What Happens During a Fast?
Intermittent fasting isn't about starving yourself but rather about cycling between periods of eating and abstaining from calories. When you fast, your body goes through several physiological changes as it exhausts its immediate sugar stores and switches to burning fat for energy. This is often referred to as metabolic switching.
- Fed State (0–4 hours after eating): Immediately after you eat, your body digests food, and blood sugar levels rise. Insulin is released to transport this sugar to your cells for energy, with excess glucose being stored as glycogen in your liver and muscles.
- Early Fasting State (4–18 hours after eating): As your body uses up the glycogen, blood sugar and insulin levels start to drop. Your body begins to access stored glycogen to convert it back into glucose for energy.
- Fasting State (18+ hours): When glycogen stores are depleted, the body shifts into ketosis, using stored fat as its primary fuel source instead of glucose. This is when significant fat burning begins. Cellular repair processes, such as autophagy, also begin to ramp up during this stage.
Popular Intermittent Fasting Schedules
There is no single answer to how many hours of not eating is considered intermittent fasting, as different methods have different timeframes. The right schedule depends on an individual's goals, lifestyle, and how their body responds.
Common Time-Restricted Eating Windows:
- 12:12 Method: This is one of the gentlest forms of IF, ideal for beginners. You fast for 12 hours and eat during a 12-hour window. For instance, finishing dinner by 8 p.m. and not eating again until 8 a.m. the next morning.
- 16:8 Method: A very popular approach, the 16:8 method involves fasting for 16 hours and restricting all food consumption to an 8-hour window. Many people achieve this by skipping breakfast. For example, eating only between noon and 8 p.m..
- 20:4 Method (The Warrior Diet): This is a more intense version where you fast for 20 hours and have a large meal within a 4-hour eating window. During the fasting period, small amounts of raw fruits and vegetables are sometimes allowed.
Weekly or Alternate-Day Fasting:
- The 5:2 Diet: With this plan, you eat normally for five days of the week. On the other two non-consecutive days, you significantly restrict your calorie intake (around 500-600 calories).
- Eat-Stop-Eat: This involves a full 24-hour fast, once or twice per week. This can be done from lunch one day to lunch the next day. This method can be challenging for beginners.
- Alternate-Day Fasting (ADF): As the name suggests, this plan involves fasting every other day. Some variations permit a small meal (under 500 calories) on fasting days.
How Fasting Duration Impacts Health Benefits
Different fasting durations trigger different physiological responses and offer varying benefits. Longer fasts generally lead to a more pronounced metabolic shift and deeper cellular cleansing.
| Fasting Duration | Key Metabolic Process | Potential Benefits |
|---|---|---|
| 12-14 Hours | Glycogen depletion begins; body shifts toward fat-burning. | Can be effective for weight loss, reduces late-night snacking. |
| 16-18 Hours | Glycogen is significantly depleted; fat-burning increases. Autophagy processes are initiated in some tissues. | Supports weight and fat loss, improves insulin sensitivity. |
| 24+ Hours | Sustained ketosis; fat is a primary fuel source. Autophagy markers peak around 24 hours. | Increased fat burning, potential for cognitive benefits, cellular repair. |
| 48-72 Hours | Deeper cellular autophagy is engaged. Must be medically supervised due to risks. | Maximal cellular cleansing and repair processes. |
Conclusion: Finding the Right Fast for You
For beginners, starting with a shorter, more sustainable fast like the 12:12 or 16:8 method is often recommended. Including your overnight sleep in the fasting window makes the process much more manageable and less daunting. Regardless of the schedule you choose, the quality of food consumed during your eating window is crucial. Intermittent fasting is not a license to eat unhealthy or ultra-processed foods. Consistent hydration with water, black coffee, or unsweetened tea is also vital during fasting periods to help with hunger and prevent dehydration. Before starting any intermittent fasting plan, especially if you have pre-existing health conditions like diabetes, it is essential to consult with a healthcare professional. They can help determine if fasting is safe for you and which approach best fits your health needs and lifestyle.
Read more from Johns Hopkins Medicine on intermittent fasting
How many hours of not eating is considered intermittent fasting?
Intermittent fasting can begin with as few as 12 hours of not eating, with many popular methods extending to 16, 18, or even 24 hours, depending on the specific protocol. The 16:8 method (16 hours fasting, 8 hours eating) is one of the most common and accessible approaches.
What happens to your body after 12 hours of fasting?
After about 12 hours of fasting, your body has used up most of its immediate glucose stores and begins the process of metabolic switching, where it turns to burning stored fat for energy. This makes it a good starting point for beginners to experience the initial benefits of fasting.
What are the different types of intermittent fasting by hours?
Common types include the 12:12 method (12 hours fast), 16:8 method (16 hours fast), 20:4 (20 hours fast in the Warrior Diet), and 24-hour fasts (Eat-Stop-Eat) performed once or twice a week. The choice depends on your experience and goals.
Can you lose weight with a 12-hour intermittent fast?
Yes, fasting for 12 hours or more can contribute to weight loss by creating a calorie deficit and initiating the metabolic shift towards fat burning. It is a mild, sustainable approach for many to reduce overall calorie intake.
What is a good intermittent fasting schedule for beginners?
A great starting point for beginners is the 12:12 method, which often aligns naturally with your sleep schedule (e.g., stopping eating after an early dinner). Once comfortable, you can gradually extend the fasting window to 14 or 16 hours.
Does drinking coffee break a fast?
No, black coffee and unsweetened tea will not break a fast as they contain negligible calories and do not trigger an insulin response. Staying hydrated with these calorie-free beverages is encouraged during fasting periods.
How does intermittent fasting differ from regular dieting?
Unlike regular diets that focus on what to eat, intermittent fasting focuses on when to eat. It doesn't restrict food types but rather the eating window, allowing for greater flexibility while still working towards health goals.
