SO WHAT IS A FAST EXACTLY?
Technically-speaking, a fast is abstaining from caloric intake for at least 8 hours.
Over the last few years, “intermittent fasting” (IF) has become a health buzz phrase, and usually indicates a habit of abstaining from caloric intake for anywhere from 12-24 hours. Two of the more popular forms of IF include the 16/8 approach, where you fast for 16 hours of the day and have an 8 hour eating window, and the 23/1 approach or one meal a day (called OMAD for short). 
In addition to IF, the practice of fasting for more than 24 hours, often called extended or prolonged fasting, has also grown in popularity.
A FAST HISTORY OF FASTING
While fasting for health may be trending, fasting as a practice is hardly new. People have been fasting for millennia for a host of reasons…
~1300-1500 BC: The Israelite leader Moses famously fasts for 40 days on two separate occasions (see Deuteronomy chapter 9).
The Jewish holiday of Yom Kippur (which falls annually on a day in September or October) commemorates the day that Moses descended the mountain with the second set of stone tablets containing the 10 commandments (see Leviticus chapter 23). While the bible does not explicitly call for fasting on Yom Kippur, many Jewish adherents traditionally fast for 25 hours on Yom Kippur (the fast generally begins just before the day starts and ends just after the day ends).
~700 BC: Some 800 years after Moses, the Israelite prophet Isaiah speaks to the people of Israel, explaining what a righteous fast before God looked like, and the promises of healing that would come from such a fast (see Isaiah chapter 68).
~500-300 BC: The ancient Greek philosopher Plato and Greek physician Hippocrates (who’s often called the Father of Modern Medicine) both extol fasting's many benefits in the 5th and 4th centuries before Christ. Plato famously said that he fasted for “greater physical and mental efficiency.”
~30 AD: Around 2000 years ago Jesus Christ is led into the wilderness to be tested spiritually while completing a 40 day fast (see Matthew chapter 4).
~325 AD: After the Council of Nicea in 325 AD, the 40 day period before Easter known as Lent starts to be observed by some Christians. Since that time Lent has generally held greater significance for those following the Catholic faith. Many Catholics fast on Ash Wednesday and Good Friday, and avoid meat on each Friday during Lent.
~624 AD: Muslims begin observing the Islamic month of Ramadan, which includes fasting from sunrise to sunset for the entire 30 day month.
~1530: Around 500 years ago, the Swiss physician Paracelsus, often called the Father of Toxicology, proclaims that, “Fasting is the greatest remedy, the physician within.”
~1870-1910: American writer and humorist Mark Twain touts the benefits of fasting, as he himself would regularly fast for 1-2 days when battling colds or fevers, with reportedly excellent results.
1911: Pulitzer Prize Winner Upton Sinclair publishes his short book The Fasting Cure, which extols the benefits of extended fasting for a host of common maladies.
1913-1948: Indian lawyer and political ethicist Mahatma Gandhi undertakes more than a dozen documented fasts, many of which last 7-21 days. [2-3]
Mid 1960s-Mid 2000s: Dozens of scientific articles are published each year on fasting and its effects on the human body.
Mid 2000s-Today: Thousands of scientific articles are published each year on fasting and fasting’s effects on human physiology. Much of the scientific evidence highlights the restorative and preventative power of proper fasting, a practice made more popular by the works of influential medical professionals like Jason Fung MD, Steven Gundry MD, Thomas Seyfried PhD and Japanese cell biologist Yoshinori Ohsumi, who won the 2016 Nobel Prize in Medicine for his work on autophagy. 
FASTING FOR WEIGHT LOSS: WHAT’S THE SCIENCE SAY?
Regarding weight loss, a 2015 systematic review of 40 studies in Molecular & Cellular Endocrinology found IF effective for weight loss, with a typical loss of around 10 pounds over 10 weeks. Obese individuals will often lose significantly more than this, with a 2-3 pounds a week weight loss a common occurrence. 
A 2020 review of 27 studies on IF came to similar conclusions. In the 4 studies where researchers looked at the 16/8 approach specifically, participants (generally obese and sometimes with type 2 diabetes) lost between 2.2% and 3.9% of total bodyweight over the course of 4-12 weeks. This is the equivalent of a 200lbs person losing approximately 0.5-1.5lbs a week.
More dramatic weight loss was experienced when 24 hour fasting was implemented. For example, a study of 40 obese participants who combined a 1 day a week fast with a high protein diet lost around 1.5-2lbs a week over 12 weeks, while a group of 26 obese participants who did alternate day fasting for 8 weeks lost around 2-2.5lbs a week! 
The bottom line: overweight and obese individuals using the 16/8 intermittent fasting approach can expect to lose about an extra 1lb a week just through IF alone (those who weigh more tend to lose more than that). That’s another 13lbs (or more) off over the next 3 months, or 26lbs (or more) over the next 6 months, just by using 16/8 IF.
