Fasting (or intermittent fasting) has become a widely discussed health practice. Although fasting clearly offers several health benefits,there are still a few experts who highlight potential drawbacks. Some will argue that fasting deprives the body of essential nutrients, while others will note its ability to activate powerful metabolic and cellular processes (e.g. autophagy). Here’s a brief overview of the expert opinions on fasting (Longer version: click here).
Fasting Methods
- Intermittent Fasting (IF):
- 16:8 Method: Fast for 16 hours, eat during an 8 hour window. This is one of the most popular fasting methods, recommended by Dr. Andrew Huberman[1] and Dr. Casey Means[2]. You can extend this to an 18:6 window for more significant fat-burning and autophagy benefits. People typically skip breakfast, eating from midday to evening.
- 20:4 Method (Warrior Diet): Fast for 20 hours, and eat all your daily calories within a 4 hour window, often in one large meal. This method mimics the eating patterns of ancient warriors, focusing on eating one substantial, nutrient-dense meal late in the day. Dr. David Sinclair[3] practices this method.
- Time-Restricted Feeding (TRF):
- TRF is similar to intermittent fasting but aligned with your circadian rhythms. The key is to eat early in the day; ideally finishing meals by sunset. This approach helps optimize metabolic health. Dr. Casey Means[2] and Dr. Andrew Huberman[3] both argue that this helps improve insulin sensitivity and reduce fat storage.
- Alternate-Day Fasting (ADF):
- Fasting every other day or severely restricting calories on fasting days (500cal). This method is used for weight loss and metabolic health. Dr. Jason Fung[4] recommends this approach for its simplicity and its ability to jump-start fat loss.
- 24-Hour Fast:
- The one-day fast is a true, full 24 hour fast. It begins from one meal to the same meal the next day (e.g. dinner-to-dinner). This method can be done once or twice a week to improve metabolic flexibility and reduce insulin resistance. Tim Ferriss[5], Dr. Jason Fund[4], and Dr. Pradip Jamnadas[6] are proponents of this method.
- 5:2 Diet:
- This approach allows you to eat normally for five days, but restrict your eating during the remaining two (non-consecutive) days. During those two days, calories are limited to 500-600 calories. This approach, popularized by Dr. Michael Mosley[7], and supported by Dr. David Sinclair[3], is easy to follow long-term. It can also provide many benefits of traditional intermittent fasting without fully abstaining from food.
- 72-Hour Fast:
- Fasting for 72 hours straight will trigger deeper autophagy and stem cell regeneration. Dr. Jason Fung[4] and Dr. Pradip Jamnadas[6] recommend this method periodically; particularly for people looking to reset their metabolism and gain regenerative health benefits.
- Prolonged Fasting (5+ days):
- A fast that lasts five-to-seven days or more will provide even deeper benefits from cellular repair and autophagy. Tim Ferriss[5] experiments with this approach, but has a very specific method that permits him to fast safely. (see Tim’s method)
- Fasting-Mimicking Diet (FMD):
- The Fasting Mimicking Diet was created by Dr. Valter Longo[8]. This five-day, low-calorie diet mimics the effects of fasting without requiring complete abstention from food. The diet allows for 1,100 calories on the first day, and 800 calories for the next four days. This diet provides similar health benefits to other true fasting methods without the as much hardship.
Tips for Success:
(as recommended by Dr. Jason Fung[4])
- Hydration: Drink plenty of water throughout your fast.
- Distraction: Stay busy to keep your mind off food
- Appetite Suppressants: Coffee and tea can help curb hunger.
- Embrace Hunger Waves:Hunger comes in waves – let them pass.
- Privacy: Don’t broadcast your fast – others may not understand the method and discourage you.
- Patience: Give yourself time. Setbacks are a part of adjusting to fasting.
Expert Opinions:
- Dr. Peter Attia[9]: According to Attia, he believes fasting leads to noticeable muscle loss in older adults, and the 16/8 method may not trigger sufficient autophagy to outweigh the downsides of fasting. He prefers hypocaloric fasting, which restricts calories for one week each month to target longevity and health improvements. However, according to his book, Outlive, this is usually preserved for individuals who are already facing obesity or other weight related illnesses.
