The Quick Answer
Intermittent fasting is a timed eating pattern that cycles between periods of eating and voluntary fasting, without specifying which foods to eat but rather when to eat them.
The most common protocols are 16:8 (16 hours fasting, 8 hours eating), 18:6, OMAD (one meal a day), and 5:2 (normal eating 5 days, restricted calories 2 days). Each protocol adjusts the ratio of fasting to eating time, with longer fasts being more restrictive but not necessarily more effective for everyone.
Important: This is educational content, not medical advice. Intermittent fasting is not appropriate for everyone. People who are pregnant, nursing, have a history of eating disorders, are underweight, or take medications that require food (such as diabetes medications) should consult a healthcare provider before starting any fasting protocol.
Intermittent Fasting Protocols Compared
| Protocol | Fasting Window | Eating Window | Difficulty | Typical Daily Meals |
|---|---|---|---|---|
| 16:8 | 16 hours | 8 hours | Low | 2-3 meals |
| 18:6 | 18 hours | 6 hours | Moderate | 2 meals |
| 20:4 (Warrior Diet) | 20 hours | 4 hours | High | 1-2 meals |
| OMAD (23:1) | 23 hours | ~1 hour | High | 1 meal |
| 5:2 | 2 days/week at 500-600 cal | 5 normal days | Moderate | Normal on eating days |
| Alternate Day Fasting | Every other day at ~500 cal | Normal on eating days | High | Normal on eating days |
16:8 -- The Standard Starting Point
Fast for 16 hours, eat within an 8-hour window. This is the most widely practiced protocol because most people already fast 10-12 hours overnight. Extending that by skipping breakfast (or dinner) adds 4-6 hours.
Meal timing example (skip breakfast):
- Last meal ends: 8:00 PM
- Fasting period: 8:00 PM to 12:00 PM next day (16 hours)
- Eating window: 12:00 PM to 8:00 PM
- Meals: Lunch at 12:00 PM, snack at 3:00 PM, dinner at 7:00 PM
Meal timing example (skip dinner):
- First meal: 7:00 AM
- Eating window: 7:00 AM to 3:00 PM
- Meals: Breakfast at 7:00 AM, lunch at 12:00 PM, light meal at 2:30 PM
- Fasting period: 3:00 PM to 7:00 AM next day (16 hours)
18:6 -- A Moderate Step Up
Fast for 18 hours, eat within a 6-hour window. This typically means two meals with a possible small snack. Example: eating window from 12:00 PM to 6:00 PM.
The additional 2 hours of fasting compared to 16:8 may not seem significant, but it eliminates an evening snacking window that many people find is their highest-calorie period.
20:4 (Warrior Diet)
Fast for 20 hours, eat within a 4-hour window. Originally described by Ori Hofmekler, the Warrior Diet allows small amounts of raw fruits and vegetables during the fasting period, with one large meal in the evening. In practice, most people eat 1-2 meals in the 4-hour window.
OMAD -- One Meal A Day
Fast for approximately 23 hours and consume all daily calories in a single meal. This is one of the more aggressive protocols. The primary challenge is consuming adequate nutrition -- particularly protein, vitamins, and minerals -- in a single sitting. Many people find it difficult to eat 1,800-2,500 calories in one meal without discomfort.
5:2 Protocol
Eat normally for 5 days per week. On 2 non-consecutive days (for example, Monday and Thursday), restrict calorie intake to 500-600 calories. This protocol does not involve complete fasting on any day and does not restrict the eating window on normal days.
The 5:2 approach, popularized by Michael Mosley, may be easier to sustain socially because you eat normally most days. The restricted days can use the 500-600 calories in one or two small meals.
Alternate Day Fasting (ADF)
Fast every other day. On fasting days, intake is restricted to approximately 500 calories (sometimes called "modified" alternate day fasting). On eating days, there are no calorie restrictions.
Research by Krista Varady and colleagues at the University of Illinois Chicago has studied this protocol extensively. ADF tends to produce a weekly calorie reduction of roughly 20-35% without requiring calorie counting on eating days.
What Happens During a Fast
The body progresses through several metabolic states as a fast extends:
0-4 hours (Fed state): The body digests and absorbs the last meal. Blood sugar and insulin levels are elevated. The body primarily uses glucose for energy.
4-12 hours (Early fasting): Insulin levels decline. The body begins using stored glycogen (glucose stored in the liver and muscles) for energy. This is the normal overnight fasting state most people experience.
12-18 hours (Glycogen depletion / Fat oxidation increase): Liver glycogen stores become depleted. The body increasingly shifts to burning fatty acids for fuel. This transition is sometimes called the "metabolic switch." Fat oxidation rates increase.
18-24 hours (Increased fat oxidation): The body relies primarily on fat for energy. Ketone production may begin to increase, providing an alternative fuel source for the brain.
24+ hours (Extended fasting): Ketone levels rise further. Research in animal models suggests increased autophagy (cellular cleanup and recycling processes) during extended fasts, though the exact timing and magnitude in humans is still an active area of research.
A note on autophagy: The claim that fasting triggers significant autophagy in humans at specific time points (often cited as "16 hours" or "24 hours") is based primarily on animal studies. The 2016 Nobel Prize in Physiology or Medicine was awarded to Yoshinori Ohsumi for discovering mechanisms of autophagy, but measuring autophagy in living humans remains technically difficult. The research is promising but not yet definitive on precise timing in humans.
