What is Fasting?

What is the difference between time-restricted, intermittent, and prolonged fasting?

This blog is a conjunction of notes from the Tim Ferris podcast Exploring Smart Drugs, Fasting, and Fat Loss — Dr. Rhonda Patrick [00:13 to 00:28] and my experiments and research.  I highly recommend listening to this podcast if you have a few hours to spare, Dr. Rhonda answers questions on how to reduce inflammation, vitamins, fasting, cold and heat exposure, and some other wellness-related subjects. 

So, I already listened to this podcast twice! It only took me about 3 hours to unpack the first 30 minutes. I chose the topic that I’ve been experimenting with and learning about over the last year. My attempt was to make notes with as much precision as possible based on the podcast. But, there is a possibility that I’ve missed a few things. Also, I added some additional information regarding intermittent fasting and fast mimicking diet. 

1) Time-Restricted Fasting Based on Circadian Rhythm

DEFINITION: This method is not about what or how much you eat, but when you eat. The time-restricted, intermittent, and prolonged fasting overlap on the feeding window.

A circadian rhythm is based on the changing environment of light during the day. It dictates when to go to sleep and wake up.  A circadian rhythm is a roughly 24-hour cycle in the physiological processes of living beings.

Dr. Rhonda explained that when healthy adults eat identical meals regarding macro-nutrient and calories at breakfast, lunch, or dinner, the post-perennial glucose increase is lowest after breakfast and highest after dinner. It means that the metabolism changes throughout the day. The metabolic genes are more active during the day.

The underline reason for this is that humans conduct most of their activities during the day and rest at night. This is due to the presence of the internal clock in the brain referred to as the suprachiasmatic nucleus or SCN.  The SCN contains about 20,000 nerve cells and is located in the hypothalamus, an area of the brain just above where the optic nerves from the eyes cross. But light is not the only external queue we have.

We also have food influencing what is known as the peripheral oscillators that occur in peripheral tissues such as in the liver and influence metabolism.  Where is light is the primary queue for circadian rhythm, timing of food intake regulates circadian rhythm in the peripheral tissues as well. That explains how the restricted time eating begins with the very first bite or drink other than water.

Even compounds that exist in black coffee such as caffeine can be reasonably expected to produce metabolic affects that influence these peripheral oscillators including activities in the liver.  

To demonstrate the importance of circadian rhythm further. These clocks regulate thousands of genes that are somewhere in the neighborhood from 10% to 15% of expressed human genome. This means that our basic metabolic physiology is meant to behave differently depending on the time of the day that it is. Even bacteria in our gut has circadian rhythms! With the invention of artificial lighting, our eating time is extended to much later in the evening. This can have very negative consequences. 

Melatonin receptors were found in the pancreas. The increase of melatonin that happens during the day inhibits insulin production. Therefore, late-night calories have a different effect on our health.

Eating late at night may reset the prefinal clock and result in misalignment of the metabolism.

This means when you wake up, your metabolism is already at the end of its cycle. That’s why time-restricted eating timing is critical. It emphasizes eating earlier in the day.

Benefits of Time-Restricted Circadian Rhythm Fasting:

  • Increases endurance
  • Helps with weight loss
  • Reduces inflammation
  • This leads to increased lean mass
  • Reduced risk of breast cancer 
  • Boosts our energy level
  • An increase in brown fat tissue
  • Promotes repairs of the cells 

2) Prolonged Fasting:

DEFINITION:  The prolonged fasting lasts between 2 to 5 days. The prolonged fasting appears to shift stem cells of the immune system from a dormant state to an active state of self-renewal. Results from experiments with mice and a Phase I human clinical trial showed that long periods of fasting significantly lowered levels of white blood cells.

Benefits of prolonged fasting (4 to 5 DAYS):

  • A dramatic increase in  Autophagy and Apoptosis followed by a massive boost in stem cell production. Autophagy clears away damaged cells to use for energy. While Apoptosis causes the damaged cells to self-destruct.  Both of these processes prevent the damaged cells from becoming cancerous cells.
  • When we clean away damaged cells, it also means that these cells are less likely to become senescent. A  senescent cell is a living cell. But it is not functioning in a way consistent with maintaining the overall health of an organ. In fact, quite the opposite. A  senescent cell can accelerate the aging of nearby cells and promote tumor growth by secreting pro-inflammatory molecules. So, a senescent cell is bad news. As we age, they are everywhere from our livers, brains, hearts, etc. And they accelerate the aging process.
  • It has a robust effect on increasing the stem cell numbers. The regenerative power of tissues and organs decline with age. It is stem cells that provide this regenerative power. Because stem cell number decline with age so does organ function. Which means anything that counters it is a win.
  • Prolonged Fasting also causes cells to clear away damaged mitochondrial and recycle the defective components for energy called mitophagy. Followed by creating new mitochondria.

3) Fasting Mimicking Diet (5 Days):

The Fasting Mimicking Diet (FMD)  is a meal program with pre-made food. It is made of plant-based ingredients to be consumed for five days. Your body is not recognizing that it is eating.  In other words, it is prolonged fasting with food.

The FMD was developed and clinically tested at the Longevity Institute at the University of Southern California. Led by Dr. Valter Longo.

The benefits of FMD are the same as prolonged fasting. 

4) Intermittent Fasting.

DEFINITION: Intermittent Fasting (IF) restricts calorie intake, unlike time-restricting fasting.  IF is a term for an eating pattern that cycles between periods of fasting and eating. You can do this by skipping breakfast, eating your first meal at noon, and your last meal at 8 pm. So, you are fasting for 16 hours every day, and restricting your eating to an 8-hour eating window. This is the most popular form of intermittent fasting, known as the 16/8 method.

 By fasting about 14 to 16 hours a day, you will give your body more than enough time to drain your glycogen stores and shift into fat-burning mode.

The benefits of Intermittent Fasting are the same as Time-Restricted Circadian Rhythm Fasting. 

My Experiments.

My schedule is flexible. So, I typically eat my last meal between 12:30 pm and 2:00 pm at the latest. I break my fast with coffee around 5:00 AM and have my first very light meal between 8:00 and 10:00 am. Every Friday I have my last meal at 10:30 AM and eat my next meal on Saturday at 10:30 am. However, I am guilty of drinking coffee. Typically, I work out, take a vigorous hike or take an Epsom salt bath on Saturday. I don’t fast during my period.

Also, I tried a Fast Mimicking Diet (FMD). I absolutely loved it.  But it comes with a high price tag. So, I’ve been working on creating my own protocol for FMD.

You have to figure out what works for you.  The easiest thing to start is either starting dinner early or skipping it altogether. If you have any medical conditions, consult a qualified doctor and discuss if fasting is right for you. Use common sense and know your physiology.  



  1. […] Practice intermittent fasting. It is a great weight-loss tool.  I promise you won’t die if you skip dinner or breakfast! You need to give your body enough time between meals.  So, try to extend the overnight fast as long as you can, by making sure you don’t eat until you are really hungry each morning. […]

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