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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

with Jason Fung

Published October 31, 2025
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About This Episode

The episode segment critiques the simplistic 'calories in, calories out' model of weight loss by explaining how the body adapts its basal metabolic rate to caloric restriction. The guest emphasizes the central role of hormones-especially insulin-in determining whether the body stores or burns fat, arguing that food is an instruction to the body, not just a source of calories. They discuss intermittent fasting, snacking patterns, exercise, and the concept of breakfast as tools or factors that influence hormonal balance and long-term weight regulation.

Topics Covered

Fasting Insulin resistance Sugar consumption Metabolic rate Exercise and weight loss Snacking habits Hormones and metabolism Obesity and weight loss Intermittent fasting Basal metabolic rate

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Quick Takeaways

  • Simply cutting 500 calories per day does not reliably produce linear fat loss because the body often compensates by burning fewer calories.
  • High-insulin eating patterns, especially frequent high-carb meals and snacks, can block access to stored body fat and lower metabolic rate during dieting.
  • Yo-yo dieting occurs when caloric restriction slows metabolism; when eating increases slightly afterward, weight often rebounds and surpasses the starting point.
  • Intermittent fasting helps lower insulin, allowing the body to draw on stored fat while maintaining a higher basal metabolic rate.
  • Exercise has many health benefits but contributes relatively little to weight loss compared to dietary and hormonal factors.
  • Frequent snacking, which became common after the late 1970s, keeps insulin elevated and reduces the time available for fat burning.
  • Contrary to common myths, short-term fasting tends to increase, not decrease, basal metabolic rate due to hormonal activation.
  • The word "breakfast" implies that a daily fasting period is normal; constantly eating removes this fasting window and favors fat storage.

Podcast Notes

Challenging the calorie model and discussing metabolism

Host introduces 'calorie deception' and low metabolism question

Host cites the book section titled 'the calorie deception' and five wrong assumptions about obesity and weight loss[1:04]
Host asks whether the common cultural idea of having a 'low metabolism' in obese people has any merit[1:20]

Guest explains the energy balance equation and why simple calorie cutting fails

Guest affirms there is merit to the idea that metabolism can be low[1:30]
Body fat is framed through the energy balance equation: body fat equals calories in minus calories out[1:38]
The common prescription "eat 500 fewer calories and lose a pound a week" is called unquestionably false[1:43]
• Guest says decades of studies show that if you eat 500 fewer calories, over time the body often burns 500 fewer calories
Basal metabolic rate (BMR) is defined as the number of calories the body expends in one day[2:05]
• Guest says almost every study for at least 50-80 years shows that eating fewer calories tends to reduce calories burned
Because eating fewer calories often leads to burning fewer calories, weight loss is limited or prevented[2:36]

How dieting alters metabolism and causes yo-yo weight changes

Metabolic slowdown during dieting and aftermath

Host asks what happens when someone diets, metabolism lowers, and then they come off the diet[2:57]
Guest says generally metabolism stays low, leading to the yo-yo dieting effect[3:02]
Example: starting at 2,000 calories in and 2,000 out, with stable weight[3:07]
Person cuts to 1,500 calories assuming body will still burn 2,000 and the 500-calorie gap will come from body fat[2:20]

Role of food type, eating frequency, and insulin in blocking fat burning

Guest describes a typical dieting pattern with 'wrong foods' and high eating frequency[3:22]
• Person eats 8-10 times a day, eats low fat, consumes lots of carbs, and frequently spikes insulin
Insulin prevents burning of body fat, a fact the guest says has been known for about 80 years[3:35]
With 1,500 calories in but high insulin from high-carb, frequent eating, the body still wants to burn 2,000 calories[3:47]
• Because insulin is high, the body cannot access body fat stores, likened to money in a bank that is closed
To balance the equation under high insulin, the body reduces its expenditure to 1,500 calories[4:15]
• Result: metabolic rate drops by 500 calories and no body fat is lost despite lower intake
Over time the person feels tired, cold, and hungry because they are burning fewer calories and producing less heat[4:47]
When the person increases intake slightly to 1,800 calories, while still only burning 1,500, they gain weight[4:57]
• Guest notes people are confused because they are eating less than their original 2,000 calories but still gaining weight
Nutritionists and doctors often accuse such patients of lying or underestimating intake instead of recognizing metabolic adaptation and hormonal effects[5:12]

Real-world example of calories focus and subsequent rebound

Host's friend who used strict calories tracking

Host describes a friend who strongly believes in calories in-calories out and posts about it online[5:44]
Friend achieved near six-pack abs while apparently eating many pizzas, using a calorie-counting approach[5:44]
After stopping the diet, there was a big yo-yo effect with weight regain[6:17]
Host connects this rebound to the guest's explanation of lowered metabolism and slight calorie increase causing weight gain[6:17]