- Dr. Casey Means[2]: Dr. Means emphasizes time-restricted feeding, with a focus on eating within a 10 hour window. She also recommends front-loading your eating habits to earlier in the day, allowing fasting to occur overnight. This, she believes, helps improve insulin sensitivity and promotes fat burning.
- Dr. David Sinclair[3]: Sinclair, a longevity expert, advocates for intermittent fasting to reduce disease risk and promote lifespan extension. He also stresses that moderate caloric restriction can significantly impact longevity. Sinclair himself fasts the entire day, with only one large meal at dinner.
- Dr. Pradip Jamnadas[6]: Dr. Jamnadas highlights fasting’s ability to clear mental fog and reduce insulin levels. His research shows processes like gluconeogenesis and ketosis begin after 12-18 hours, with autophagy peaking after 72 hours.
Key Processes:
- Autophagy: A cellular recycling process where old, damaged cells are broken down and replaced. It typically begins after 24 hours of fasting and peaks around 48-72 hours.
- Ketosis: Starting after 18 hours of fasting, ketosis occurs when the body begins burning fat for energy, improving metabolic health and fat loss.
- mTOR Inhibition: Fasting inhibits mTOR, a pathway related to growth and aging. Long fasts promote FOXO gene activation, aiding cellular repair and potentially extending lifespan.
Footnotes:
[1]: Dr. Andrew Huberman: Huberman is a Stanford neuroscientist and host of the Huberman Lab podcast, is known for his research on neuroplasticity and health optimization.
[2]: Dr. Casey Means: Means is a Stanford-trained physician (ENT) and co-founder of Levels Health, a company dedicated to metabolic optimization. She’s also a NYT bestselling author with her book, Good Energy. Her book explores how metabolic health impacts overall well-being.
[3]: Dr. David Sinclair: Sinclair is a globally renowned expert in aging and genetics. He is also a professor at Harvard Medical School, where he co-directs the Paul F. Glenn Center for the Biology of Aging. His book, Lifespan: Why We Age – and Why We Don’t Have To, has been praised for its complex scientific findings.
[4]: Dr. Jason Fung: Fung is a nephrologist specializing in fasting and metabolic health. He is internationally known for his work in treating type 2 diabetes and obesity through dietary interventions. Fung was trained at the University of Toronto and performed his residency at UCLA. Similar to others mentioned, Fung is the author of two well-known books, The Obesity Code and The Diabetes Code. Both books provide guidance on managing metabolic diseases through nutrition.
[5]: Tim Ferriss: Tim is the only non-doctor cited, but yet, still provides remarkable insights. He’s an entrepreneur and best-selling author known for his groundbreaking books on lifestyle design and productivity, including the 4-Hour Workweek and The 4-Hour Body. A graduate of Princeton, Tim is known as the “human guinea pig” due to his self-experimentations and “biohacking” methods.
[6]: Dr. Pradip Jamnadas: Dr. Jamnadas is a cardiologist and founder of Cardiovascular Interventions in Orlando, Florida. His extensive work is well-known in preventative cardiology and his deep dive into fastings impact on metabolic health has made him an influential speaker and educator.
[7]: Dr. Michael Mosley: Mosley is a British medical doctor that trained at the prestigious Royal Free Hospital Medical School in London. He became well-known globally after publishing his book, The Fast Diet, which introduced the world to the concept of intermittent fasting.
[8]: Dr. Valter Longo: Longo is the Director of the USC Longevity Institute and professor of gerontology and biological sciences at the University of Southern California. He received his PhD in biochemistry at UCLA and is considered one of the foremost experts in aging and nutrition. His Fast-Mimicking Diet has been hailed for its revolutionary approach to achieving the benefits of fasting without nutritional deprivation. His book, The Longevity Diet, combines decades of research on extending lifespan through diet.
[9]: Dr. Peter Attia: Dr. Attia is a Stanford-trained physician with a focus on longevity and metabolic health. Attia completed his surgical residency at Johns Hopkins and later shifted his focus to preventative medicine and the science of extending healthspan. His work is focused on applying scientific rigor to maximize longevity, with his best-selling book, Outlive: The Science and Art of Longevity, emphasizing practical approaches to living healthier longer.