Dr. Jason Fung, a Canadian nephrologist, has become a leading voice in the discussion surrounding intermittent and extended fasting. His work, notably in his books The Obesity Code and The Diabetes Code, challenges the traditional calorie-centric view of weight loss. Instead, he posits that elevated insulin levels drive obesity and metabolic dysfunction. By controlling insulin through periods of not eating, his fasting schedules aim to restore metabolic health and promote fat loss naturally.
The Core Philosophy: Insulin Control
The foundation of Jason Fung's fasting schedules is the concept of hormonal balance. When we eat, particularly carbohydrates, our bodies produce insulin to help cells absorb glucose for energy. Over time, frequent eating and high-carb diets can lead to chronically high insulin levels and insulin resistance, where cells no longer respond effectively to the hormone. This forces the pancreas to produce even more insulin, and the body begins to store excess glucose as fat.
Fung's protocols use fasting to lower insulin levels, allowing the body to access stored fat for energy. This approach not only helps with weight loss but can also reverse insulin resistance, a key factor in conditions like type 2 diabetes.
Common Fasting Schedules Recommended by Dr. Fung
Fung does not prescribe a single schedule but rather advocates for a flexible approach, allowing individuals to choose what best fits their lifestyle and goals. Here are some of the most common protocols outlined in his work:
- Time-Restricted Eating (TRE): This involves restricting your daily eating to a specific window. A common and easy starting point is a 10-hour eating window (e.g., 9 a.m. to 7 p.m.). This aligns with circadian rhythms and is an effective way to introduce fasting.
- 16/8 Method: A more advanced form of TRE, this involves fasting for 16 hours and eating during an 8-hour window. Many people find it easiest to do this by skipping breakfast and having their meals between noon and 8 p.m..
- 24-Hour Fast (Eat-Stop-Eat): This involves abstaining from food for a full 24 hours, typically done from dinner to dinner or lunch to lunch, once or twice a week. This protocol allows the body to move into deeper states of autophagy and fat burning.
- Alternate-Day Fasting (ADF): As the name suggests, this involves alternating between days of eating normally and days of fasting. For many, a modified version is used, allowing up to 500-600 calories on fasting days.
- Extended Fasts (48-72 hours): For individuals with more severe insulin resistance or specific health goals, Fung may recommend longer, medically supervised fasts. These prolonged fasts can significantly accelerate benefits like autophagy and metabolic resetting.
Comparison of Jason Fung's Fasting Schedules
| Feature | Time-Restricted Eating (e.g., 16/8) | 24-Hour Fast | Alternate-Day Fasting (ADF) | Prolonged Fast (48-72 hrs) |
|---|---|---|---|---|
| Frequency | Daily or several times per week | 1-2 times per week | Every other day | Periodically, with medical guidance |
| Duration | 16-hour fast, 8-hour eating window | Full 24 hours | 36-40 hour fast alternating with eating day | 48-72 hours or longer |
| Beginner Friendly | Yes, easier to start with a smaller window like 12/12. | Moderate, requires building up tolerance. | Difficult for most beginners. | Not for beginners; requires strict medical oversight. |
| Calorie Intake | Only during the 8-hour window. | None on fasting days, normal on eating days. | None or very low (500-600 kcal) on fast days. | None during the fast. |
| Key Benefit | Improved insulin sensitivity, sustainable weight loss. | Deep fat burning and autophagy activation. | Potent fat burning and metabolic improvement. | Maximum autophagy and rapid metabolic reset. |
What to Consume During Fasting
During a fasting window, the goal is to consume zero calories to keep insulin levels low. This means sticking to non-caloric beverages. Staying hydrated is crucial for managing hunger and avoiding side effects like headaches.
Approved fasting liquids include:
- Water (plain or with a pinch of sea salt for electrolytes)
- Herbal teas (unsweetened)
- Black coffee (plain)
- Bone broth (especially during longer fasts, provides electrolytes and minerals without significant calories)
Breaking the Fast and Eating Windows
What you eat is just as important as when you eat. To support the metabolic changes initiated by fasting, Fung recommends a low-carbohydrate, healthy-fat, and moderate-protein diet during eating windows.
Focus on whole, nutrient-dense foods like:
- Non-starchy vegetables (leafy greens, broccoli)
- Healthy fats (avocado, olive oil, nuts, seeds)
- Moderate protein (fish, poultry, eggs)
- Limited grains and sugars
How to Begin Jason Fung's Fasting Protocol
- Start Gradually: For beginners, start with a simple time-restricted eating schedule, such as fasting for 12 or 14 hours and gradually extending it.
- Stay Hydrated: Drink plenty of water and other non-caloric beverages to manage hunger and stay hydrated.
- Listen to Your Body: Pay attention to how your body feels. If you experience dizziness, weakness, or other concerning symptoms, adjust your schedule.
- Combine with Low-Carb Diet: To maximize the benefits of fasting, combine it with a low-carb, whole-food diet during eating periods.
Conclusion
Dr. Jason Fung's fasting schedules are a powerful tool for improving metabolic health by addressing the root cause of insulin resistance. His approach provides a flexible, accessible framework, from gentle time-restricted eating for beginners to medically supervised extended fasts for more intensive purposes. By focusing on when to eat rather than simply what or how much, it empowers individuals to reset their bodies and achieve sustainable health improvements. However, it is essential to consult a healthcare professional before starting any new fasting regimen, especially if you have existing health conditions or are on medication.
Learn more from Dr. Fung's published works, including The Complete Guide to Fasting.
