Ultimate Guide to Fasting

Our founder used Basis to track his body's response to a 72-hr fast.

Caloric restriction is the practice of taking fewer calories than total daily energy needs. Caloric restriction without malnutrition is the most effective non-pharmacological intervention that enhances longevity and healthspan in numerous species. It has been proven to reduce oxidative stress through various pathways and a growing body of evidence shows that sustained periods of caloric restriction without malnutrition improves risk factors involved in the pathophysiology of type 2 diabetes, cardiovascular diseases, cancer, and neurological disorders in humans.

But chronic caloric restriction is difficult to sustain and newer dietary strategies such as intermittent fasting and protein restriction have emerged as alternative approaches that still improve markers of aging. The most popular type of caloric restriction applied today is a 16:8 Time-Restricted Feeding schedule i.e. no eating for 16 hours, eat for 8 hours. While difficult for some to get started, Intermittent Fasting is generally easy to adopt and has important short and long-term health benefits such as metabolic, cardiovascular, and digestive health benefits.

Multi-day fasting is another form of fasting with a higher detoxifying effect that activates important an important cellular restoration process called autophagy. Autophagy is a super useful process that recycles, cleans up, and rids your body of damaged and misfolded proteins. When your cells are constantly fed, they aren't worried about efficiency and restoration - they're thinking that 'times are good, we don't need to work hard anymore.' In a well-fed state, your cells are only concerned with growing. Besides putting all their emphasis on growth, your well-fed cells also turn other genes off such as those related to fat metabolism, stress resistance, and damage repair.

During fasting (starvation), things are very different. We have a well-preserved starvation “program” that kicks our cell into a completely different state when food, particularly glucose or sugar, isn’t around. Your body reacts to the newfound stress by waking up cell functions to protect you and as a result, starts acting more efficiently and dynamically. This 'reactivation' of the cellular system leads to lowered inflammation, increases your brain's resistance to stress, and many other important long-term health benefits.

So, what happens to your body on a fast?

At Basis, we wanted to put the science to the test and really see what happens in real-time to your body when you’re following a prolonged fast. So our founder George volunteered to be the guinea pig by spending last week doing a 3-day water fast and tracking his vitals in real-time with Basis and a continuous glucose monitor to see the impact it has on his body.

For data sharing purposes, he also did a pre- and post-fast blood panel. You can see a table with the tested biomarkers below. He also provided body composition data because most people think of fasting for weight loss purposes.

Disclaimer: Speak with your doctor before undertaking any type of fasting. Your doctor will advise you if any pre-existing condition or abnormalities in your biomarkers could create complications.

Without further ado, let’s jump into the Basis real-time data along with an explanation of the cellular process that happens as you hit different intervals in your fast.

72-HR Water Fast


Low: 75  
High: 119  
Average: 94
Glucose Variability: 10.31

Heart rate

Low: 49
High: 96
Average: 62

What's Happening

12 hours - Hello ketosis.

In this state, your body starts to break down and burn fat. This process creates ketones which serve as an alternative energy source when glucose isn’t available.

18 hours - Fat-burning mode.

I'm now generating significant ketones and I can now begin to measure blood ketone levels above baseline values. Under normal conditions, the concentration of ketones in your plasma ranges below 20 mg/dL but when you fast this concentration can reach 80+ mg/dL. My ketones measured at 32 mg/dL compared to 80 mg/dL for my glucose.

24 hours - Autophagy

Autophagy can only happen when your glucose stores are significantly low. In humans, that usually happens after 24 hours of fasting.



Low: 69
High: 107  
Average: 85
Glucose Variability: 9.96

Heart rate

Low: 45
High: 97
Average: 62

What's Happening

48 hours - Peak growth hormone

Growth hormone helps preserve lean muscle mass and reduces fat tissue accumulation, particularly as we age. It also appears to promote wound healing and cardiovascular health. After two days of fasting, the large number of circulating ketone bodies and the impact of ghrelin, the hunger hormone, promote increased growth hormone secretion.

At this point in the fast, my glucose line is virtually flat, ranging between 75 - 85 mg/dL and I'm preserving my energy for essential activities and work.



Low: 69
High: 90
Average: 78
Glucose Variability: 8.27

Heart rate

Low: 50
High: 120
Average: 64

What's Happening

72 hours - Checkpoint.

At the 3-day mark, I've hit a key milestone in cellular regeneration. My body has broken down old immune cells and generated new ones. Getting those Covid defenses up! Interestingly, through this same mechanism, prolonged fasting for at least 72 hours has been shown to also preserve healthy white blood cell or lymphocyte counts in patients undergoing chemotherapy.

Last ketone measurement before breaking the fast. Approaching the 80 mg/dL along with a 79mg/dL glucose level measurement.

Bonus stage: Refeeding!

The fast isn’t done until you get some food back in your body. The most important thing to remember is that you should resist the temptation to chow down on a pizza or a burger because as amazing as that may seem in the moment, and trust me the temptation is always there, after 3 days of just sipping on water, it will feel absolutely terrible afterward. Your stomach has for all-intensive purposes shrunk and hasn’t gone through the process of digesting anything for 3 days. You’ll get bad stomach cramping and spend quite a bit of time in the bathroom if you eat a carb-heavy meal.

Beyond this practical reason, it’s important to break your fast with a nutritious, balanced meal that will further enhance the all-around cell and tissue health-boosting exercise you just undertook.

Want to try out this same experiment?

Sign up for Basis below and we'll get you setup with everything you need to try it out.

Looking for some extra motivation?

Join our discord server and join an active 72 hr water fasting challenge.

Found this post useful? Consider supporting us!