What is the diet where you only eat during certain hours? A guide to intermittent fasting

November 23, 2025 •

4 min read

According to Johns Hopkins Medicine, after hours without food, the body exhausts its sugar stores and starts burning fat—a process called metabolic switching. This fundamental biological shift is the basis for what is the diet where you only eat during certain hours, more commonly known as intermittent fasting (IF).

Quick Summary

Intermittent fasting is an eating pattern that cycles between voluntary periods of eating and fasting. It focuses on when to eat rather than what, using various structured schedules.

Key Points

Metabolic Switch: The core principle of IF is triggering your body to shift from burning sugar to stored fat for energy.
Flexible Schedules: Popular IF methods like the 16:8, 5:2, and Alternate-Day Fasting offer flexible approaches to fit different lifestyles.
Focus on Food Quality: The benefits of intermittent fasting are maximized by consuming nutrient-dense, whole foods during your eating window.
Not for Everyone: Certain populations, including pregnant women, those with diabetes, and a history of eating disorders, should avoid IF or consult a doctor.
Consult a Professional: Always speak with a healthcare provider before starting IF to ensure it's a safe and appropriate option for your health.
Beyond Weight Loss: Potential benefits of IF extend beyond weight management to include improved insulin sensitivity, cellular repair, and heart health markers.

What Is Intermittent Fasting?

Intermittent fasting (IF) is an eating pattern that cycles between periods of eating and voluntary fasting. Unlike many diets that dictate what you should eat, IF focuses on when you eat. By condensing your daily food intake into a specific window of time, and abstaining from food for the remaining hours, you prompt your body to change how it uses energy. Time-restricted eating (TRE) is the most popular form of IF, where you restrict all calorie intake to a consistent window of 6 to 12 hours each day. The practice has gained significant attention not only for weight management but also for its potential effects on metabolic health, inflammation, and cellular processes.

The Science Behind Metabolic Switching

The key to understanding how intermittent fasting works lies in metabolic switching. After several hours without food, your body depletes its stores of readily available glucose (sugar) and begins burning stored fat for energy instead. This change in fuel source triggers numerous physiological responses that can benefit overall health. For many people, a normal eating pattern of three meals plus snacks throughout the day never allows for this switch to occur, keeping their body running constantly on glucose. IF works by prolonging the period without food, forcing the metabolic switch to activate more regularly. This is not just about weight loss; this process also activates cellular repair mechanisms, including autophagy, where the body cleans out old and damaged cells to promote regeneration.

Popular Intermittent Fasting Schedules

Numerous IF schedules exist, each offering a different balance of fasting and eating. Your choice should depend on your lifestyle, goals, and how your body responds.

The 16:8 Method

Also known as time-restricted eating (TRE), this is one of the most common and manageable forms of IF.

How it works: Fast for 16 hours and consume all your calories within an 8-hour window. A popular schedule is eating between 12 p.m. and 8 p.m., which involves skipping breakfast. Many find this manageable as a large portion of the fasting happens during sleep.
What to eat: During the 8-hour window, focus on nutrient-dense meals with plenty of whole foods, lean proteins, healthy fats, and fiber to promote satiety.

The 5:2 Diet

This approach involves a more flexible weekly schedule rather than daily time-restriction.

How it works: Eat normally for five days of the week. On the other two non-consecutive days, restrict calorie intake to a very low amount, typically 500-600 calories.
What to eat: On fasting days, choose high-fiber and high-protein foods like lean protein and vegetables to help manage hunger.

Alternate-Day Fasting (ADF)

As the name suggests, this method involves alternating between days of eating normally and days of fasting.

How it works: Follow your normal diet one day, then either completely fast or severely restrict calories (to about 25% of your normal intake) the next day.
What to eat: Adherence to ADF can be challenging. Focus on consuming healthy, satiating foods during non-fasting days to prevent overeating.