Those who utilize at least one 24 hour fast per week can expect even greater weight loss results (1.5-2.5lbs/week on average), an effect that is amplified by the addition of a high protein diet.
FASTING FOR HEALING & PREVENTION:
WHAT’S THAT SCIENCE SAY?
A 2010 study found that fasting-induced autophagy “has been recognized as a crucial defense mechanism against cancer, infection and neurodegenerative diseases.” Infection here would include conditions like pneumonia, influenza, COVID and the common cold, and neurodegenerative diseases would of course include conditions like Alzheimer’s, dementia and Parkinson’s. 
A 2014 study from the journal Cell Metabolism found that fasting “helps reduce obesity, hypertension, asthma and rheumatoid arthritis” and that fasting “has the potential to delay aging.” 
A 2017 study from Ageing Research Reviews found fasting effective for insulin resistance, cardiovascular disease and diabetes, and reported promising results in using intermittent fasting to combat multiple sclerosis and cancer. 
Cardiovascular diseases such as heart disease, hypertension and stroke…
Cancer and diseases that center around immunity/infection such as COVID, pneumonia, influenza and the common cold…
Asthma and other chronic respiratory diseases such as COPD…
Neurodegenerative diseases such as Alzheimer’s, dementia, multiple sclerosis and Parkinson’s…
Blood sugar, insulin and pancreatic diseases such as diabetes and insulin resistance…
Systemic metabolic conditions such as obesity…
Inflammatory musculoskeletal diseases such as arthritis…
Even accelerated aging…
These diseases and conditions make up the vast majority of the disease burden in the United States, and here we have scientific evidence that smart fasting practices can improve ALL of them!
HOW FASTING WORKS
Modern science and clinical research are also helping us understand more precisely how fasting exerts its therapeutic effects on the body, which includes…
• Changes to gene expression, including DNA repair
• Cellular repair and regeneration, including positive changes to the mitochondria, to stem-cell regeneration and with autophagy, a process where cells digest and remove old and dysfunctional intracellular proteins
• Hormonal optimization, including boosts to Human Growth Hormone and lowered insulin levels and
• Positive changes to general metabolism, including regulating glucose and glycogen levels, as well as triggering ketogenesis, where body fat and triglycerides are broken down into ketones and free fatty acids, to be used by the cells for fuel.
For example, a 2022 article in the prestigious British Medical Journal (BMJ) highlighted some of the known benefits of intermittent fasting, including its ability to reduce systemic inflammation and ROS production (free radical production that drives oxidative stress leading to cellular and tissue degeneration). 
WHAT METHOD OF FASTING DO I MOST RECOMMEND?
ANSWER: 16/8 & PERIODIC 24 HOUR FASTS
For many of my clients I really like the 16/8 approach, as I think it provides a good mix of some of the main benefits of fasting (ex. accelerated autophagy, detox, inflammation reduction, digestive system improvement, accelerated fat burning, etc.) while also giving us a significant enough eating window where we can still have a nutritional intake that promotes accelerated metabolism, thermogenesis and anabolic growth. For example, with 16/8 this might look like an eating window from 9am to 5pm.
For those who want some of the benefits of IF but struggle with how tight the 8 hour eating window is, we’ll sometimes do a 14/10 approach, which extends the eating window (ex. 9am to 7pm).
Additionally, I’ve seen people get better results when their eating window is shifted to earlier in the day (ex. 9am to 5pm) versus later (ex. 1pm to 9pm). At a minimum, I usually want my clients to refrain from eating the 3 hours before bedtime, mostly in order to improve sleep quality.
I also really like the effects that a weekly, biweekly or even monthly 24 hour fast provides. For these, I generally recommend starting the fast around lunchtime.
…BUT WHAT ABOUT 18/6, 20/4, OMAD, ALTERNATE DAY FASTING, THE 5:2 APPROACH & EVEN PROLONGED FASTING???
Several studies (such as the 2020 review of 27 IF trials I already mentioned) have highlighted some of the positive health and weight loss effects of other fasting approaches such as 5:2 (which involves eating normally for 5 days a week while fasting for 2 days a week). 
And then there’s this fascinating 2019 study of more than 1400 participants who engaged in prolonged fasts of 4-21 days. Listen to what the research author’s report…
“Significant reductions in weight, abdominal circumference and blood pressure were observed in the whole group. A beneficial modulating effect of fasting on blood lipids… and further general health-related blood parameters was shown. In all groups, fasting led to a decrease in blood glucose levels to low norm range... An increase in physical and emotional well-being and an absence of hunger feeling in 93.2% of the subjects supported the feasibility of prolonged fasting. Among the 404 subjects with pre-existing health-complaints, 341 (84.4%) reported an improvement. Adverse effects were reported in less than 1% of the participants.” 
