Fasting Physiology
Hey friend!
I can’t seem to go anywhere these days without someone asking me about fasting. I’ve talked about fasted cardio in the past, but not about the different fasting protocols and their effectiveness (or lack thereof) for weight management. I wanted to do a two-part series talking about what happens to the body when you fast (this letter) and then move into specific fasting protocols (next week).
Let’s go.
What happens in the body as you fast?
0-3 Hours
In the first few hours after eating a meal, your body is still trying to break down, ingest, and store the nutrients of the last eaten meal. Specifically:
- Ingested food is broken down into component parts – glucose, amino acids, and fatty acids.
- Nutrients enter the blood.
- Nutrients are shuttled to where they need to go.
- Tissues pull nutrients as needed for use.
- The remaining nutrients are stored in the form of glycogen, fat, or muscle.
After 3 to 4 hours, your blood levels will have returned to normal levels.
4-12 Hours
After about 4 hours, your body is no longer anabolic (in a growth phase) and instead enters a catabolic state (favoring breakdown). The body must begin to rely on stored nutrients to continue to operate.
Glucose is the body’s preferred fuel source. As such, your body will start breaking down stored glycogen to keep your blood sugar levels steady.
12-24 Hours
Your stored glycogen levels start to get low. Depending on how much glycogen you started with, your levels will start to get low at different rates. In general, blood sugar levels can be expected to be ~20% lower after 12 to 24 hours of fasting.
As your glycogen stores start to deplete, your body slowly starts to look for other fuel sources to make up the difference. The ‘backup’ fuel of choice is fat, in the form of ketone bodies.
Note: Your body will also try to turn to your stored protein (from muscle). That sucks. However, we can protect ourselves by engaging in resistance training and eating enough protein. Those two things tell your body that you think your muscle is important, and that it shouldn’t be broken down for fuel.
24-72 Hours
After 24 hours, your glycogen stores are significantly depleted. Your body is beginning to turn more and more to stored fat for energy. Fatty acids are pulled form your adipose tissue and transported to the liver for conversion into ketone bodies. By 72 hours, your body will be relying on ketones as a primary fuel source.
Note: While the rest of the body will rely on ketones, the brain still needs glucose to function. Your body begins the process of converting non-carbohydrate materials into glucose. Specifically, your body can convert parts of amino acids and fatty acids into glucose. This process will create approximately 80g of glucose per day, to be used by the brain.
We also see an interesting change in hormone production over this timeframe. Ghrelin, the hormone responsible for making you hungry, starts to decrease by day 3. This is in agreement with the observation that most people are very hungry on days 1 and 2 of a fast, but by day 3 the hunger begins to subside.
72-120 Hours
After day 3, we enter the prolonged fasting period. Blood glucose levels stay low, hunger is suppressed, and the body continues to increasingly rely on ketones. Ketone levels continued to rise until a sufficient amount is available to fuel the body.
120+ Hours
After day 5, you will enter a steady state of ketosis. Your body will also decrease its level of various anabolic (growth) hormones.
It should be noted that some ketones may continue to raise as time goes on. One study showed that ketone levels do not plateau until day 20-25.
Please note: Extended fasts should not be done without first consulting a medical professional!!
Some thoughts
I’d like to point out that your body doesn’t really think it’s ‘fasted’ when you don’t eat for a few hours. The human body is incredibly resilient. If it changed how it operated after a single missed meal, the human race would have perished a long time ago. It is only in modern times that we can consistently eat multiple meals in a day for the entirety of our lives. Ancient humans didn’t know when their next meal would be. Their body’s, and yours, were built to handle these situations.
Any magical claims made to various intermittent fasting protocols are mostly BS (and we’ll talk more in detail about this next week). As you can see from the above information, the changes inside the body are relatively modest. It’s not until after a few DAYS of fasting where your body starts to shift its metabolism around.
…
Thanks for reading this week. Please shoot me a message with any feedback or article suggestions that you may have. I’d love to hear from you. Seriously. My email is caseythomasrd@gmail.com and I read 100% of what you send me (sometimes despite my better judgement).
Enjoy the rest of your day!