Intermittent Fasting Overview - Joel Prowting

Posted : 10 February 2017

​Read Health Adviser Joel Prowting's blog on Intermittent Fasting. What are your thoughts on Intermittent Fasting? Let us know.

​*Disclaimer: This blog was written and reflects the opinions and ideas of the author and not that of Bupa.


Intermittent fasting is a dietary strategy that can be used to help people lose and keep off excess body weight. As with any diet that creates a negative calorie balance, it can cause weight loss to occur. The question is, is it more effective for weight loss than daily calorie restriction? Are there any physiological advantages to it? And who could benefit the most from it?

Part one of this blog will explain what exactly Intermittent Fasting is, the different types that exist, the current science behind it and popular examples that you may have heard of already.

Part two of this blog will provide practical recommendations on how to do Intermittent Fasting and provide specific examples of IF for an average male and female.

What is intermittent fasting?

Dictionary definitions:

• Intermittent = “occurring at irregular intervals; not continuous or steady”

• Fasting = “abstain from all or some kinds of food or drink; to eat only sparingly or of certain kinds of food”

Fasting (in various forms) has been used for thousands of years for both religious and health reasons(1). A decade ago, the suggestion of using fasting as a weight loss tool would have been met with incredulous replies of “...but you’ll go into starvation mode!”, with the notion that if you didn’t eat regularly; fat storage (lipogenesis) would go into overdrive and actually result in weight gain. However, it is in fact overall calorie balance that dictates weight gain and loss(2). In recent years, popularity has grown for using fasting in a weight management context, with the introduction of strategies such as the “5:2 diet”.

Essentially, IF involves restricting energy intake for a pre-defined period of time, followed by ad libitum (as desired) periods of eating(3). By using this approach, a weekly calorie deficit (= weight loss) can be achieved by having a few days of significant calorie restriction as opposed to a small amount of calorie restriction each day (see diagram below). Within the last decade, numerous studies have been carried out in order to determine the effectiveness of Intermittent Fasting as a dietary strategy.






Types of Intermittent Fasting

The three main categories of Intermittent Fasting that exist within the scientific research are: alternate day fasting (ADF), whole day fasting (WDF) and time restricted fasting (TRF) (4).

1. Alternate day fasting = involves a 24 hour fasting period (0-25% of normal calorie intake) alternated with a 24 hour normal feeding period (ad libitum)

2. Whole day fasting = involves one to three 24 hour fasting periods (0-25% of normal calorie intake) during a week of otherwise normal calorie intake

3. Time restricted fasting = involves a fasting period of 16-20 hours and a feeding period of 4-8 hours each day

What does the research say?

A study by Klempel(5) and colleagues in 2012 concluded that calorie restriction achieved using Intermittent Fasting was an effective strategy for weight loss and reducing Coronary Heart Disease (CHD) risk in obese women. In 2014, a study by Barnosky(6) and colleagues compared Intermittent Fasting to daily calorie restriction, concluding that: “...preliminary findings show promise for the use of Intermittent Fasting and Alternate Day Fasting as alternatives to daily calorie restriction for weight loss and type 2 diabetes risk reduction in overweight and obese populations...”.

An extensive review published recently by Seimon(3) and colleagues in 2015 sought to find out whether Intermittent Fasting type diets provided any physiological benefits when compared to daily calorie restriction. They looked at the outcomes of 40 different studies and found that there were no significant differences between Intermittent Fasting and daily calorie restriction in terms of weight loss, fat loss, waist circumference reduction and improvements in glucose homeostasis (blood sugar control).

Strengths of Intermittent Fasting

A strength of IF is that it in most people, it does not lead to disordered eating and overconsumption on non-restricted days(7), meaning that a negative weekly calorie balance can be achieved during free living conditions (or people’s day to day routines). The number of people that are able to stick with Intermittent Fasting is similar to that of daily calorie restriction, meaning that it can be used as an alternative for individuals who find daily calorie restriction difficult to stick to(8). In practice, this means that somebody can lose weight without having to drastically alter the types of food that they are eating.

Weaknesses of Intermittent Fasting

The study by Harvie and colleagues in 2013 noted that 12.6% of the subjects (18 out of 142 women) could not stick to the IF protocol, meaning that this approach may not work for everybody. Furthermore, Intermittent Fasting does not require (although it certainly helps!) individuals to eat healthier foods (i.e. fruits, vegetables, lean proteins, fibre etc) as is often the case with many usual dietary strategies. This could lead to nutrient deficiencies developing, and may compromise health despite improved body composition (on the flip side, people that are picky eaters may prefer IF for this very reason!). Finally, at present there isn’t very much data that looks at the effectiveness of IF over a long term period (>6 months), meaning that it is hard to say whether this approach can be sustained for long durations(8).

Popular Intermittent Fasting Approaches

Arguably, the most well known form of Intermittent Fasting is the “5/2 Diet” (popularised by Michael Mosley during the Horizon Documentary “Eat, Fast and Live Longer”). The 5/2 diet is a version of “Whole Day Fasting” (WDF), where a person eats normally for 5 days of the week, and then has 2 non-consecutive fasting days (<500 kcals) in the week. For an average male (2500kcals required per day), this would equate to a weekly calorie deficit of approximately 4000kcals, or around 1.1lbs (0.5kg) of fat loss per week*. This would be equivalent to reducing daily calorie intake by 571kcals per day. The 5/2 diet can be modified in a number of ways to influence rates of fat loss, with examples shown in part 2 of this blog post.

