#353 – AMA #72: Fasting: benefits for body composition and disease prevention, potential risks, and Peter’s updated practical framework

We discuss:

• Why the topic of fasting is being revisited [1:45];
• The importance of being open to changing one’s opinion in science and medicine as new evidence emerges [4:15];
• Defining fasting and how it fits in the framework of dietary restriction, time-restricted feeding, and calorie restriction [7:30];
• The most common types of fasting [11:45];
• Why someone might choose fasting over continuous caloric restriction [19:00];
• The biochemical effects of fasting: fuel partitioning, autophagy, and more [21:00];
• Autophagy: a cellular cleanup process stimulated by fasting and energy depletion [26:00];
• Comparing the physiological effects of a ketogenic diet to those of long-term fasting [27:45];
• How long fasting takes to induce ketosis and boost autophagy, and how autophagy is measured [29:45];
• At-home tests for ketones [33:45];
• Reducing senescent (“zombie”) cells and inflammation through autophagy [35:00];
• Examining the evidence for the long-term health benefits of fasting beyond short-term metabolic effects [37:15];
• The common health claims associated with fasting across metabolic, neurological, oncological, autoimmune, and longevity domains [40:30];
• The impact of fasting on body weight and body composition [42:00];
• Weight regain and potential metabolic consequences of fasting [44:00];
• The risk of lean mass loss with fasting and how to mitigate it [47:15];
• A practical framework for using fasting as a strategy to improve body composition [52:15];
• How extended fasting compares to caloric restriction in improving insulin sensitivity in people with insulin resistance or type 2 diabetes [54:00];
• Is there any evidence that fasting can extend lifespan? [55:45];
• Examining whether fasting improves cardiovascular disease risk beyond the effects of weight loss and calorie reduction [58:15];
• Exploring whether prolonged fasting can reduce cancer risk [59:45];
• The potential benefits of fasting or fasting-mimicking diets during cancer treatment: emerging evidence and practical considerations [1:02:15];
• Fasting for protection against neurodegenerative diseases: early evidence of therapeutic benefit and open questions [1:10:45];
• Various fasting methods for autoimmune diseases: potential symptom relief with cautious application [1:14:00];
• Fasting and type 1 diabetes: risks, evidence, and careful dietary alternatives [1:16:45];
• Fasting and gut disorders: potential benefits outside of flare-ups [1:18:45];
• The primary risks and downsides associated with fasting [1:19:45];
• The essential vitamins and minerals to supplement during prolonged fasting [1:21:30];
• Who should avoid fasting and why [1:22:15];
• Fasting’s role in health: where it shows promise and where it likely offers no distinct advantage over caloric restriction [1:23:45]; and
• More.

Show Notes

Why the topic of fasting is being revisited [1:45]

The topic of Fasting:

• First time fasting was covered in AMA #11, released January 2020.
• This episode is AMA #72, highlighting how long it’s been since they fully addressed the topic.

Rationale for Revisit

• Fasting is a highly requested and frequently misunderstood topic.
• Though touched on in various episodes since AMA #11, this is the first full dedicated revisit.
• Peter’s thinking has evolved since then, including patient application and personal practice.

Episode Structure Preview

• Clarifying Terminology
  • Acknowledgment that confusion arises from inconsistent definitions (e.g., “fasting” means different things to different people).
  • First goal of the episode: define terms and types of fasting protocols.
• Main Content Themes
  • Reasons people choose to fast and the purported or actual benefits.
• Topic areas include:
  • Body composition
  • Metabolic health
  • Lifespan / longevity
  • Cardiovascular disease
  • Cancer (prevention and treatment)
  • Neurodegenerative diseases (e.g., Alzheimer’s, Parkinson’s)
  • Autoimmune conditions (e.g., arthritis)
• Risks and Downsides
  • Will explore potential negative effects of fasting and who should be cautious.
• Practical Application
  • The episode will close with a framework to:
    • Help listeners decide how (or if) to use fasting.
    • Show how Peter uses fasting personally and with his patients.

The importance of being open to changing one’s opinion in science and medicine as new evidence emerges [4:15]

Why Changing Opinions Matters

• Peter is known for changing his mind when new evidence emerges.
• References the “Strong Convictions, Loosely Held” theme used in milestone episodes like #200 and #300.
• This episode focuses on fasting, a topic where Peter’s views have evolved over time based on new data and clinical experience.
• Many public figures—and people in general—become overly attached to their initial beliefs and resist change.
• When someone changes their mind publicly, people often criticize them by saying, “How could you say this if you used to say that?”

Peter’s Response: A Framework for Intellectual Flexibility

• Everyone—not just scientists or doctors—should be open to changing opinions in light of new information.
• In today’s world, especially in politics, changing your mind is often seen as a weakness. The more firmly someone sticks to their opinion, the more credible they appear.
• Changing your mind should be seen as a strength, not a flaw.
• Science is inherently probabilistic, not absolute. Even facts that feel like certainties (e.g., gravity) are grounded in probabilities.
• Medicine and science evolve constantly. No belief should be held with rigid certainty.

Examples of Changing Positions

• Peter received backlash for changing his position on GLP-1 agonists (e.g., Ozempic, Wegovy).
• He quotes economist John Maynard Keynes: “When the facts change, I change my mind. What do you do, sir?”

Guiding Principle

• Peter is not wed to being right—he is wed to knowing what is right.
• He recommends that others adopt the same orientation: focus on truth and growth, not ego or consistency.
• Constant evolution of understanding is the best way to stay aligned with reality.

Defining fasting and how it fits in the framework of dietary restriction, time-restricted feeding, and calorie restriction [7:30]

Establishing a Foundational Nutrition Framework

• Energy balance is essential: consuming too much or too little is harmful.
• Adequate protein intake is a constant requirement for maintaining lean mass.
• The ratio of carbohydrates to fats is less important as long as total energy intake is appropriate.
• Sufficient micronutrient intake and avoiding toxins are also fundamental.
• For over 80% of people (based on Peter’s clinical experience), maintaining energy balance requires effort and strategy.

Peter’s DR/TR/CR Framework

• DR (Dietary Restriction): Deliberately restricts a specific dietary element.
  • Can involve limiting carbs, fats, animal products, or processed foods.
  • No single best method—many forms can work if the restriction is significant enough.
• TR (Time restriction): Limits the eating window each day without necessarily changing food quality or quantity.
  • Also referred to as intermittent fasting.
  • Narrowing the eating window often naturally reduces caloric intake.
• CR (Caloric Restriction): Focuses on reducing total calories consumed, regardless of the type or timing of food.
  • May not involve food type or timing changes—just smaller portions.
• These strategies can be combined.
  • Example: During peak cycling performance, Peter used both DR and TR, eating one snack and one meal daily, with significant dietary constraints, resulting in being 16–17 pounds lighter than his current weight.

Clarifying the Term “Fasting”

• “Fasting” lacks a standardized definition and is used broadly.
• It can describe:
  • Complete water-only fasting for multiple days.
  • Daily time-restricted feeding (e.g., 16:8 or 18:6 fasting windows).
• The discussion in the episode will further explore various fasting protocols, their definitions, and their pros and cons.

The most common types of fasting [11:45]

In this figure, you can see what an ADF protocol might look like—the pink squares on this over the course of a seven-day week show you days of either complete or significant caloric restriction:

{end of show notes preview}