What is the Jason Fung diet?

October 4, 2025 •

3 min read

Dr. Jason Fung, a Canadian nephrologist, has proposed a diet that challenges the conventional 'calories in, calories out' model by focusing on hormonal regulation, particularly insulin, as the key driver of weight management. He advocates for a combination of intermittent fasting and a low-carbohydrate diet to reverse insulin resistance, a condition central to his theory of obesity.

Quick Summary

The Jason Fung diet combines scheduled intermittent fasting with a low-carb, whole-foods approach to manage insulin levels. It is designed to promote weight loss, improve metabolic health, and help with conditions like type 2 diabetes by controlling hormonal responses rather than restricting calories.

Key Points

Insulin Focus: The diet treats obesity and metabolic issues as hormonal problems stemming from high insulin levels.
Two-Pillar Approach: It combines scheduled intermittent fasting with a low-carb, whole-foods diet to control insulin.
Flexible Fasting: It offers various intermittent fasting protocols, such as 16:8 and 24-hour fasts, that can be adapted to individual needs.
Food Quality Matters: Emphasis is placed on consuming healthy fats, moderate protein, and non-starchy vegetables during eating periods.
Potential for Type 2 Diabetes Reversal: The diet's insulin-reducing effects can be highly beneficial for managing and reversing type 2 diabetes.
Consult a Doctor: Medical supervision is strongly advised, especially for individuals with pre-existing health conditions or those on medication.

Understanding the Philosophy Behind the Jason Fung Diet

At its core, the Jason Fung diet posits that weight gain and metabolic syndrome are primarily driven by elevated insulin levels, not just calorie intake. Dr. Fung argues that persistently high insulin puts the body in a fat-storing state and inhibits fat burning. The diet's goal is to lower insulin through strategic eating and fasting, allowing the body to use stored fat for energy.

The Core Components of the Diet

The Jason Fung diet centers on two main strategies: intermittent fasting and a low-carbohydrate, whole-foods diet.

Intermittent Fasting (IF)

Intermittent fasting involves alternating between periods of eating and fasting to allow insulin levels to drop and promote fat burning. Recommended methods vary, including:

16:8 Method: Fasting for 16 hours and eating during an 8-hour window.
24-Hour Fast: Fasting from dinner one day to dinner the next, done once or twice weekly.
Alternate-Day Fasting: Alternating between normal eating days and fasting days, which may include up to 500 calories.
Extended Fasts: Longer fasts (48-72 hours) should only be done under medical supervision.

Low-Carbohydrate, Whole-Food Nutrition

During eating periods, the focus is on minimizing refined carbohydrates and sugars that cause insulin spikes. The diet emphasizes nutrient-dense, whole foods, including healthy fats, moderate protein, and non-starchy vegetables.

Foods to include:

Healthy fats: Avocados, nuts, seeds, olive oil.
Moderate protein: Fish, poultry, eggs, lean meats.
Non-starchy vegetables: Leafy greens, broccoli, peppers.
Fiber-rich foods.

Foods to avoid:

Processed foods and refined grains.
Sugary drinks and added sugars.
Fast food and commercial snacks.

Comparison: Jason Fung Diet vs. Traditional Calorie Counting


Feature	Jason Fung Diet	Traditional Calorie Counting
Primary Mechanism	Hormonal regulation, focusing on lowering insulin.	Energy balance, focusing on eating fewer calories than expended.
Focus	When and what you eat (especially avoiding refined carbs).	The quantity of food eaten, regardless of type, though some diets prioritize macronutrients.
Fat	Encourages healthy fats to increase satiety and provide energy during fasting.	Often restricts fat intake in favor of a low-fat approach.
Sustainability	Adaptable fasting schedules and focus on whole foods may feel sustainable for some.	Can lead to hunger and difficulty maintaining long-term due to constant restriction.
Key Outcome	Improved insulin sensitivity and metabolic health.	Weight loss, but often with risks of slowing metabolism and regaining weight.

Benefits and Potential Downsides

Potential Benefits

Significant Weight Loss: Lowered insulin allows the body to effectively burn stored fat.
Improved Insulin Sensitivity: Can be effective in managing and potentially reversing type 2 diabetes and prediabetes.
Better Metabolic Health: Intermittent fasting may reduce inflammation and oxidative stress.
Reduced Hunger: Healthy fats and controlled eating windows can lead to decreased cravings and increased satiety.
Cellular Repair: Fasting can trigger autophagy, a process of clearing out damaged cells.

Potential Downsides

Initial Adaptation Phase: Beginners may experience temporary hunger, headaches, fatigue, or mood swings.
Nutritional Deficiencies: A poorly planned low-carb diet could lead to deficiencies if not carefully managed.
Scientific Claims: Some critics suggest certain claims, particularly regarding metabolism, may be overstated.
Not for Everyone: Not recommended for pregnant/breastfeeding women, individuals with eating disorders, or those with certain medical conditions. Consultation with a healthcare professional is essential before starting.

Conclusion: A Hormonal Approach to Nutrition

The Jason Fung diet provides an alternative perspective on weight management, focusing on insulin regulation through intermittent fasting and a low-carbohydrate, whole-foods diet. This approach aims to reduce insulin levels and reverse insulin resistance, showing promise for individuals with metabolic issues like type 2 diabetes. However, results vary, and it is not universally suitable. Adherence to principles and personalized medical guidance are crucial. Further long-term studies are needed, but for many, it offers an effective health strategy. For detailed information, Dr. Fung's book The Obesity Code is a valuable resource.

Frequently Asked Questions

The main idea is that obesity is a hormonal disorder caused by high insulin levels, not simply a calorie imbalance. The diet aims to lower insulin through intermittent fasting and a low-carbohydrate diet.

The diet's core components are intermittent fasting, which schedules eating periods, and a low-carbohydrate, whole-foods approach for eating windows.

No, it is not suitable for everyone. Pregnant or breastfeeding women, individuals with a history of eating disorders, and those with certain medical conditions should avoid it or use extreme caution under medical supervision.

Recommended foods include healthy fats (like avocados and nuts), moderate protein (fish and poultry), and non-starchy vegetables. Processed foods, refined carbs, and sugars are discouraged.

Fasting lowers insulin levels, which helps the body switch from burning glucose to burning stored fat for energy. It also triggers cellular repair processes, such as autophagy.

While both emphasize low carbs, the Jason Fung diet's key differentiator is its strategic use of intermittent fasting to regulate insulin, making it more flexible and focused on meal timing than on achieving deep ketosis.

Individuals may experience headaches, hunger, or fatigue during the initial adaptation phase. It is recommended to start slowly, stay hydrated with water and electrolytes, and listen to your body, adjusting the fasting schedule as needed.