Practical Meal Timing: A 16:8 Week
Here is what a typical week might look like on a 16:8 schedule (noon to 8 PM eating window):
| Time | Activity |
|---|---|
| 7:00 AM | Wake up. Black coffee or plain tea (zero calories). |
| 8:00 AM - 11:30 AM | Work, exercise, or daily activities. Water, black coffee, or plain tea as desired. |
| 12:00 PM | First meal: protein, vegetables, healthy fats, complex carbohydrates. |
| 3:00 PM | Optional snack: fruit, nuts, yogurt. |
| 7:00 PM | Second/third meal: balanced dinner. |
| 8:00 PM | Eating window closes. |
| 8:00 PM - 12:00 PM next day | Fasting period (16 hours). Water and zero-calorie beverages. |
This schedule is flexible. Some people shift the window earlier (10 AM to 6 PM) or later (2 PM to 10 PM) depending on their daily routine and social commitments.
Common Mistakes
1. Overeating During the Eating Window
Restricting the eating window does not automatically create a calorie deficit. If you consume 3,000 calories in 8 hours instead of 3,000 calories in 16 hours, your total intake has not changed. Intermittent fasting works for weight management primarily when it naturally reduces total calorie consumption.
2. Not Drinking Enough Water
During the fasting period, you are not getting water from food. Many people become mildly dehydrated without realizing it, leading to headaches and fatigue they attribute to the fast itself. Aim for at least 8 cups (64 oz / ~2 liters) of water per day, and more if you exercise.
3. Starting Too Aggressively
Jumping directly into OMAD or alternate-day fasting without any prior experience often leads to abandoning the practice within days. Start with 12:12 (12 hours fasting, 12 hours eating), progress to 14:10, then 16:8. Allow 1-2 weeks at each level before advancing.
4. Ignoring Nutritional Quality
Fasting is a timing strategy, not a nutrition strategy. Eating processed, nutrient-poor food during your eating window undermines the purpose. Focus on adequate protein (at least 0.7 grams per pound of body weight), vegetables, fruits, whole grains, and healthy fats.
5. Fasting Through Important Signals
Feeling mildly hungry is normal during the adjustment period (1-2 weeks). Feeling dizzy, shaky, unable to concentrate, or extremely weak is not normal and means you should eat. Fasting should not override your body's distress signals.
Who Should Not Fast
Intermittent fasting is not appropriate for:
- Pregnant or nursing women -- Caloric restriction during pregnancy or lactation can affect fetal development and milk supply.
- People with eating disorders -- Fasting can reinforce restrictive eating patterns and worsen disordered eating.
- People with diabetes (especially Type 1) -- Fasting can cause dangerous blood sugar fluctuations. Consult an endocrinologist first.
- Children and adolescents -- Growing bodies need consistent nutrition.
- People taking medications that require food -- Some medications must be taken with meals at specific intervals.
- Anyone who is underweight -- Further caloric restriction is not appropriate.
Source: National Institute on Aging. Research on Intermittent Fasting.
Frequently Asked Questions
Can I drink coffee during a fast?
Plain black coffee with no sugar, cream, or milk is generally considered acceptable. It contains negligible calories and does not trigger a significant insulin response. Adding cream, sugar, or flavored syrups introduces calories and may break the fast.
Will fasting slow my metabolism?
Short-term fasting (under 72 hours) does not appear to reduce metabolic rate. Some research suggests it may temporarily increase metabolic rate through norepinephrine release. Metabolic slowdown is associated with prolonged caloric restriction over weeks, not with the timing pattern of intermittent fasting.
Which fasting schedule is easiest for beginners?
The 16:8 protocol is the most common starting point. Since most people already fast 10-12 hours overnight, extending that by skipping breakfast is a manageable first step. Start with 14:10 if 16:8 feels too aggressive initially.
Can I exercise while fasting?
Moderate exercise during a fast is generally safe for healthy adults. Light cardio and moderate strength training are well-tolerated. High-intensity or long endurance sessions may be harder due to lower glycogen availability. If you feel dizzy or weak, stop and eat.
How many calories break a fast?
There is no universally agreed number. A strict fast means zero calories. Some practitioners allow up to 50 calories (a splash of cream) and consider the fast intact. Protein and carbohydrates trigger more insulin response than fat.
Is intermittent fasting safe for women?
Research on women is more limited. Some women report menstrual irregularities with aggressive protocols. A moderate approach (14:10 or 16:8) is generally better tolerated. Women who are pregnant, nursing, or trying to conceive should consult a healthcare provider.
Does intermittent fasting cause muscle loss?
Not more than conventional calorie restriction, provided protein intake is adequate (0.7-1.0 grams per pound of body weight per day) and resistance training is maintained during the eating window.
Can I do intermittent fasting every day?
Many people practice 16:8 daily as a long-term pattern. Whether this is appropriate depends on individual health, activity level, and how you feel. Some prefer 5-6 days per week with flexibility on weekends.
What should I eat to break a fast?
For fasts under 24 hours, most regular foods are fine. For longer fasts, start with something easy to digest -- a small portion of protein and vegetables, or broth. Avoid breaking an extended fast with a large, high-carbohydrate meal.
Will intermittent fasting help me lose weight?
It can, primarily by making it easier to maintain a calorie deficit. Research shows it produces similar weight loss to conventional calorie restriction when total calories are equal. If you compensate by overeating during your eating window, you will not lose weight.
Related Tools
- Fasting Calculator -- Calculate your fasting and eating windows based on your chosen protocol
- Calorie Needs Calculator -- Estimate your daily caloric needs based on age, weight, height, and activity level
- BMR Calculator -- Calculate your basal metabolic rate to understand your baseline energy expenditure