with carbs and protein (amino acids), the cleaner your fast is, i.e., no ingestion of even things like bone broth and just sticking to water, black coffee, and tea, the less likely mTOR will be stimulated.
Stage 4: Ketosis (2 days - 1 week)
This stage lasts from 2 days to 1 week after a meal. At this point the brain isn't able to rely primarily on glucose for energy as there isn't enough glucose being produced in the liver by glycerol (from fat). So the liver now takes fat (triglycerides) and turns it into ketones which can cross the blood brain barrier and fuel the brain. This can supply up to 70% of the energy the brain needs.
Stage 5: Protein Conservation (>1 week)
This stage lasts for one week and beyond after a meal. This is where you are supplying your body with energy almost exclusively from stored body fat. Protein is still being broken down to produce glucose (as remember some cells can only use glucose for energy) but primarily the energy needs are being supplied by fat. During this stage hunger really starts to disappear as measured by hunger hormones such as ghrelin being low. The reason for this is is because the body is feeding itself from its own body fat. That's why people can do extended fasts like 30+ days and survive and be fine.
So there you have it. Hopefully that was helpful. Comment below with your thoughts, even if you disagreed as I'd love to hear from you.