Ben Greenfield’s Top Two Recommended Ways To Fast For Longevity

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This is a guest post from WellnessFX practitioner, Ben Greenfield. Greenfield is an ex-bodybuilder, Ironman triathlete, Spartan racer, coach, speaker and author of the New York Times Bestseller “Beyond Training: Mastering Endurance, Health and Life.” Ben blogs and podcasts at, and resides in Spokane, WA with his wife and twin boys. To learn more about Ben’s practice, education, and specialties, skip to the bottom! 

While they are unlikely to be familiar with popular fasting acronyms such as CR, IF, DTRF, ADF or FMD, centenarians in Blue Zones like Nicoya, Sardinia and Okinawa tend to eat relatively small portions of whole foods, consuming a low to moderate calorie diet by being mindful of their hunger and avoiding calorie-dense, fat- and sugar-laden processed and packaged foods.

Okinawans practice the traditional cultural rule of hara hachi bu, which means eating only until they are about 80% full. Typically, most meals are consumed within an 8- to 12-hour feeding window throughout the day, referred to by researchers as a “compressed feeding window.” This is the same window observed in many of the new, Westernized forms of fasting- or caloric-restriction-based diets, most of which involve at least 12 to 16 hours of not consuming calories during a given 24-hour period.

So why do all these different forms of fasting seem to work so well? You’re about to discover why, along with a quick primer on fasting and the two methods of fasting that I’ve personally found to be most effective for me and my clients.

Fasting: What You Need To Know

Intermittent fasting (IF) might be the most popular way to get the benefits of caloric restriction without necessarily starving yourself. It involves alternating cycles of eating and fasting.

One type of IF, called alternate-day fasting (ADF), involves alternating days of feasting and fasting, usually by eating regularly one day and then fasting for 24 hours until dinner on the following day, often for two to three cycles per week.

Daily time-restricted feeding (DTRF) is another type of IF, and in this scenario, all your food is consumed within a 3- to 12-hour period every day. A 16-8 fasting diet involves consuming all food within an 8-hour window and fasting for the remaining 16 hours of the day.

Then there’s the “fasting-mimicking diet” (FMD), made popular of late by the researcher Valter Longo, author of the book “The Longevity Diet.” In this scenario, rather than abstaining from food completely as you would during a traditional fast, you consume small amounts of food throughout the day (about 40% of your normal intake), and, surprisingly this seems to produce many of the same therapeutic benefits of traditional fasting. This would be an example of caloric restriction that does work.

A typical FMD lasts about three to five days and is repeated two to four times throughout the year.

The most up-to-date research on fasting suggests that the benefits all come down to your mitochondria, your tiny cellular power plants. Inside your cells, mitochondrial networks generally alternate between two states called “fused” and “fragmented.”

Caloric-restricted diets and fasting promote homeostasis and induce a healthy fluctuation between these fused and fragmented states, allowing the mitochondria to live longer. These diets also increase fatty acid oxidation, allowing for less free radical production and less damage occurring to your cells and the mitochondria contained within them.

Finally, it’s important to realize that many of the benefits of fasting come from a long period of time between meals, and not extreme calorie restriction (AKA being hungry, cold and libidoless your entire life).

Two of the most common pieces of advice for losing weight and getting in shape are 1) move more and 2) eat less (restrict calories). The idea behind restricting calories and moving more is to expend more energy than you consume. But the fact is that restricting calories can result in low energy levels, which would technically make it harder to move more, and lower metabolic rate via thyroid dysregulation when implemented long term. So – especially for highly active individuals, such as endurance athletes and strength athletes – reducing calories simply isn’t the best option. There are more efficient ways to burn fat and even live longer that don’t involve reducing your overall caloric intake – particularly the fasting methods you’re about to learn.

For example, I personally implement fasting in two ways: 1) a daily, intermittent overnight fast of 12 to 16 hours, leaving me with an 8- to 12-hour daily compressed feeding window, and 2) a weekly or bi-monthly 24-hour fast from dinnertime to dinnertime. To maintain the adequate availability of recovery nutrients during these fasts, I’ll “cheat” with occasional low-calorie recovery- or energy-enhancing nutrients such as essential amino acids, exogenous ketones, minerals, a multi-vitamin and bone broth. As an active athlete, this approach, compared to quarterly 3- to 5-day water fasts or an FMD approach, is a far more sustainable scenario for frequent training and high levels of physical activity.

Populations that need to take a careful approach to fasting are extremely lean individuals with low stores of essential body fat, people prone to eating disorders, and women with “adrenal fatigue” or hormonal imbalances.

