If you have been trying to lose weight you may have heard a lot about intermittent fasting lately.  Is this the new way to lose weight? How does it differ from reducing calories?

Calorie restriction is reducing average daily caloric intake below what is typical, without deficiency of essential nutrients.  With intermittent fasting, a person does not eat during certain times of the day or week.

How does intermittent fasting work?

Unlike some of the other popular diets for weight loss, intermittent fasting does not focus on what you eat but rather the timing and frequency of when you eat. It is described as “eating patterns in which individuals go extended time periods (e.g., 16–48 hours) with little or no energy intake, with intervening periods of normal food intake, on a recurring basis.”1  Essentially, you are reducing the number of hours in a day or throughout the week that you are eating. Then, on “feast days” you will make up for some of the calories missed on fasting days.

There are different versions of the diet including:

Alternate Day Fasting: zero calorie intake on fasting days (water, coffee, and zero-calorie beverages are allowed).  Eating is unrestricted every other day.

5:2 Fasting: Calories are limited to 500 for 2 days a week.  For the other 5 days of the week, a normal diet is resumed.

Time Restricted Fasting: window of food intake <8-10 hours intervals per day but does not necessarily have calorie-restriction.

How does this compare to simply restricting calories for weight loss?

In a 2017 randomized clinical trial of alternate day fasting and calorie restriction (25% daily calorie restriction, approximately 1,500 calories per day) produced similar amounts of weight loss (5-6%) over 52 weeks in 100 adult participants who were either overweight or obese.2  In studies less than 12 weeks long, this type of fasting has also shown similar weight reductions (4-8%) when compared to calorie restriction.3

The 5:2 fasting diet has been shown to have the same results of weight loss (4-8%) in adult who are overweight or obese when compared to calorie restriction in short and long-term trials.4,5

A review of 9 trials ranging from 5-12 weeks in length of time restricted found no significant difference in weight loss (3-4%) compared to a calorie restricted diet.4

The main reason why people lose weight with intermittent fast is because it helps reduce daily caloric intake. But what about days or times when someone is not fasting?  Wouldn’t someone indulge and eat more calories? Findings from trials indicate that alternate day and time restricted fasting reduce overall caloric intake by approximately 10-30% relative to baseline.4  Study participants typically only consume an extra 10-15% of their calorie needs (approx. 200-300 kcal) on feast days relative to their calculated energy needs.  The same appears to occur with time restricted eating, most individuals are not able to consume the amount of food they previously ate within their 4-8 hour eating window.4

To-date, there are no well-controlled clinical trials supporting the benefits of time restricted fasting compared to calorie restriction. Therefore, future studies will need to be conducted to determine the effectiveness of time restriction as an intervention for weight loss.

A closer look at calorie restriction

The National Institutes of Health conducted a multicenter, randomized controlled trial on calorie restriction.  In this study 218 healthy young and middle-aged, normal-weight or slightly overweight adults were randomly divided into two groups. People in the experimental group were told to follow a calorie-restriction diet for 2 years, while those in the control group followed their usual diet. Participants following a calorie restricted diet reduced their intake by 12% on average. Those on the calorie restricted diet had on average a 10% weight loss over 2 years. Compared to participants in the control group, those in the calorie-restriction group had reduced risk factors (lower blood pressure and lower cholesterol) for age-related diseases such as diabetes, heart disease, and stroke.7

The bottom line

You may be interested in trying either approach for weight loss as both have been proven to help with weight reduction.  Whichever eating pattern you try, it’s important to make sure it provides you with a safe level of nutrition. You ultimately want to find an eating approach that not only works well for you, but also one that you find to be sustainable.  Talk with your healthcare provider about the benefits and risks based on your personal medical history before you make any significant changes to your eating pattern.


1.   Rynders CA, Thomas EA, Zaman A, Pan Z, Catenacci VA, Melanson EL. Effectiveness of Intermittent Fasting and Time-Restricted Feeding Compared to Continuous Energy Restriction for Weight Loss. Nutrients. 2019;11(10):2442. Published 2019 Oct 14. doi:10.3390/nu11102442

2.   Trepanowski JF, Kroeger CM, Barnosky A, et al. Effect of Alternate-Day Fasting on Weight Loss, Weight Maintenance, and Cardioprotection Among Metabolically Healthy Obese Adults: A Randomized Clinical Trial. JAMA Intern Med. 2017;177(7):930-938. doi:10.1001/jamainternmed.2017.0936

3.   Catenacci VA, Pan Z, Ostendorf D, et al. A randomized pilot study comparing zero-calorie alternate-day fasting to daily caloric restriction in adults with obesity. Obesity (Silver Spring). 2016;24(9):1874-1883. doi:10.1002/oby.21581

4.   Varady KA, Cienfuegos S, Ezpeleta M, Gabel K. Clinical application of intermittent fasting for weight loss: progress and future directions. Nat Rev Endocrinol. 2022;18(5):309-321. doi:10.1038/s41574-022-00638-x

5.   Sundfør TM, Svendsen M, Tonstad S. Effect of intermittent versus continuous energy restriction on weight loss, maintenance and cardiometabolic risk: A randomized 1-year trial. Nutr Metab Cardiovasc Dis. 2018;28(7):698-706. doi:10.1016/j.numecd.2018.03.009

6.   Lowe, Dylan A et al. “Effects of Time-Restricted Eating on Weight Loss and Other Metabolic Parameters in Women and Men With Overweight and Obesity: The TREAT Randomized Clinical Trial.” JAMA internal medicine vol. 180,11 (2020): 1491-1499. doi:10.1001/jamainternmed.2020.4153

7.   National Institute of Aging.  Calorie Restriction and Fasting Diets. Available at: https://www.nia.nih.gov/news/calorie-restriction-and-fasting-diets-what-do-we-know