Long-term harm of calorie-restriction dieting

Guest agrees that when metabolism is lowered and intake goes up slightly, all the lost weight comes back, "and then some"[6:07]
Guest calls this form of dieting very detrimental over the long term and likely to make you gain weight[6:17]

Hormonal perspective on dieting and intermittent fasting

Food as energy and as hormonal instructions

Guest emphasizes that you must think about more than calories; you must consider hormones as instructions to the body[6:36]
Food contains both energy (calories) and instructions about what to do with that energy[6:36]

Contrasting two 1,500-calorie approaches: frequent eating vs intermittent fasting

Example is repeated: 2,000 calories in and out, then dropping to 1,500 to diet[6:45]
In the alternative approach, the person practices intermittent fasting so insulin can fall[6:50]
• Insulin rises when you eat and falls when you don't; when insulin falls, the body takes energy out of storage
With 1,500 calories coming in and low insulin, the body wants to burn 2,000 calories and can take 500 from body fat[7:21]
• Equation balances at 1,500 calories from food plus 500 from fat equals 2,000 burned, allowing fat loss
In the high-insulin scenario, by contrast, 1,500 in forced the body to reduce burning to 1,500 because it cannot access fat[7:21]
Guest says insulin is the hormonal signal that "opens the doors" so body fat can come out[7:49]

Why people can't access large fat stores

Guest points out that people may have 200,000-300,000 calories of body fat yet struggle to burn it[7:21]
The reason given is that they haven't activated the right hormones (primarily by lowering insulin) to access those calories[8:13]

What happens when you stop dieting after intermittent fasting

In the intermittent fasting scenario, 1,500 eaten plus 500 from body fat yields 2,000 burned[8:18]
If the person later returns to 2,000 calories of intake, they neither gain nor lose weight, same as before dieting[8:30]
In the high-insulin restriction scenario, increasing to even 1,800 calories led to weight gain due to reduced metabolic rate[8:30]
Guest stresses that the difference was never the absolute calories (2,000 vs 1,500), but the hormonal context, especially insulin[8:34]
Guest notes that cortisol and other hormones also play important roles in metabolism alongside insulin[8:50]

Exercise, calories burned, and weight loss

Common belief about exercising to earn more calories

Host raises the idea that one can burn 1,000 calories exercising to "reserve" more calories to eat[9:05]

Guest's view on exercise and its limited impact on weight loss

Guest says exercise's effect on weight loss is probably very small, though it is good for many other aspects of health[9:31]
• Exercise benefits cited include flexibility, strength, and core fitness
Main reason for limited weight-loss effect: the absolute number of calories burned by typical exercise is low[9:31]
• Guest notes that most middle-aged people do only quick walks or 30-45 minutes three times a week, not 8 hours of high-intensity training
On a treadmill, the calorie counter goes up very slowly; 30 minutes may only show about 120 calories burned[10:01]
• Guest compares 120 calories to "a couple of cookies" to illustrate how small it is versus daily intake
When normal usage is 2,000 calories, adding 100 or so from exercise is a small percentage increase[10:13]

Exercise-induced hunger and compensatory eating

Guest says another problem is that exercise tends to cause increased eating[10:39]
During exercise, appetite is actually reduced, a phenomenon called exercise-induced anorexia[10:55]
• Example: in the middle of a basketball game, people do not suddenly feel very hungry because blood is in the muscles and hunger is not the focus
After exercise, there is a rebound where people are hungrier, leading them to eat more[11:12]
Guest notes decades of studies show this pattern of reduced hunger during exercise and increased hunger afterward[11:07]

Harvard study on children's activities, TV, and mild exercise

Guest describes a Harvard study measuring calorie differences in children during various activities[11:30]
Watching TV produced about +100 calories per hour on average[11:45]
Mild exercise showed about the same, roughly +100 calories per hour[11:54]
• Guest infers that mild exercise must be causing children to eat more for the caloric balance to be similarly positive
Conclusion: being hungrier after exercise can make it difficult to lose weight[12:11]

Diet's dominance over exercise in weight loss

Guest states that about 95% of weight loss is diet, as mentioned in the book[12:14]
He stresses this does not mean people should not exercise; everyone should exercise for health[12:29]
For weight loss specifically, the main focus must be on foods eaten, not only calorie quantity but type and hormonal effects[12:35]
How often one eats also matters because constant eating versus infrequent eating creates different hormonal balances[12:48]