For these populations, the risks and stresses of fasting may outweigh many of the benefits. My podcast with Dr. Jason Fung, which derives its name and ideas from his bookThe Complete Guide To Fasting: How To Heal Your Body Through Intermittent, Alternate-Day, and Extended Fasting,” is an excellent resource that takes a deep dive into these concepts.

The 2 Best Ways To Fast For Longevity

While IF acts as an umbrella term that basically means cycling between a period of fasting and a period of non-fasting for a certain amount of time, IF is typically taken to mean either a 12-hour compressed feeding window followed by 12 hours of fasting or an 8-hour compressed feeding window followed by 16 hours of fasting. As you just learned, I personally incorporate a weekly intermittent fasting practice, which is a form of the feast-famine cycling you can read about below.

There is a dizzying variety of fasting protocols:

  • The 16-8 method where you fast for 16 hours each day
  • The Eat-Stop-Eat protocol where you do a 24-hour fast once or twice a week
  • The Warrior Diet where you fast during the day and eat a huge meal at night.

But for the purposes of overall wellness and longevity, in addition to the intermittent fasting strategy I discussed earlier, I am a big fan of the following two fasting approaches.

Approach 1: Feast-famine cycling

What it is: My friend Dr. Dan Pompa, author of “The Cellular Healing Diet,” calls the ancient healing strategy of “diet variation” a “feast-famine” cycle. It is based upon the lifestyle and dietary habits of our ancestors. Many ancient cultures and modern Blue Zones follow patterns of fasting and feasting according to the availability of foods.

Humans have not always had grocery stores and Uber Eats-esque services that provide food of all kinds 24-7. Instead, we have traditionally been forced to follow the seasons of food availability. For example, in the winter, many Native American tribes would subsist on fats, especially the organ fat from the kidneys, liver and gut, and they would give their dogs the muscle meat because what they cherished most was the caloric density and nutrients from the organs of the animals. In the summer, these same tribes would eat more roots and berries, prickly pears, beans, pods, mustard seeds, cholla blossoms, acorn squashes, pumpkins and a variety of gourds – a much higher carbohydrate intake compared to the winter consumption of saturated fats.

In his book “Nutrition and Physical Degeneration,” Weston A. Price describes this practice as follows:

“…They cycled between the summer cultivation of starches and fruits and far greater reliance on fats from animals (particularly organ meats and marrow) during winter… the successful nutrition for nine months of the year was largely limited to wild game, chiefly moose and caribou. During the summer months the Indians were able to use growing plants. During the winter some use was made of bark and buds of trees. I found the Indians putting great emphasis upon the eating of the organs of the animals, including the wall of parts of the digestive tract… These Indians obtain their fat-soluble vitamins and also most of their minerals from the organs of the animals. An important part of the nutrition of the children consisted in various preparations of bone marrow, both as a substitute for milk and as a special dietary ration.”

How it works: Science has since shown that periods of caloric restriction, fasts and forced, seasonal feast-famine dietary variations promoted health in these indigenous cultures by forcing the body to use fat for fuel instead of glucose as a primary source, and also to give the digestive system a rest and the cells a chance to engage in the natural cell turnover known as autophagy.

How to do it: Dr. Pompa’s “5-1-1” rule is a great place to start. For five days of the week you follow a lower carb, ketogenic-style diet or, especially if you are attempting to lose weight, a calorie-restricted diet, one day of the week you do a 24-hour fast (dinnertime to dinnertime), and the remaining day of the week you do a feast day, eating any of your favorite healthy foods, especially carbohydrates “ad libitum.” The 5-1-1 rule can be modified regarding fasting and feasting days based on body fat, schedule and physical activity levels. For some, a 4-2-1 or 2-2-3 approach also works.

Approach #2: Alternate-day fasting

What it is: Alternate-day fasting (ADF), in its strictest form, requires you to fast for one full 24-hour day, then eat for the following 24 hours. It is more effective than caloric restriction in maintaining long-term weight loss and allows your body to enter a powerful state of autophagy. There are different approaches to using ADF in terms of how many days a week you implement it, but one of the most effective approaches was developed by Valter Longo, and is outlined in detail in his book “The Longevity Diet.” His approach involves a 5-day program that he calls the “fasting-mimicking diet” (FMD), which you’ll learn about in just a moment.

As a word of caution, similarly to IF, ADF may be harmful to children, the elderly and people who already have low body fat percentages.

