What is the rule for intermittent fasting and how to get started?

December 30, 2025 •

3 min read

According to research from Johns Hopkins, the human body is naturally equipped to go for extended periods without food, a capability that dates back to our hunter-gatherer ancestors. But what is the rule for intermittent fasting in a modern context? It's an eating pattern, not a conventional diet, that focuses on when to eat, not just what to eat.

Quick Summary

Intermittent fasting cycles between periods of eating and fasting, without dictating specific food types. The core concept involves restricting calorie intake to defined time windows, with popular methods ranging from daily time-restricted eating to weekly fasting days.

Key Points

Flexible Approach: Intermittent fasting isn't a single diet but an eating pattern with different rules and methods, such as 16/8, 5:2, and Eat-Stop-Eat.
Timing, Not Food: The core principle is focusing on when to eat, cycling between eating and fasting windows, rather than what specific foods to avoid.
Stay Hydrated: Consuming calorie-free beverages like water, black coffee, and unsweetened tea is permitted and encouraged during fasting periods.
Prioritize Whole Foods: During your eating window, maximize benefits by choosing nutrient-dense whole foods over processed and sugary options.
Listen to Your Body: Start slowly, perhaps with a 12-hour fast, and adjust the schedule as needed, as side effects like hunger and headaches can occur initially.
Not For Everyone: Certain individuals, including those who are pregnant, breastfeeding, or have a history of eating disorders, should avoid IF without medical supervision.

The rules for intermittent fasting (IF) are less about what you eat and more about when you eat. This eating pattern focuses on alternating between periods of eating and fasting, a process that can lead to metabolic switching, where your body burns stored fat for energy. The 'rules' are determined by the specific IF method chosen. Key to all methods are consistency, eating nutrient-dense foods during eating windows, and consuming only calorie-free beverages during fasts.

Popular Intermittent Fasting Methods

There isn't one single rule for intermittent fasting, but several popular methods offering different schedules:

The 16/8 Method

This common method involves fasting for 16 hours and eating within an 8-hour window. Skipping breakfast and eating from noon to 8 p.m. is a typical approach. Many find this schedule easy as much of the fast occurs during sleep.

Common 16/8 Timelines:

9 a.m. to 5 p.m.: For early risers.
10 a.m. to 6 p.m.: A balanced option.
12 p.m. to 8 p.m.: Skips breakfast.

The 5:2 Diet

This method involves eating normally for five days and restricting calories to 500-600 on two non-consecutive days. Men typically aim for 600 calories, women for 500. Fasting days can be chosen for flexibility.

The Eat-Stop-Eat Method

This involves a 24-hour fast, once or twice weekly. Fasting from dinner to dinner is an example. Only calorie-free drinks are allowed. This method can be challenging initially.

Alternate-Day Fasting

This approach alternates between fasting and normal eating days. Some variations allow about 500 calories on fasting days. This is a more intense method, not generally recommended for beginners.

Essential Intermittent Fasting Rules

Regardless of the method, certain principles are crucial for safe and successful IF:

Stay Hydrated: Drink plenty of water, black coffee, and unsweetened tea during fasts.
Listen to Your Body: Adjust your schedule if you feel unwell. IF should be sustainable.
Prioritize Nutrient-Dense Foods: Focus on whole foods during eating windows.
Consistency is Key: Allow time for your body to adapt.
Plan Your Meals: Planning helps avoid poor food choices.
Start Slowly: Beginners can begin with a 12-hour fast that includes sleep time.

Comparison of Intermittent Fasting Methods


Feature	16/8 Method	5:2 Diet	Eat-Stop-Eat	Alternate-Day Fasting
Time Frame	Fast daily for 16 hours, eat within an 8-hour window.	Eat normally 5 days, restrict calories 2 non-consecutive days.	Fast for 24 hours, once or twice per week.	Fast (or modify fast) every other day.
Calorie Restriction	Calorie intake is naturally reduced due to shorter eating window.	Restrict to 500-600 calories on fasting days.	No calories on fasting day.	Restrict or eliminate calories on fasting days.
Best For	Beginners, flexible schedules, and those who dislike breakfast.	Individuals who prefer specific calorie-restricted days.	Experienced fasters looking for a weekly reset.	Advanced users looking for a more intense regimen.
Sustainability	Very high, considered a lifestyle change by many.	Moderate, requires planning around fasting days.	Low for beginners, side effects can be intense initially.	Low, can be challenging and extreme for many.

Who Should Not Try Intermittent Fasting?

IF is generally safe for healthy adults but not for everyone. Consult a healthcare professional before starting, especially if you are:

Pregnant or breastfeeding
Have a history of eating disorders
Under 18
Have diabetes, particularly type 1
Have underlying medical conditions like low blood pressure

Conclusion: Finding the Right Rules for You

Intermittent fasting is a flexible eating pattern focused on when you eat. Success depends on finding a method that fits your lifestyle. Prioritize nutrient-rich foods during eating windows and stay hydrated while fasting. With planning and consistency, IF can be sustainable. Always listen to your body and consult a doctor to ensure it's right for you.

For more information on IF and its benefits, refer to resources from Johns Hopkins Medicine.

Frequently Asked Questions

During your fasting period, you can drink calorie-free beverages like water, black coffee, unsweetened tea, and plain sparkling water.

No, skipping breakfast is a common practice with methods like the 16/8 schedule and is not unhealthy, as long as you consume a balanced, nutritious diet during your eating window.

Intermittent fasting is not known to cause significant muscle loss, especially when combined with a balanced diet high in protein and regular strength training. In fact, some studies show it can help preserve muscle mass while burning fat.

Beginners can start with a 12-hour fasting window, which often includes the time you are asleep. As your body adapts, you can gradually increase the fasting duration.

The consensus is to avoid artificial sweeteners, as they can potentially trigger an insulin response and break your fast. Stick to truly calorie-free options.

Initial side effects can include hunger, headaches, irritability, and fatigue, as your body adjusts to the new eating pattern. These symptoms typically subside within a few weeks.

The best method depends on your lifestyle. The 16/8 method is great for beginners, while the 5:2 diet offers flexibility with two restricted-calorie days. Experiment with different approaches to find what is most sustainable for you.