Shift from three meals to frequent snacking and its metabolic impact

Data on meal frequency changes from 1977 to 2003

Host cites an American survey of more than 60,000 adults and children[12:59]
In 1977, most people ate three times a day[13:03]
By 2003, most people were eating five to six times a day[13:07]

Low-fat, high-carb advice and unintended snacking

Guest recounts that in 1977, official advice was to eat lots of carbs (55-60%) and less fat[13:28]
In 1977, typical pattern was breakfast, lunch, and dinner with no snacks[13:40]
• If a child wanted an after-school snack, parents would often say it would ruin dinner
• If someone wanted a bedtime snack, they were told to eat more at dinner instead
As people adopted high-carb, low-fat diets, a new pattern emerged with more frequent hunger[13:28]
Guest gives example of two slices of bread with jam in the morning: little satiety, insulin and glucose spike then crash[14:05]
• As sugars and insulin fall mid-morning, people become hungry again and seek low-fat muffins or similar snacks
At lunch, a big plate of pasta creates another spike-crash cycle, leading to ravenous hunger by 3 pm and more snacks like crackers[14:33]
By 2003, mid-morning, mid-afternoon, and bedtime snacks become routine, increasing eating occasions to five or six per day[14:41]
People believed low-fat eating was healthy, so the increased snacking became "institutionalized"[14:53]

Change in cultural view of snacks and the logic of fasting vs constant eating

Previously, snacks were seen as an indulgence, not something good for you, but later were promoted as something everyone should do regularly[14:59]
Guest says a norm emerged that people should never be without food for more than an hour and a half[15:11]
He explains a simple framework: when you eat, insulin goes up and you store calories; when you don't eat or fast, insulin goes down and you burn calories[15:23]
Given that logic, he argues it makes no sense to eat all the time if you want to lose body fat[15:42]
To lose fat, you need to extend the period when you are not eating, remove snacks, and allow insulin to stay low long enough to permit fat burning[15:42]

Origins and controversy of modern intermittent fasting advocacy

Guest's role in popularizing intermittent fasting for weight loss

Host says the guest is cited as the founder of modern intermittent fasting, especially as a tool for weight loss[16:03]
Host notes intermittent fasting has existed for thousands of years, but the internet credits this guest with bringing it into medical and public discussion around 2013-2014[16:18]
Guest says that in 2013-2014, almost nobody was discussing intermittent fasting medically in terms of body processes and pros/cons[16:25]
For years he felt like a lone voice arguing that fasting is a useful tool for weight loss[16:40]
At the time, many people believed intermittent fasting was extremely bad for you[16:49]

Investigating myths about intermittent fasting

Guest says he reviewed the literature and found many claims about fasting being harmful were not supported[17:10]
Common myths included that fasting would make you gain weight, be tired, and be hungry[17:02]
He counters that there is extensive data over 2,000 years of fasting use and these negative claims are simply not true[17:10]
He notes this is why there was such little medical discussion about fasting as a tool before he began promoting it[17:10]

Professional backlash and personal observations about fasting

Guest says he was attacked from all sides: doctors, dietitians, and others who feared he would cause harm[17:27]
He recalls colleagues admitting they themselves routinely went 24 hours without eating during training without ill effects[17:48]
• Examples include being busy in the operating room or emergency room and skipping meals
Guest realized that as a doctor he regularly tells patients to fast for surgery, postoperative periods, and fasting blood work[18:07]
He found it inconsistent that fasting is standard for medical procedures but is discouraged for weight loss[18:09]

Basal metabolic rate during fasting versus calorie restriction

The 'starvation mode' myth

A major myth he identifies is that fasting will cause basal metabolic rate to fall into 'starvation mode'[18:36]
Concern is that if metabolic rate falls too low during fasting, people will gain weight when they resume eating[18:52]

Evidence that metabolic rate rises with short-term fasting

Guest describes a study design: measure a person's caloric burn before fasting and after four days of zero food[18:57]
Example numbers: day zero basal metabolic rate is 2,000 calories per day[18:52]
After four days of no food, the body is burning 2,200 calories per day[18:57]
Conclusion: basal metabolic rate did not go down; it went up during fasting[18:27]
He says multiple studies show this increase in metabolic rate with fasting[19:25]
By contrast, calorie-restricted, low-fat diets and eating all the time cause metabolic rate to drop, making weight loss very hard[19:29]

