Intermittent Fasting and Weight Loss

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What is Intermittent Fasting?

guest post by registered dietitian, susan macfarlane

Fasting has been an integral part of many different religions for centuries. Fasting has gained popularity more recently among scientists and weight-loss enthusiasts alike for its touted benefits of improving insulin sensitivity, delaying aging, improving cardiovascular health, and reducing body weight. However, fasting may not be as risk-free as purported, with the potential for disordered eating, negative metabolic adaptations, and nutrient deficiencies being cited as potential pitfalls of this latest dietary fad.

Various approaches to fasting have gained popularity in mainstream media, including the 5:2 diet (5 days of “normal” eating with 2 days of calorie restriction), alternate day fasting, and intermittent fasting (daily food intake is restricted to a small window of time, for example, 12-8 pm, while restriction lasts anywhere between 12-24 hours). Although intermittent fasting is often promoted alongside a ketogenic diet, there are no official “rules” regarding what can be eaten during non-fast times.

The Science on Fasting

Given the popularity of fasting, one would expect there to be a vast amount of high-quality scientific studies to support it. However, as is often the case with emerging diet trends, there is a serious lack of quality research from which to make conclusions on the safety, effectiveness, and most-importantly, long-term outcomes of fasting protocols. Case in point, a 2018 systematic review and meta-analysis investigating the impact of fasting protocols on weight loss, cardiovascular health, and blood glucose control was only able to identify 6 randomized/pseudo-randomized trials that met inclusion criteria, and of these trials, only 3 were determined to have a high-quality design. Nonetheless, this meta-analysis did reveal some interesting findings that certainly peaked my interest as a dietitian:

  • Compared to calorie-restricted diets (which reduce calorie intake by ~25% per day), fasting protocols resulted in a similar rate of weight loss that was equally sustainable at 12 months follow-up.
  • Fasting protocols resulted in greater reductions of waist circumference and fat mass compared to calorie-restricted diet in 2 studies.
  • Insulin concentrations were significantly better in fasting protocols compared to calorie-restricted trials in 2 studies, although no differences in glucose and cholesterol levels were seen between the dietary approaches.
  • Ratings of quality of life were similar between calorie-restricted diets and fasting protocols in most studies.
  • Adverse outcomes, such as food preoccupation, headaches, and low- energy levels, were not different between fasting and calorie-restricted diets
  • Rates of adherence were similar between calorie-restricted and intermittent fasting protocols.


Collectively, these results suggest that fasting protocols are as effective as calorie-restricted diets when it comes to weight loss and may be a viable approach for the right individual. However, we are still left with many unanswered questions about fasting protocols, specifically:

  • Is the diet sustainable in the long-term (i.e. 5-10 years)?
  • Are there any weight rebound effects once the diet is stopped?
  • Are any nutrients at risk of deficiency?
  • Does the risk of disordered eating increase?

Who Should Try Intermittent Fasting

The one thing that everyone can (or should) agree on when it comes to weight loss is there is no “one size fits all” approach. Given that obesity is a chronic medical condition that has the potential to negatively impact social, emotional, and physical health, lifelong interventions that are clear, easy-to-follow, and effective are needed. Fasting protocols certainly may be one approach to weight management, but before signing-up, it’s important to determine if this diet change is right for you by considering the following:

Can you follow a fasting protocol long-term?

Weight loss occurs during fasting because there is an overall decrease in the number of calories that a person consumes. And with any significant weight loss, there is always a reduction in metabolism (a consequence of eating less food and having a lower body mass). If a weight loss protocol is stopped, and the person returns to eating the way he or she did before the intervention, weight rebound will inevitably occur. As such, it’s essential to think about the approach you will use to maintain lost weight, should you decide to stop fasting.

Will social obligations interfere with your ability to fast?

If your work requires you to wine and dine clients, or you maintain an active social life, you may find fasting more challenging to implement and sustain. (And any weight loss approach that interferes with social obligations is not one that is likely to be followed for very long.) That said, it is possible to adapt a fasting protocol to fit your social life by altering the day or time that you decide to fast.

Are you an eating model for others?

This consideration is arguably the most important since it can have long-term impacts on the eating habits of children. As any parent knows, children can easily pick up on and replicate our behaviour – both good and bad. If your child (who undoubtedly looks up to you) sees you skipping meals, they may believe that to be like you, they should also skip meals. In addition, they might consider the food they are eating is “bad” or feel like there is something wrong with them for needing meals when mom or dad do not. A good rule of thumb when adopting any dietary change is to consider if such an approach would be appropriate in children. If you wouldn’t want your child to eat this way, then you probably shouldn’t either. 😊

Do you have a history of disordered eating?

From the few studies I have seen, it appears that fasting doesn’t lead to disordered eating in the short-term, but there is a big question mark when it comes to long-term fasting and disordered eating. Given that fasting has the potential to lead to subjective/objective binges, the creation of food rules, and a reduced recognition/response to hunger and fullness cues, fasting should be avoided by anyone actively struggling with disordered eating or who has been diagnosed with an eating disorder in the past.

Using Cronometer To Track Your Fasts

Upgrade to Cronometer Gold to gain access to our Fasting feature which makes it easy to keep track of your fasting periods and chart them against biometrics, exercise and nutrient intake to help identify trends in your health. 


Harris L, Hamilton S, Azevedo LB, Olajide J, De Brún C, Waller G, et al. Intermittent fasting interventions for treatment of overweight and obesity in adults: a systematic review and meta-analysis. JBI Database System Rev Implement Rep. 2018 Feb;16(2):507-547. Abstract available from:

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