While I recognize that ALL these fasting approaches can have significant health benefits, many of these are also more difficult to consistently execute due to factors like a tighter eating window (1-6 hours vs 8-10 hours), more frequent 24 hour fasting days (ex. alternate day fasting and the 5:2 approach) and the overall difficulty of prolonged fasting (especially if attempted without professional or personal support). It’s for these reasons that I tend to recommend fasting approaches like 16/8 and the occasional 24 hour fast more often.
7 TIPS FOR SUCCESSFUL FASTING
TIP 1: Pick a fasting approach that’s likely to work for your lifestyle, and don’t be afraid to adjust if/when your lifestyle changes. I’ve had clients who love a 16/8 approach that delays breakfast to mid/late morning (10/11am) but still gives them time to eat dinner with family, and I’ve had other clients with crazy busy work schedules who like the simplicity that OMAD provides. But remember, try to balance the benefits of fasting (autophagy, detox, digestive system improvement, inflammation reduction, etc.) with an eating window (and nutrient intake) that’s conducive for healthy metabolism and building/maintaining lean muscle mass.
TIP 2: Get some professional and/or personal support. This may be a health coach or nutritionist on the professional side, or a spouse, close friend, workout partner or even members of a FB group on the personal side.
TIP 3: Settle on the details of the schedule. Let’s say you’re going to try 16/8. Is your eating window 9a-5p or 12p-8p? Also, decide how many meals (and snacks) you’re going to have during your eating window (one of the reasons I don’t love OMAD is that people tend to absolutely gorge themselves during that small eating window, which isn’t great for your digestive system).
TIP 4: Prepare for fasting. A lot of this centers around picking your zero calorie beverages such as flat water, sparkling water, Vitamin Water Zero, and coffee/tea. I also strongly suggest a good electrolyte supplement to add to your beverage (my personal favorite is LMNT electrolytes).
TIP 5: If doing a 24 hour fast (or longer), try to stay physically active during your fast (of course rest if you need to rest). If exercising, consider scaling down the intensity. Zone 2 training and activities like hiking, SUP, light jogging, some forms of yoga, swimming and biking can be a great fit for times of prolonged fasting.
TIP 6: Recognize and embrace the spiritual and psychological opportunities that can come with fasting, along with the health benefits. Fasting can be a time of dynamic spiritual growth where addictions and negative behavior patterns get broken and spiritual and psychological health and wellbeing improve significantly.
TIP 7: Pair smart fasting with a smart nutritional plan that includes which healthy foods to eat (and which not-so-healthy foods to avoid), along with daily macronutrient targets. Apps like MyFitnessPal can be tremendously helpful here.
5 www.sciencedirect.com/science/article/abs/pii/S0303720715300800 (2015 review of 40 studies on IF)
6 www.ncbi.nlm.nih.gov/pmc/articles/PMC7021351/ (2020; review of 27 IF studies; 4 studies using 16/8 approach had results as follows: 10 ppl, older + obese, 4 wks, 2.2% bodyweight lost [ex. 200lbs person = 4.4lbs in 4 weeks or 1.1lbs/wk]; 10 ppl, obese + T2D, 2 wks, 1.4% bw lost [ex. 200lbs person = 2.8 lbs in 2 weeks or 1.4lbs/wk]; 46 ppl, obese, 12 wks, 3.2% bw lost [0.5 lbs/wk]; 54 ppl obese + T2D, 12 wks, 3.9% bw lost [0.7 lbs/wk]; 1 study using 1 day fast + high protein diet had this result: 40 ppl, obese, 12 wks, 1 day fast <500 cal), ~1.7lbs/wk; 1 study using alternate day fasting had this result: 26 ppl, obese, 8 wks, lost 8.7% bw [17.4lbs in 8 weeks or 2.2 lbs/wk])
7 www.ncbi.nlm.nih.gov/pmc/articles/PMC3106288/ (2010 study on fasting)
8 www.ncbi.nlm.nih.gov/pmc/articles/PMC3946160/ (2014 study on fasting)
9 www.ncbi.nlm.nih.gov/pmc/articles/PMC5411330/ (2017 study on fasting)
10 www.ncbi.nlm.nih.gov/pmc/articles/PMC8744103/ (2022; “IF has been shown to facilitate weight-loss and improve glycemic control among other metabolic processes…[it’s thought that IF primarily works by] increased ketosis [and] reduction in reactive oxidative species production. IF has been proposed to reduce systemic inflammation and may be of benefit in patients with certain chronic conditions associated with inflammation, such as metabolic syndrome.)
11 www.ncbi.nlm.nih.gov/pmc/articles/PMC6314618/ (2019 study of more than 1400 participants who engaged in prolonged fasts of 4-21 days)