Extended fasting means going longer than a typical overnight or time-restricted fast. It is an optional, more advanced strategy—not a required starting point.
In this short lesson, Dr. Tro Kalayjian introduces extended fasting as Lever 10 in Toward Health’s Ten Levers framework. He explains why people should first establish a satiating way of eating, become comfortable with time-restricted eating, and involve their medical team before attempting longer fasts.
What this lesson covers
- Why extended fasting comes after nutrition and meal-timing fundamentals
- How people may progress gradually from time-restricted eating toward longer fasts
- Why medications may need adjustment during fasting
- How common fasting schedules differ from one another
Before attempting a longer fast
Discuss extended fasting with your physician or medical team before beginning, particularly if you take medications that affect blood glucose or blood pressure. Medication needs can change when food intake changes substantially.
The appropriate approach depends on the individual. Longer fasting should not be treated as a test of willpower, and it is not necessary for everyone pursuing metabolic health or weight-loss goals.
Important terminology clarifications
The video discusses several related strategies. They are not interchangeable:
- 5:2 intermittent energy restriction: Five usual-eating days and two energy-restricted days each week.
- 4:3 intermittent fasting: Four usual-eating days and three nonconsecutive, substantially energy-restricted days each week in the studied protocol.
- Alternate-day fasting: Alternating a fasting or markedly energy-restricted day with a feeding day.
- Five-day modified or fasting-mimicking protocol: A defined low-energy program followed for five consecutive days, generally as a periodic cycle. It is different from 5:2 fasting.
- Protein-sparing modified fast (PSMF): A structured, very-low-energy, low-carbohydrate, low-fat intervention that uses prescribed protein to help preserve lean mass. It generally requires planned micronutrient and electrolyte support plus medical monitoring.
- Autophagy: A cellular process that degrades and recycles intracellular components, including damaged proteins and organelles. It does not mean “making new cells.”
Clinical trials have evaluated these as distinct interventions. Their results should not be generalized automatically from one fasting schedule to another.
Video transcript
The last lever I want you to consider, and I mentioned this before, is the power of extended fasting. Once you’re comfortable with time-restricted eating, I want you to consider extended fasting. Extended fasting is an age-old practice used for religious purposes for thousands of years. It is very safe, but it’s important to know that if you’re on medications, this is something that you want to discuss with your doctor before you do.
Many of our patients and clients in our programs employ 24- or 48-hour fasts, and some do longer fasts throughout the year. Absolutely, there’s been evidence published in major medical journals showing that extended fasts can improve metabolic health and improve cellular turnover. They get rid of the old cells that are ready to retire.
And promote autophagy, which is making new cells. You don’t want to do this without making sure your baseline diet is a satiating diet. You may want to experiment with time-restricted eating first. You may want to try doing one meal a day. And, as I said before, make sure before you go to an extended fast that you talk to your medical team to make sure it’s appropriate for you.
Several medications need to be adjusted when you’re going to fast. I’ll talk briefly about protein-sparing modified fasts. Some people like this approach where they eat in a very-low-carbohydrate and low-fat approach and eat protein without any restriction. That’s called a protein-sparing modified fast, and that’s also been shown, maybe one or two days a week, to help improve with weight loss if weight is an issue.
Back to extended fasts, this is something that’s been published in the New England Journal of Medicine, in JAMA, and in the British Medical Journal. They’ve shown that five-day fasts, which is fasting for five days where you have less than 400 to 800 calories per day—maybe in the form of bone broth, maybe in the form of electrolytes and a salad, and that’s it for the day for five days straight—and then two days a week you go back to a normal approach. This approach of five and two has been shown to have many improved cardiovascular, weight, and metabolic-health benefits.
Alternate-day fasting, which is fasting for about 24 to 48 hours two or three times a week, has also been shown to help with weight loss and cardiovascular approaches. But with all of these, you don’t want to just rush into them. If you’re eating 16 hours a day right now, you may want to start to limit it to eight hours for a month, and then the next month lower that to four hours.
Then experiment with maybe one or two 48-hour—24-hour fasts, excuse me—one-meal-a-day days back-to-back to see how your body feels before you try a longer 24- or 48-hour fast. There is plenty of literature showing that there are benefits to periodic long fasts, which is the five and two, or periodic four and three fasts, which is eating for four days a week and fasting for three. But before you pick up fasting, I suggest you start with time-restricted eating.
And honestly, talk with your coach, talk with your nutritionist, and talk with your doctor about whether fasting is appropriate for you and how to do it safely.
References
- Harvie MN, Pegington M, Mattson MP, et al. The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women. International Journal of Obesity. 2011;35(5):714–727. https://doi.org/10.1038/ijo.2010.171
- Trepanowski JF, Kroeger CM, Barnosky A, et al. Effect of Alternate-Day Fasting on Weight Loss, Weight Maintenance, and Cardioprotection Among Metabolically Healthy Obese Adults: A Randomized Clinical Trial. JAMA Internal Medicine. 2017;177(7):930–938. https://doi.org/10.1001/jamainternmed.2017.0936
- Catenacci VA, et al. The Effect of 4:3 Intermittent Fasting on Weight Loss at 12 Months: A Randomized Clinical Trial. Annals of Internal Medicine. 2025;178(5):634–644. https://doi.org/10.7326/ANNALS-24-01631
- Wei M, Brandhorst S, Shelehchi M, et al. Fasting-mimicking diet and markers/risk factors for aging, diabetes, cancer, and cardiovascular disease. Science Translational Medicine. 2017;9(377):eaai8700. https://doi.org/10.1126/scitranslmed.aai8700
