5 mins read

Ultimate Guide to Fasting

Discover the essential guidelines to fasting and see a deep-dive into a multi-day fast.

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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.

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.

Types of intermittent fasting schedules

Here are seven types of fasting schedules:

  1. The 16/8 Method: This is the most popular intermittent fasting schedule, where you fast for 16 hours and eat during an 8-hour window each day. For example, you could eat between noon and 8 pm, and then fast for the remaining 16 hours. This schedule is easy to follow and can be a good starting point for beginners.
  2. The Circadian Rhythm Fasting Method: This is another from of intermittent fasting options in which, you stop eating from sun down to sunset, to match it with your Circadian Rhythm.
  3. The 5:2 Diet: With this schedule, you eat normally for 5 days per week, and then restrict your calories to 500-600 for 2 non-consecutive days. This schedule allows for flexibility in your eating habits, but can be difficult for some people to stick to.
  4. The Alternate Day Diet: With this schedule, you eat normally one day, and then restrict your calories to 500-600 the next day. This schedule can be difficult to maintain long-term, but can be effective for weight loss.
  5. The Eat-Stop-Eat Method: With this schedule, you fast for 24 hours once or twice per week. For example, you could eat dinner on Monday, and then not eat again until dinner on Tuesday. This schedule is more intense than other intermittent fasting schedules and can be difficult for some people to follow.
  6. The One Meal a Day (OMAD) Diet: With this schedule, you eat one large meal per day, and fast for the remaining 23 hours. This schedule is very restrictive and can be difficult for some people to stick to, but can be effective for weight loss.
  7. The Spontaneous Meal Skipping Diet: With this schedule, you don't follow a specific eating plan, but instead allow yourself to skip meals when you're not hungry or don't have time to eat. This schedule is less structured than other intermittent fasting schedules, and can be more flexible and sustainable for some people.
  8. Multi-day Fasting: 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.

What can you consume during a fast?

During a fast, you will not be consuming any solid food. However, there are still certain beverages and foods that you can consume to help you stay hydrated and nourished while fasting. Here are some examples of what you can consume during a fast:

  1. Water: The most important thing to consume during a fast is water. Water isn't just allowed, it's essential for maintaining hydration and supporting the body's normal functions during your fasted hours. It's important to drink plenty of water during a fast to help your body flush out toxins and waste products. Electrolytes, as long as they contain very few calories (<20) are also allowed and helpful during a fast.
  2. Black coffee: Yes,coffee is allowed during a fast, as long as it's black so  it contains very few calories and no carbohydrates or protein. However, be careful not to add any sweeteners or cream, as these can break your fast.
  3. Green tea: Green tea is another beverage that is allowed during a fast, as it contains very few calories and no carbohydrates or protein. Green tea is also rich in antioxidants, which can support the body's natural detoxification processes.
  4. Bone broth: Bone broth is a nourishing and hydrating beverage that can be consumed during a fast. It is rich in minerals and collagen, which can support the body's natural healing and repair processes.
  5. Lemon water: Lemon water is a refreshing and hydrating beverage that can be consumed during a fast. Lemon water is also rich in vitamin C and other antioxidants, which can support the body's natural detoxification processes.
  6. Apple cider vinegar: Apple cider vinegar is another beverage that can be consumed during a fast. It is rich in acetic acid, which can support the body's natural detoxification processes and help to balance the body's pH levels.

What should you eat before a fast?

No matter what you eat before starting your timer, you’re likely to see a lot of benefits from fasting. But you can make things a lot easier on yourself—and potentially achieve even greater benefits—by eating certain foods and avoiding others.

Much of fasting’s magic happens when your body enters ketosis, a state where your liver glycogen is depleted and your body uses something called “ketone bodies” for fuel instead of glucose. If you really want to get the most out of your fast, and get into ketosis even faster, you can start your timer with your liver glycogen either partially or completely depleted by eating Keto. That means avoiding carbohydrates pretty much entirely (with the exception of fiber).

Consider cleaning up your diet and altering your macronutrient distribution about two days to one week prior to your fast, depending on the duration of the fast you’re planning. This means cutting out the sugar and processed carbs, and beginning to decrease the total amount of carbs you consume

A typical pre-fast (non-keto) meal would include brown rice, baked chicken, a cooked veggie like carrots or broccoli, a tossed salad and fresh fruits for dessert. Those fruits, veggies and brown rice provide fiber to help youfeel fuller a little longer and, of course, offer tons of vitamins and minerals.

What's a good meal to break a fast?

Depending on how long of a fast you just completed (Intermittent fasting vs. multi-day fast) you are going to find yourself at a different level of hunger and craving burgers, pizzas and cakes. You're probably thinking, “I’ve gotten all the great benefits of fasting, now I can reward myself”.

In all honesty, this is the last thing you want to do. And the most important reason is that it would also cause some digestive issues. So, which are good foods to break a fast?

  • Soups, like bone broth;
  • Eggs; The protein found in eggs is helpful when breaking a fast as it will help prevent the loss of muscle. So, if your goal is to lose fat and gain muscle, then you should definitely consider eating some eggs when breaking your fast. Eggs are also rich in leucine, an amino acid lauded for its role in muscle growth. Apart from this, there are many other reasons why eggs should be your go-to. For one, they are easily digestible, low in calories, protein-rich, and the fact that they are so filling makes them reduce appetite.
  • Avocado; avocado might just be the best choice for someone who’s breaking a fast. The fruit is low in calories and rich in healthy fats, antioxidants, vitamins, minerals, folate, potassium, and copper.
  • Leafy greens;
  • Nuts; Nuts are great as they are vitamin-rich and help to restore nutrient balance in the body after a fast.
  • Fish; Fish is a great source of protein, vitamin D, and healthy fats. For this reason, fish should definitely be on your list of the best foods to break a fast.
  • Watermelon; Watermelons are made up of more than 90% water, they are great fruits to consume while breaking a fast as the water makes them easy to digest and helps hydrate you.

The bottom line is that while breaking a fast, you want to avoid foods that contain large amounts of sugar, complex carbs, fiber, and fat.

So, what happens to your body on a multi-day 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 one of our team members completed a 3-day water fast and tracked 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 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.

I broke my fast with the following:

  1. One slice of watermelon
  2. 15 mins later; salad with organic spinach, macadamia nuts and Extra Virgin Olive Olive (EVOO)
  3. 20 mins later; 2 egg omellete with organic chicken, kefalotiri cheese and avocado.

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.


5 mins read
Ultimate Guide to Fasting

Discover the essential guidelines to fasting and see a deep-dive into a multi-day fast.