Hormonal explanation for increased metabolic rate during fasting

Guest calls the explanation basic medical physiology[19:48]
When you don't eat, insulin falls, allowing the body to start using stored calories[19:58]
At the same time, sympathetic tone (fight-or-flight), cortisol, and growth hormone levels go up[20:07]
• These hormones tell the body to start pulling calories out of storage and activate it
He argues that fasting is an activation state: the body becomes more energized to seek food[20:50]
• Example: a hungry wolf is highly activated and dangerous, while a lion that has just eaten is lethargic and wants to lie down
Thus he concludes that metabolic rate goes up, not down, during fasting, contrary to the starvation mode myth[20:43]

Reframing food as instruction and daily fasting via breakfast

Food as hormonal instruction

Host reflects that he now sees food as an instruction that alters hormones and thus bodily responses[20:59]

Questioning the necessity of breakfast

Host mentions reading that the guest thinks most people do not need breakfast[21:06]
Guest says the idea that you must eat as soon as you get up is false[21:12]

Meaning of 'breakfast' and normal daily fasting pattern

Guest explains that you will always 'break your fast' at some meal, which is why it is called breakfast[21:23]
The term 'breakfast' implies the body should have a fasting period every day[21:32]
Normal pattern: a feeding period where insulin rises and calories are stored, followed by a fasting period when insulin falls and stored calories are used[21:38]
Example: stop eating at 6 pm and eat again at 8 am, yielding a 14-hour fasting period where the body is not eating and thus using stored calories[21:58]
He argues that if you eat all the time, the body continuously stores energy and never has a period to burn it, leading to weight gain[22:11]

Clip context

Indication this is a replayed segment

Narrator states that what was just heard was a most replayed moment from a previous episode and directs listeners to the full episode via a link[22:21]

Lessons Learned

Actionable insights and wisdom you can apply to your business, career, and personal life.

1

Focusing solely on cutting calories without considering hormones and food timing can backfire because the body often compensates by lowering its metabolic rate.

Reflection Questions:

  • • What current eating habits of mine might be encouraging my body to reduce its energy expenditure instead of maintain or increase it?
  • • How could I start evaluating my diet not just by calorie count but by how it affects my energy levels and hunger over the day?
  • • What is one small change I can make this week to reduce chronic calorie restriction and instead support a more stable metabolism?
2

Insulin and other hormones determine whether the body stores energy or burns stored fat, so food choice and meal frequency are as important as total calories for weight regulation.

Reflection Questions:

  • • Which of my regular foods are most likely to spike insulin and keep it elevated throughout the day?
  • • How might reducing the number of times I eat each day change my ability to access stored body fat over the next month?
  • • What specific adjustment to my meal composition or timing could I experiment with to encourage lower insulin periods?
3

Intermittent fasting can create windows of low insulin that allow the body to draw on fat stores while maintaining or even increasing basal metabolic rate.

Reflection Questions:

  • • When during my typical day would it be most practical to extend the time between my last meal and my next meal?
  • • How could I gradually test a simple fasting window (like 12-14 hours overnight) and track how it affects my hunger, mood, and weight?
  • • What support or routines (sleep schedule, hydration, activities) would I need in place to try intermittent fasting safely and consistently?
4

Frequent snacking, even on low-fat foods, can keep insulin elevated, shorten fasting periods, and undermine long-term fat loss despite eating "healthy" by conventional advice.

Reflection Questions:

  • • In what situations do I tend to snack out of habit rather than true hunger during the day?
  • • How might my weight and energy change if I removed just one habitual snack and allowed a longer gap between meals?
  • • What strategies (like changing my environment or planning meals differently) could help me rely less on between-meal snacks?
5

Exercise is vital for health but contributes relatively little to weight loss compared with diet and hormonal balance, so training plans should be paired with thoughtful nutrition strategies.

Reflection Questions:

  • • Am I expecting my exercise routine to compensate for dietary choices that keep my insulin high and my metabolism sluggish?
  • • How could I redesign my health plan so that my diet does most of the work for weight management while exercise supports strength and wellbeing?
  • • What one change could I make this week to align my eating habits more closely with the goals I'm pursuing through exercise?
6

Short-term fasting does not necessarily slow metabolism; in many cases it activates the body, increasing energy expenditure through hormonal changes.

Reflection Questions:

  • • How do my assumptions about "starvation mode" influence my willingness to leave gaps between meals or skip a snack?
  • • In what ways could I safely test a short fasting period and observe whether I feel more or less energetic and focused?
  • • What indicators (such as alertness, warmth, or performance) could I monitor to see how my body responds to brief periods without food?

Episode Summary - Notes by Reagan

Most Replayed Moment: Calories In, Calories Out Is A Myth! Why Most Diets Fail - Dr. Jason Fung
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