Comparison of Intermittent Fasting Methods


Method	Fasting Duration	Eating Window	Best Suited For	Potential Benefits
16:8 (TRE)	16 hours daily	8 hours daily	Beginners, daily routine	Weight loss, metabolic health
5:2 Diet	2 low-calorie days/week	5 normal eating days/week	Less daily restriction, flexibility	Weight loss, insulin sensitivity
ADF	Every other day	Every other day	Weight loss focus, experienced fasters	Significant weight loss, metabolic improvement

Risks and Considerations Before Starting

While intermittent fasting can be beneficial, it is not suitable for everyone and comes with potential side effects. Before beginning any new dietary regimen, it is crucial to consult with a healthcare professional, especially if you have pre-existing health conditions.

Potential side effects: Common side effects during the adjustment period include hunger pangs, headaches, fatigue, dizziness, irritability, and difficulty concentrating.
Risk of disordered eating: Focusing heavily on eating schedules and ignoring natural hunger cues can be a slippery slope toward disordered eating for susceptible individuals.
Impact on specific populations: IF is not recommended for children and teens under 18, pregnant or breastfeeding women, or individuals with a history of eating disorders or type 1 diabetes.
Cardiovascular concerns: Some preliminary studies have raised questions about a potential link between certain time-restricted eating patterns and increased cardiovascular risk, highlighting the need for more long-term research.

How to Optimize Your Eating Window

For intermittent fasting to be effective and sustainable, the quality of your food during the eating period is paramount. This is not a license to eat junk food. Maximizing the benefits requires a focus on nutrient-dense, whole foods.

Prioritize lean protein: Protein promotes satiety, helping to manage hunger and preserve lean muscle mass. Examples include fish, poultry, beans, and lentils.
Include healthy fats: Found in avocados, nuts, seeds, and olive oil, healthy fats are crucial for hormone production and nutrient absorption.
Eat complex carbohydrates and fiber: Whole grains, fruits, and vegetables provide sustained energy and essential fiber for digestive health.
Stay hydrated: Drink plenty of water throughout the day, including the fasting period. Unsweetened coffee and tea are also generally acceptable during a fast.

Conclusion: Is Intermittent Fasting Right for You?

Intermittent fasting, particularly time-restricted eating, is a popular dietary pattern that offers potential benefits for weight loss and metabolic health by leveraging the body's natural metabolic cycles. It's a method that works for many by simplifying their eating routine without rigid calorie counting. However, it requires careful consideration of individual health, lifestyle, and potential risks. It is essential to choose a schedule that fits your life, not the other way around. Most importantly, consult with a doctor or a registered dietitian before starting to ensure the approach is safe and appropriate for your specific needs, focusing on fueling your body with healthy, nutrient-dense foods during your eating windows.

For more in-depth information, you can read the resource provided by Johns Hopkins Medicine on Intermittent Fasting.

Frequently Asked Questions

The main difference is that intermittent fasting focuses on when you eat, rather than restricting what you eat. This eating pattern cycles between periods of voluntary eating and fasting, whereas traditional diets often focus on daily calorie restriction or specific food types.

During the fasting window, you can typically drink calorie-free beverages, such as water, black coffee, and unsweetened tea. Staying hydrated is important throughout the entire day, but these beverages are generally acceptable during a fast.

While any calorie-restrictive diet carries some risk of muscle loss, including adequate lean protein during your eating window and maintaining physical activity can help preserve muscle mass. Some studies even suggest IF can support fat loss while maintaining muscle.

The time to see results varies depending on the individual, the chosen schedule, and overall dietary habits. Some people may notice changes in a few weeks, while others may require a few months to see significant weight loss.

The 16:8 method is often considered the most beginner-friendly. It involves fasting for 16 hours, with a large portion of that time spent sleeping, and eating within an 8-hour window, making it a relatively simple adjustment.

Yes, it can be dangerous for certain individuals. It is not recommended for children, pregnant or breastfeeding women, people with type 1 diabetes, or those with a history of eating disorders. Always consult a healthcare provider before beginning.

With IF, calorie counting is not strictly required. The limited eating window often leads to a natural reduction in overall calorie intake. However, it is important to eat a balanced, nutritious diet and avoid overindulging in unhealthy foods during your eating period.