Multi-day fasting is relatively low risk (for most people) and offers promising longevity benefits. It is very effective for weight/fat loss and metabolic health and preventing diabetes and other metabolic conditions. There are also promising longevity benefits such as cancer and Alzheimer’s prevention (although this is less proven). The general idea is that when you undergo the stress of fasting, your body activates a whole host of protective processes that end up improving health and longevity related biomarkers (similar to how the stress of exercise creates health benefits).

Below is an abbreviated list of health, lifestyle, and mental benefits, as well as potential risks for people with certain conditions. This page will be continually updated as the field develops. Additional scientific studies and resources will be linked as they are encountered.

Effective major health benefits 

Lots of evidence and easy to measure

Weight loss & fat loss

• This one is undisputed and easy to measure. If you don’t eat for multiple days you will lose weight and you will lose fat. Your body is burning energy (somewhere around 15 times your bodyweight in calories per day) and not replenishing it. I usually lose 0.75 to 1.0 lbs of fat loss per day of fasting (with exercise).
• But isn’t it just water weight? Yes you do lose a lot of water weight quickly, and also gain it back quickly, but you will also lose fat. 
• If the weight isn’t coming down during the fast it could be: measuring weight at inconsistent times (for example, I am usually 3-5 lbs heavier before bed than when I wake up in the morning), you just drank a bunch of water, or maybe you took a lot of salt and are retaining water that. Keep going and you will see the fat/weight come down (especially on Day 4+).
• Will you be able to keep it off once you finish? It depends on what time frame you are referring to and what you are eating, and how quickly you put on fat, but in my experience, yes. A pound of body fat is around 3500 excess calories (big generalization). If you continue overeating by 500 calories a day, it would take 7 days to put on a pound of fat. If you lose 4 pounds of fat on a 4-day fast. It would take you a month to regain the fat.
• Measurable with scale, tape measure, and DEXA scan.

Reverse Type 2 Diabetes, metabolic syndrome, cardiovascular disease

• Eating too many carbs and exercising too little causes insulin resistance. That means your body is not very effective at using a certain quantity of insulin to bring down sugar, and requires more insulin than normal to bring down the blood sugar. The problem is that insulin is the body’s storage hormone – the more there is, the more you are going store fat and the less you are going to burn fat. Keep going down this path and the amount of insulin your body can produce will no longer be enough, and your blood sugar will remain elevated (pre-diabetes, and type 2 diabetes). This has a bunch of terrible effects on your health. 
• When you fast, your insulin goes way down, your blood sugar goes way down and your body becomes more insulin sensitive (less insulin does the job). Ideally, you would want your body to use the lowest amount of insulin to get the job done of storing nutrients and lowering blood sugar.
• Fast regularly and watch the metrics improve.
• Ways to measure: Continuous glucose monitor, Hemoglobin A1C blood test, oral glucose tolerance test

Speculative major health benefits 

Very promising, but more research needed & hard to measure

Cancer prevention

• The theory is that a large percentage of cancer cells can only use glucose as a good source. But your brain and the rest of your body can use ketones. When you are fasting and no longer consuming glucose, you are essentially starving out (or slowing the growth of) cancerous and precancerous cells.
• Everyone continually has pre-cancerous cells in their body regardless of if there are observable symptoms. The idea is that if you do an extended fast periodically, you can continually clear out cancer cells to delay or prevent serious cancers.
• Chemotherapy has been shown to be more effective when you are in fasted or keto state.

Alzheimer’s, dementia, and prevention of cognitive decline 

• When in a deep fast, some degree of autophagy happens, where your body breaks down and reuses old and damaged cells. This potentially includes Alzheimer’s associated cells like beta amyloid plaques and tau tangles that “gum up” your brain.
• Additionally, some have referred to Alzehimer’s as “diabetes of the brain” or “Type 3 Diabetes,” where the brain’s ability to use insulin diminishes cognitive ability. Using fasting to increase insulin sensitivity could lower this risk.
• Yet another theory is that inflammation is a central mechanism in Alzheimer’s disease. Animal studies have indicated that fasting can reduce inflammation and potentially slow processes related to cognitive decline.

