The Diary of a CEO with Steven Bartlett: Most Replayed Moment: Calories In, Calories Out Is A Myth! Why Most Diets Fail – Dr. Jason Fung
Release Date: October 31, 2025

Episode Overview

In this most replayed segment of The Diary of a CEO, Steven Bartlett sits down with Dr. Jason Fung, renowned nephrologist, author, and pioneer in intermittent fasting, to dismantle the widely accepted belief in "calories in, calories out" as the foundational principle of weight loss. Dr. Fung explains the hormonal intricacies behind fat storage and burning, the pitfalls of conventional dieting, why most diets fail, and the real science behind effective, sustainable weight management. The conversation is frank, practical, and myth-busting—packed with actionable insights and a new way to look at food, fasting, and metabolism.

Key Discussion Points & Insights

The Myth of “Calories In, Calories Out”

• [00:34] Dr. Fung: Scientific consensus from decades of studies shows that simply eating fewer calories does not guarantee weight loss because the body adapts by lowering its basal metabolic rate (BMR).
  • "Every single study that we've done over the last 50 years shows that if you eat 500 fewer calories, eventually your body will just burn 500 fewer calories." — Dr. Fung [00:41]
• The energy balance equation (body fat = calories in - calories out) is not static; calories consumed and calories burned interact dynamically, primarily through hormonal signaling.
• Dieting by calorie restriction often leads to a reduction in metabolic rate, resulting in feelings of fatigue, cold, and hunger, which frequently precede yo-yo weight regain.

The Role of Insulin & Hormones

• Not all calories are equal: The hormonal response to different foods is crucial. High-carb/high-insulin foods prevent the body from accessing stored fat, even in calorie deficit situations.
  • "Insulin prevents you from burning body fat. It's like the bank is closed and you can't take [calories] out." — Dr. Fung [02:33]
• When calorie intake is reduced but insulin is kept high (e.g., many small carb-heavy meals), the body adapts by reducing BMR instead of burning fat.
• The choice of food and meal timing can either open or lock the doors to fat storage.

Why Most Diets Fail: The Yo-Yo Effect

• Extreme restriction lowers metabolism; returning to even modestly higher calorie intake (but with the same “wrong” foods) leads to quick weight regain, sometimes even surpassing original weight.
  • "This form of dieting is actually, over the long term, probably going to make you gain weight." — Steven Bartlett [05:19]
  • "Oh, it's very detrimental…we all know yo-yo dieting is very detrimental." — Dr. Fung [05:24]

Intermittent Fasting: The Missing Tool

• Fasting lowers insulin, allowing access to stored fat. With low insulin, calorie deficits are met by body fat stores, not by metabolic slowdown.
  • "When you actually fast, your metabolic rate went up...not down." — Dr. Fung [16:54]
• Intermittent fasting mimics ancestral eating patterns and is historically normal. It effectively counters the metabolic damage of frequent eating.

Exercise: Benefits Beyond Weight Loss

• Exercise is essential for physical and mental health, but its contribution to weight loss is limited. The number of calories burned during typical sessions is small, and exercise often increases appetite after workouts.
  • "In terms of weight loss, it's actually a very, very small effect." — Dr. Fung [08:13]
  • "The only way [mild exercise is calorie-neutral] is that exercise causes you to eat more." — Dr. Fung [10:50]

The Culture of Constant Eating

• From 1977 to 2003, eating frequency in the US shifted from three meals per day to five or six—largely due to a push for frequent, carb-based snacking and fear of dietary fat.
  • "As we started to eat a lot of carbs…you get hungry at 10:30, you go around looking for a low-fat muffin..." — Dr. Fung [12:27]
• Frequent eating keeps insulin perpetually elevated, locking away body fat and inhibiting fat burning.
  • "If you want to lose body fat, you actually need to extend the period of time that you're not eating." — Dr. Fung [14:38]

The Breakfast Question

• The idea that one must eat as soon as they wake up is a myth.
  • "The whole idea that you need to eat as soon as you get up is just false...Break fast...tells you that your body should have a fasting period every day." — Dr. Fung [20:16]

Notable Quotes & Memorable Moments

On Calorie Restriction & Metabolism

• "You eat 500 less, your body burns 500 less, and you're not losing any body weight." — Dr. Jason Fung [00:55]
• "Over time, you get tired because you're burning fewer calories. You don't have enough energy to generate body heat. So you're cold, you're tired, you're hungry." — Dr. Jason Fung [03:35]

On Hormonal Regulation

• "Food contains calories, energy, but it contains instructions as to what to do." — Dr. Jason Fung [05:28]
• "Insulin is a hormone that goes up when you eat...When insulin is low, it's going to allow fat burning." — Dr. Jason Fung [06:26, 14:56]

On Fasting and Health

• "I actually tell people to fast all the time...So why is it, for weight loss, you shouldn't fast? That doesn't make any sense." — Dr. Jason Fung [16:50]
• "If you want to lose weight, dropping that basal metabolic rate is death." — Dr. Jason Fung [17:36]
• "When you don't eat, your body is activating itself...those hormones are going to start telling your body to start pulling calories out." — Dr. Jason Fung [18:18]

Key Timestamps

• [00:34] Why “calories in, calories out” model fails
• [02:33] Insulin’s role as the metabolic gatekeeper
• [05:17] The yo-yo effect: why calorie restriction backfires
• [06:26] Intermittent fasting as a hormonal lever
• [08:13] Why exercise does little for weight loss
• [12:15] Cultural shift: from three meals to constant snacking
• [15:06] The rise and rationale of intermittent fasting
• [16:54] Fasting increases, not decreases, metabolic rate
• [20:16] Breakfast is a cultural, not biological, mandate

Tone & Takeaways

The conversation is direct, challenging, and grounded in both clinical observation and physiological evidence. Dr. Fung emphasizes:

• The failure of one-size-fits-all calorie math.
• The paramount importance of hormonal cues—especially insulin—in determining whether the body stores or burns fat.
• The efficacy and historical normalcy of intermittent fasting as a health and weight loss tool.
• Listeners are urged to reconsider their relationship with food, the timing and frequency of meals, and the real impacts of exercise.

Final insight: Rethink not just what or how much you eat, but also when and how you eat—because the instructions you give your body matter more than the simple calorie count.

For more, listen to the full episode linked in the show notes.