Dr. Jason Fung: The Founder of Modern Intermittent Fasting Reveals the Truth About Weight Loss and Fasting

In this video interview, Dr. Jason Fung, a pioneer of modern intermittent fasting, discusses the benefits of extending fasting periods for weight loss. He explains that fasting allows insulin levels to decrease, which leads to fat burning and calorie reduction. Dr. Fung faced criticism when he first introduced the concept of fasting for weight loss, but he backed it up with evidence from centuries of intermittent fasting practices. He also addresses myths surrounding fasting, such as the belief that it lowers metabolic rate and causes increased hunger. Additionally, he explains the role of fiber in maintaining a healthy diet and the insufficient evidence supporting protein as a weight loss tool. Overall, Dr. Fung hopes his work can help reverse type 2 diabetes and change the perception of weight loss strategies.

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Key Insights:

  • Extending the fasting period and eliminating snacks can help in losing body fat.
  • Intermittent fasting as a tool for weight loss was not widely discussed from a medical standpoint until recently.
  • Intermittent fasting was initially seen as harmful, but the founder discovered evidence to the contrary.
  • Fasting actually increases basal metabolic rate, contrary to the myth of slowing it down.
  • There are no strict rules for fasting; it can range from 12-14 hours to multiple days without eating.
  • Fiber deficiency arose from the processing of food, as removing fiber made it more appealing and caused insulin spikes.
  • Unprocessed foods, high in fiber, are better for regulating insulin response compared to highly processed foods.
  • Protein, while spiking insulin, is an inefficient macronutrient for storing energy and is rare to find in pure form in nature.
  • Fasting is more effective than juice fasting as it helps regulate insulin levels without overdosing on sugar.
  • The greatest gift to the world for the speaker would be helping people reverse type 2 diabetes and challenging the existing understanding of weight loss based on caloric intake.


If you want to lose body fat, you actually need to extend the period of time that you’re not eating, in other words, extend your fasting period and get rid of all the snacks. This allows your insulin levels to stay low, which promotes fat burning and the utilization of calories stored in the body. I am often credited as the founder of modern intermittent fasting. In 2013-2014, nobody was really talking about intermittent fasting from a medical standpoint and its benefits for weight loss. Many myths and misconceptions surrounded intermittent fasting, causing people to believe it was bad for them. However, based on historical data and medical knowledge, I started promoting intermittent fasting as a tool for weight loss.

During that time, I faced a lot of criticism and attacks from doctors and dietitians who believed that fasting was harmful. Ironically, many of them admitted to practicing some form of fasting themselves, such as fasting before surgery or for medical tests. This disparity between medical practices and weight loss recommendations didn’t make sense to me, so I started looking into the physiology of fasting.

One prevalent myth was that intermittent fasting would slow down the metabolism, leading to weight gain once you resume eating. However, studies have shown that fasting actually increases the basal metabolic rate, making weight loss easier. When you fast, insulin levels decrease, while other hormones like cortisol and growth hormone rise, signaling the body to use stored calories for energy. This metabolic activation is why fasting is effective for weight loss.

There are various ways to practice fasting, including a 12-14 hour fast (which is a baseline), a 16-hour fast, a 24-hour fast, or even multiple days without eating. The body is capable of using its stored energy (body fat) during periods of fasting, making it a natural and efficient process.

Leptin resistance is a condition where the body becomes less responsive to the hormone leptin, which regulates hunger and appetite. The reasons for leptin resistance are complex, but it can be influenced by hormonal imbalances caused by factors such as insulin levels. Balancing hormones through fasting and healthy eating habits can help alleviate leptin resistance.

Regarding fiber, processed foods often lack fiber, making them more appealing due to their faster digestion and subsequent release of insulin. Slower digestion from high-fiber foods, such as beans and whole grains, results in a gentler insulin response. Therefore, unprocessed foods are generally better for controlling insulin levels.

Protein can also affect insulin levels to some extent, but its impact on weight gain is less significant compared to carbohydrates. Protein is a less efficient macronutrient for energy storage, as it is not easily converted into stored fat like glucose and carbohydrates.

The key to managing insulin levels is to make healthier food choices and limit the frequency of meals. While fasting is one approach, simply changing the types of food consumed, favoring unprocessed options, can also have a positive impact on insulin release.

Juice fasting is not a recommended form of fasting, as it often involves consuming high amounts of sugar. Juices with low sugar content, like cucumber or kale juice, can be healthier options due to their vitamin contents. However, it is generally more effective and beneficial to opt for traditional fasting rather than juice fasting.

My hope is that my work will contribute to reversing type 2 diabetes and changing the narrative around weight loss. Breaking the stigma and understanding the role of hormones in weight management is crucial. I want to encourage people to think beyond calories in and calories out and explore the potential of natural methods, such as fasting, to achieve their health goals. It may seem like an audacious goal, but if I can help even a fraction of the millions of people struggling with weight-related issues, it will be a significant accomplishment.

(Note: This transcript has been edited for clarity and readability)