“Alternate Day Fasting” (ADF) has been popularised by a book created by researcher Dr Krista Varaday (University of Illinois) called “The Every Other Day Diet”. It is similar to the 5/2 diet, involving fasting and feeding days, however these days are alternated. For example, Monday = normal day, Tuesday = 500kcal, Wednesday = normal day... etc. This strategy has the potential to cause faster rates of fat loss because there a more fasting days. So for an average male (2500kcal required per day), this would equate to a weekly calorie deficit of approximately 6000kcals, or around 1.7lbs (0.78kg) of fat loss per week. However, sticking to this diet may be more difficult for some individuals because of the larger calorie deficit.

Finally, “Time Restricted Fasting” (TRF) is a popular technique within the fitness/exercise community. The most well known strategies include the “Warrior Diet” (18h of fasting/4h of feeding) and “Leangains diet” (16h of fasting/8h of feeding). By limiting the eating window to a certain amount of time within a day, it becomes much more difficult to over consume calories. For example, if eating between 2pm and 10pm, 3 medium/large sized meals are more likely to keep you full (i.e. less snacking between/after meal times). Whereas if eating your first meal at 7am and then eating your last meal at 6pm (11 hour window), it is more likely that an individual will get hungry between meals, leading to snacking and increased overall calorie intake. TRF may not work effectively for everybody, as it is still possible to eat too many calories within the small time frame. For some individuals however, it can help them to deal with the increased appetite often associated with dieting.


The take home message is that different forms of Intermittent Fasting have demonstrated their effectiveness in the scientific literature, and thus can be chosen on the basis of personal preference(9). As the study by Harvie and colleagues in 2013 succinctly put it: “No single dietary approach will be appropriate and feasible for all, and there is unlikely to be a panacea given the complexity of weight management, but the addition of a further choice of intervention may be helpful.”

In summary:

• Intermittent Fasting (IF) is a weight loss strategy that involves fasting for a certain time period, followed by eating normally for a certain time period,

• The three most common/researched forms of IF are: Alternate Day Fasting (ADF), Whole Day Fasting (WDF) and Time Restricted Fasting (TRF),

• Most of the research currently supports the use of Intermittent Fasting as an effective dietary strategy, demonstrating an equivalent effect on weight loss and health markers when compared to continuous daily calorie restriction,

• Using any of the aforementioned strategies can result in fat loss, which provides individuals with a choice when considering which strategy to use,

• Individuals should use the strategy that they find the easiest to stick to in the long term.


* Appendix

3500kcal deficit = 1lb fat loss (10)
4000kcal/3500kcal = 1.14lb fat loss per week
6000kcal/3500kcal = 1.71lb fat loss per week



1) Sweileh, N., Schnitzler, A., Hunter, G. R., & Davis, B. (1992). Body composition and energy metabolism in resting and exercising muslims during Ramadan fast. The Journal of Sports Medicine and Physical Fitness, 32(2), 156-163.
2) Redman, L. M., Heilbronn, L. K., Martin, C. K., De Jonge, L., Williamson, D. A., Delany, J. P., & Ravussin, E. (2009). Metabolic and behavioral compensations in response to caloric restriction: implications for the maintenance of weight loss. PloS One, 4(2), e4377.
3) Seimon, R. V., Roekenes, J. A., Zibellini, J., Zhu, B., Gibson, A. A., Hills, A. P., ... & Sainsbury, A. (2015). Do intermittent diets provide physiological benefits over continuous diets for weight loss? A systematic review of clinical trials. Molecular and Cellular Endocrinology, (418), 153-172.
4) Tinsley, G. M., & La Bounty, P. M. (2015). Effects of intermittent fasting on body composition and clinical health markers in humans. Nutrition Reviews, 73(10), 661-674.
5) Klempel, M. C., Kroeger, C. M., Bhutani, S., Trepanowski, J. F., & Varady, K. A. (2012). Intermittent fasting combined with calorie restriction is effective for weight loss and cardio-protection in obese women. Nutrition Journal, 11(1), 1.
6) Barnosky, A. R., Hoddy, K. K., Unterman, T. G., & Varady, K. A. (2014). Intermittent fasting vs daily calorie restriction for type 2 diabetes prevention: a review of human findings. Translational Research, 164(4), 302-311.
7) Harvie, M., Wright, C., Pegington, M., McMullan, D., Mitchell, E., Martin, B., ... & Camandola, S. (2013). The effect of intermittent energy and carbohydrate restriction v. daily energy restriction on weight loss and metabolic disease risk markers in overweight women. British Journal of Nutrition, 110(08), 1534-1547.
8) Headland, M., Clifton, P. M., Carter, S., & Keogh, J. B. (2016). Weight-Loss Outcomes: A Systematic Review and Meta-Analysis of Intermittent Energy Restriction Trials Lasting a Minimum of 6 Months. Nutrients, 8(6), 354.
9) Aragon, A. (2015) Intermittent fasting and body composition: After over a decade of research, where are we today? Alan Aragon’s Research Review, December; p13-15.
10) Bortz, W. M. (1968). Predictability of weight loss. The Journal of the American Medical Association, 204(2), 101-105.



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