How it works: Long periods of fasting like this can stimulate adaptive responses such as autophagy, weight loss and a long-term reduction of insulin-like growth factor 1 (IGF-1), glucose and markers of inflammation and aging. Longo’s FMD approach, which is often referred to as “alternate-day modified fasting,” works by producing many of the same physiological effects as a strict 4-day water fast, which consists of abstaining from all food and drink except for water and occasionally black coffee and tea.

Unlike the water fast, the FMD provides your body with optimal nourishment while not activating the pro-aging pathways, including the growth hormone and IGF-1 pathways activated by normal diets. When following a FMD, your body does not recognize that it is being fed, so believes that it’s in an extended period of severe fasting. This can trigger a strengthening of the immune system, the replacement of old T-cells to combat particular invasive microbes and an improvement in insulin sensitivity.

How to do it: It couldn’t be more straightforward. In a traditional ADF, you follow the protocol as described above for one or two days. Longo’s form of the diet, on the other hand, is not devoid of calories but rather low in calories and particularly low in protein. His formula is based on meals of about 360 calories each, derived 9% from protein, 44% from fat and 47% from carbohydrates. There are three meals on the first day amounting to a total of 1,090 calories and two on each of the next four days amounting to a total of 725 calories. Longo owns a company called L-Nutra that pre-measures and pre-packages these meals as Prolon meal kits, which consist of vegan foods from natural sources.

I am not convinced that the minimalist meal kits Longo’s company sells as part of the official ADF program are necessary, and I think they are, in fact, somewhat poor on the nutrient-density scale. But with wild plant intake and small doses of superfoods such as spirulina, chlorella, bone broth, berries, seed or nuts – along with a few dietary biohacks such as essential amino acids and ketone estes – you can easily put together your own minimalist calorie alternative to the packaged version. Finally, a word of warning: extremely long periods of fasting like this can stimulate powerful adaptive responses such as autophagy and weight loss, but it’s usually not recommended to pursue this approach or an FMD approach for more than a few days at a time.


Forms of FMD, ADF and weekly or bi-monthly 24-hour fasts are potent strategies to enjoy the benefits of caloric restriction without necessarily suffering from energy restriction or caloric depletion. But given that you shouldn’t follow many of these diets for more than a few days at a time, it is helpful to have another fasting strategy so you can continue to enhance weight loss and autophagy on a more regular basis. Should you wish to supplement these strategies or forego the rigors of these types of fasting, you can, of course, engage in an overnight intermittent fast every day to fast all year long without metabolic issues. This type of fasting will cause the same effects as the other methods while being far more sustainable.

The primary takeaways are that:

  1. When it comes to caloric restriction, timing your caloric intake is often more important than actually limiting your calories and can still allow you to you tap into the long life enjoyed by centenarian populations.
  2. Feast-famine cycling in the form of a 5-1-1 scenario works very well for many people, especially when combined with an FMD a few times a year, such as Valter Longo’s program.
  3. A daily intermittent fast is the “lowest-hanging fasting fruit” for most people, especially if they’re just getting started with fasting.

I hope this has been helpful! Visit for even more fasting tips and tricks.

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About Ben Greenfield, CPT, Sports Nutritionist

Location: Washington
Practice & Philosophy:
I’m a nutritionist and fitness trainer in Washington state who specializes in exercise science, fueling for endurance events such as triathlon or marathon, and optimizing fat loss and performance through dietary, supplement, exercise and lifestyle strategies.

Specialties include: Weight Management, Gastrointestinal (Including Intestinal Health, Food Allergies, Gluten-Free), Sports, Athletic Fitness, Paleo Diet, Sleep, Energy and Fatigue. Additional specialties include: Endurance sports, Crossfit, Triathlons. 

Education & Credentials
University of Idaho Bachelor’s Exercise Science University of Idaho Master’s Exercise Science/Nutrition NSCA Certified Personal Trainer (C-PT) NSCA Certified Strength & Conditioning Coach (CSCS) ISSN Certified Sports Nutrition (C-ISSN) Superhuman Coach Ce

Additional Info
I work with: Endurance athletes, including triathletes, marathoners, runners, cyclists and swimmers. Weight lifters, bodybuilders, Crossfitters and individuals seeking body improvement. Overweight or obese individuals. I look at all components, including health/longevity. For example, eating a high amount of carbohydrates prior to a workout can help you perform better, but chronic long term use of that strategy can result in blood sugar problems, weight gain, or nutrition deficiencies. So I believe that in considering strategies to achieve your goal, you must consider not only acute performance benefits, but also potential mental, digestive, hormonal and metabolic side-effects.

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