Other potential health benefits

Other plausible health benefits with reasonable evidence

• Longevity benefits – There have been numerous animal studies on the effect of caloric restriction to extend lifespan. It is less clear the degree to which this applies to humans. The opposite of caloric restriction– caloric excess– clearly negatively affects longevity.

• Autophagy – Autophagy is derived from Greek and means “self-eating.” The body performs garbage collection on cells that are not functioning optimally (often aging cells), and recycles the materials to use for new structures or energy. This process accelerates when in a catabolic or fasted state. We evolved to be able to do this, and most modern humans do not. Autophagy could provide the repair to delay certain disease states as we age.

• Gut system reset – During a fast, you are giving your stomach, gut, and digestive system rest. It can function as a reboot of your gut microbial environment. It can be used to manage or prevent issues related to ulcerative colitis and IBS.

• Exercise performance – Improvement will likely only be applicable to certain situations, and in other is could decrease performance. One such case of improvement is the ability to hold breath longer. Utilizing energy from fat/ketones uses less Oxygen per ATP released. Navy SEAL divers have been able to hold breath 2-3x longer than normal while in Ketosis. Some ultra marathon runners follow ketogenic diets to be able to sustain long races without bonking. Anecdotally, when fasting, I feel like I have an unlimited capacity to walk.

Lifestyle Benefits

• Less money on food – If you are not eating, you are not consuming food at home, or getting delivery, or eating at restaurants.

• Less time on food – Tally up all the time you spend each day planning meals, acquiring food, preparing food, eating food, and cleaning up after food. While fasting all of this goes away.

• Forces you to stop drinking alcohol for that time period – When fasting, you are not eating, and you are also not drinking. You don’t feel like drinking alcohol. In my experience, if you are deep into a fast and you go to a concert or show with friends, you will be totally fine not drinking, even if you would normally beeline to the bar.

• Allows you to be more loose in the types of foods that you eat – Once you know that you have the ability to fast, and counteract cheat meals, or day(s), you can be more flexible with what you eat. Traveling or someone else chose the restaurant? It is okay to have those burgers and fries or that pasta plate at a fun dinner with friends if you can fast it away later. You don’t need to maniacally track all of your macros and eat clean every single meal. 

• More time to focus on a particular project – I love deep fasts for deep work. There are no distractions from food and less decisions to be made. Is your significant other out of town for the weekend? Perfect time for a fast to work on your book, side hustle, or to just reflect.

Mental or Intangible Benefits

• Elevated mental clarity – A lot of people reporting being a slightly elevated mental state when they reach full ketosis. You just feel energized and that you can concentrate and just hammer through some work. For me personally, the feeling comes in waves. Some parts of the day I feel great and hyper focused, and other parts of the day I zone out.

• Mental empowerment – This was a surprising one. Going multiple days without eating is a big accomplishment. Not many people in the modern world do that. It is hard to explain without going through it, but you feel mentally empowered after the fast. It is incredibly empowering to know that at any instant, someone could be like “Sorry, but there won’t be any food available for the next 5 days” and you will be totally fine with it.
  • Confidence – You have confidence in knowing how your body works.
  • Will power – I believe it increases your overall willpower capacity and has the ability to bleed into other physical, mental, and nutritional challenges in your life.
  • Control – You have this amazing sense of control over your food. No longer are you a slave to your food.

• Being in tune with your ancestors – Our ancestors were made to fast. We evolved and survived because of our ability to fast. Modern living has completely detached that from our experience. Going through Day 2+ of a fast is a primal human experience. 

• It will make you appreciate food more – You may think that I am “anti-food” because I preach the fasting lifestyle. I have found that I enjoy food even more because of fasting. While fasting, your senses are slightly heightened. If I am walking down the street all of the smells from restaurants are extra intense. You may find yourself making lists of all the restaurants or recipes you want to try. Your first few meals after the fast will be glorious, and you will have less guilt when enjoying them.

• Stable energy levels – Because you are not eating meals throughout the day, spiking your blood sugar, and then crashing it, and your stomach using blood supply to digest food, your blood sugar stays remarkable stable. Many report not having that post-lunch crash.

• Gaining positive adaptations (it being easier the next time you do it) – Long fasting gets easier the more you do it. You get into ketosis faster. Your body is more fat adapted. You don’t get the keto flu. You don’t get as hungry as usual. You can keep up your normal social and exercise routine. You can work while fasting.