Fasting vs. Intermittent Energy Restriction: Understanding the Differences and Benefits

Intermittent fasting (IF) and intermittent energy restriction (IER) are two popular approaches for weight loss, metabolic health, and overall wellness. While both involve controlled eating patterns, they differ in how they manage calorie intake and fasting windows. In this blog post, we’ll explore the differences between fasting and intermittent energy restriction, their potential benefits, and how to determine which approach may be best for your health goals.

1. What is Intermittent Fasting (IF)?

Intermittent fasting (IF) involves cycling between periods of eating and fasting without necessarily focusing on calorie intake during eating windows. The primary idea is to restrict when you eat, not how much you eat. IF comes in various forms, including the 16:8 method, where you fast for 16 hours and eat during an 8-hour window, and alternate-day fasting, where you alternate between fasting and non-fasting days.

a. How Intermittent Fasting Works

Intermittent fasting helps shift the body from using glucose (sugar) as its primary energy source to burning stored fat for fuel, a state known as ketosis. This metabolic shift can promote weight loss, reduce inflammation, and improve metabolic markers like insulin sensitivity.

Key Benefits of IF:

• Enhances fat burning by promoting ketosis
• Improves insulin sensitivity and blood sugar regulation
• Reduces inflammation and oxidative stress
• May promote cellular repair and autophagy (a process that clears out damaged cells)

A study in The New England Journal of Medicine found that intermittent fasting can help reduce body fat and improve metabolic health by optimizing insulin sensitivity and promoting fat oxidation .

2. What is Intermittent Energy Restriction (IER)?

Intermittent energy restriction (IER) is another eating pattern that focuses more on calorie intake rather than specific fasting windows. IER involves alternating between days of normal calorie consumption and days of significantly reduced caloric intake, often referred to as "fasting days" or "low-calorie days."

For example, the 5:2 method of intermittent energy restriction involves eating normally for five days of the week and consuming only 20-25% of your typical daily calories (around 500–600 calories) on two non-consecutive days.

a. How Intermittent Energy Restriction Works

On energy-restricted days, calorie intake is sharply reduced, which puts the body in a calorie deficit without the complete absence of food. The calorie reduction still triggers some of the same metabolic processes seen in fasting, such as fat burning and improved insulin sensitivity, without prolonged periods of complete food abstinence.

Key Benefits of IER:

• Helps manage calorie intake while still allowing some food on restricted days
• Promotes fat loss by creating a sustained calorie deficit
• May improve metabolic health markers, such as cholesterol and triglycerides
• Provides flexibility in managing hunger and cravings on fasting days

Research published in Obesity Reviews shows that intermittent energy restriction can lead to similar weight loss and health benefits as continuous calorie restriction, with greater flexibility for individuals .

3. Comparing Fasting and Intermittent Energy Restriction

While both intermittent fasting and intermittent energy restriction can lead to weight loss and metabolic improvements, they differ in how they approach food intake, fasting periods, and overall flexibility.

a. Fasting Focuses on Timing

Intermittent fasting is all about when you eat rather than how much. As long as you stay within your eating window, calorie counting is less emphasized. This can make it simpler for people who prefer not to track calories but want the benefits of fasting, including improved metabolism and fat burning.

b. IER Focuses on Caloric Intake

Intermittent energy restriction, on the other hand, focuses more on how much you eat on specific days. By reducing calorie intake on "fasting days," IER creates a calorie deficit without requiring strict adherence to specific eating windows. This can be more appealing for individuals who want the flexibility to eat smaller amounts of food on their fasting days rather than completely abstaining.

c. Flexibility and Lifestyle Suitability

• Intermittent Fasting: This approach is ideal for people who can comfortably fast for long periods or skip meals without feeling overly hungry. It requires more structure around meal timing and may require adjustments based on work schedules or social events.

• Intermittent Energy Restriction: IER is better suited for those who prefer to reduce calories rather than avoid food entirely. It allows flexibility with meal timing and offers the psychological comfort of still being able to eat on restricted days, even if in smaller portions.

4. Health Benefits of Both Approaches

While both intermittent fasting and intermittent energy restriction are effective for weight management, they also offer other health benefits. Let’s explore how both methods positively impact health beyond weight loss.

a. Improved Metabolic Health

Both IF and IER can improve insulin sensitivity, lower blood sugar levels, and reduce the risk of type 2 diabetes. They help regulate blood sugar by promoting fat metabolism and reducing reliance on glucose for energy.

b. Heart Health Benefits

Intermittent fasting and intermittent energy restriction both support heart health by lowering cholesterol and triglyceride levels, reducing blood pressure, and decreasing markers of inflammation. A study in The American Journal of Clinical Nutrition found that both fasting and energy-restricted diets led to significant improvements in cardiovascular health markers .

c. Cellular Repair and Anti-Aging

Intermittent fasting, in particular, is known for promoting autophagy—a process where the body removes damaged cells and regenerates new ones. This process supports cellular health, reduces inflammation, and may slow the aging process. While IER may not promote autophagy to the same extent, its calorie-restriction benefits can still enhance overall cellular function and longevity.

5. Choosing the Best Approach for You

When deciding between intermittent fasting and intermittent energy restriction, consider your lifestyle, health goals, and personal preferences. Here are a few factors to help guide your decision:

• Your Daily Schedule: If you have a busy or unpredictable schedule, intermittent energy restriction might offer more flexibility since you don’t have to adhere to strict fasting windows.

• Hunger Tolerance: If you struggle with extended periods of fasting but still want the metabolic benefits, IER may be easier since you can eat small amounts on "fasting days."

• Weight Loss Goals: Both methods are effective for weight loss, but if you prefer not to count calories, intermittent fasting may be a better fit. On the other hand, if tracking food intake helps you stay on target, intermittent energy restriction could be more suitable.

• Mental and Physical Adaptation: Intermittent fasting may take more time to adapt to, especially when adjusting to longer fasting periods. IER may feel less restrictive since you are not completely avoiding food, making it easier for those who find fasting mentally challenging.

Conclusion

Both intermittent fasting and intermittent energy restriction offer powerful tools for improving health, promoting weight loss, and boosting metabolic function. Whether you prefer fasting windows or calorie-restricted days, both methods can fit into a balanced lifestyle and provide numerous health benefits. The key is finding the approach that aligns with your personal preferences and health goals. Regardless of which method you choose, staying consistent and mindful of your body’s needs is crucial for long-term success.

Sources:

1. Longo, V. D., & Panda, S. (2016). Fasting and Intermittent Energy Restriction for Metabolic Health. Cell Metabolism, 23(6), 1048–1059.
2. Mattson, M. P. (2017). Intermittent Fasting and Energy Restriction for Weight Loss: Comparing the Two Methods. Annual Review of Nutrition, 37, 395-418.
3. Harvie, M., & Howell, A. (2017). The Role of Intermittent Energy Restriction in Weight Loss and Metabolic Health. Obesity Reviews, 18(2), 51-58.
4. Patterson, R. E., & Sears, D. D. (2017). Metabolic Effects of Intermittent Fasting and Energy Restriction. Annual Review of Nutrition, 37, 371-393.
5. Anton, S. D., Moehl, K., & Donahoo, W. T. (2018). Intermittent Fasting and Caloric Restriction: Health Benefits of Two Approaches. Frontiers in Endocrinology, 9, 2103.