What's the Deal with Intermittent Fasting?

What is it? 

Intermittent fasting (IF) is a diet trend that has been around for some time, and while some fad diets come and go (we are looking at you celery juice and the master cleanse), IF has not made its way out the door quite yet. IF is a style of eating that focuses more on when you eat versus what you eat. There are several different types which all ultimately lead to a window of eating and window of fasting. The details of the timing of these windows can vary. We will discuss some of the more popular types along with some benefits and contraindications.

History of Fasting:

The concept of fasting is not new. Humans have been fasting since early times due to food scarcity or religious reasons. 

When food was not available, humans had no choice but to fast. 

The Greeks fasted as a way to get rid of illness, and they also believed that taking food may allow the entry of demonic forces. Other religions like Islam, Christinatity and Buddhism believe fasting is a way to purify the soul. Muslims fast during Ramadan to practice self-restraint and self-reflection.

In today’s world, IF is a moderate way of incorporating fasting into one’s lifestyle. While it may not be followed due to food scarcity or religious reasons, a lot of people have an interest in it to feel and look better. It has gained a large popularity among those looking to lose weight.

Types of Intermittent Fasting

  • Alternate day fasting: someone will alternate the days they eat their regular calories as a normal day and fast with water only the following day. So, in a week it would look like: day 1- eat normal, day 2- fast, day 3- eat normal, day 4- fast… repeat. This style may be hard for people to adhere to, especially if they are active. A more realistic version of this may be what is called a Modified alternate day fasting. Every other day, either 500 calories or 25% the amount of normal caloric intake is consumed. On non-fasting days, a regular eating pattern is followed. So, in a week it would be: day 1- eat normal, day 2- eat 25% of regular calories, day 3- eat normal, day 4- eat 25% of regular calories… repeat.
  • 5:2 (Twice-a-week) method: On 5 days of the week, a regular eating pattern is followed. On the other 2 days, which are the fasting days, calories are limited to 500. This can be achieved by either consuming one 500 calorie meal, or 2 meals that are between 200-300 calories each. The 2 fasting days can be on any 2 days of the week, as long as there is at least 1 non-fasting day between them.
  • Time-restricted (16:8 and 14:10): This is the most common form of IF. There is a window of time that one should fast, or be able to consume calories throughout the day. The 2 most common include the 16:8 which allows an 8 hour window to eat, and 14:10 which allows a 10 hour window to eat. There is no set calorie restriction with this style of IF, the focus is simply to eat only within the eating window, which in theory would lower the amount of daily calories eaten in a day since the amount of time you are eating is lowered.
  • The 24-hour fast: No food is consumed for a full 24 hours either 1 or 2 days during the week. On non-fasting days, a regular eating pattern is followed.

So why would someone want to IF?

One of the most common reasons is to lose weight. Unlike the ancient times when food was scarce and drive throughs were nonexistent, humans now have 24/7 access to food. We can pretty much eat anytime we want. Our food industry has also come a long way and we have the luxury of being able to fill our pantries with packaged foods, or stock our freezers with ready-to-eat meals and other frozen items. 

Other people look to IF as a way to limit mindless eating. While having 24/7 access to food sounds great in theory, it also requires a lot of self control. Eating multiple snacks throughout the day and eating late at night may not solely contribute to weight gain, but consuming excess calories will. Therefore, assigning a time window when eating is allowed may help reduce overall calorie consumption if done properly.  

Let’s look at some existing research on IF!

IF and Exercise:

Exercise is an important thing to consider when looking at IF. One reason for this is that pre- and post-workout nutrition is vital for good performance and desirable results. Additionally, both the types of food and the timing of meals are important when following an exercise regimen. So, one may ask, “Is it safe to exercise when IF and can I still get good results?”  

Research is conflicting when looking at the effects of IF and exercise and more research is needed to further assess how safe and effective it is. When looking at a review of the literature, it was found that IF either decreased performance or had no effect on it. However, most studies have been done in sedentary or untrained individuals so it cannot be applied to trained athletes. In trained and untrained subjects, both body weight and body fat percentage decreased, indicating that IF had little effect on weight loss and reduction in body fat. 

IF and Weight Loss:

If you google “Intermittent Fasting” on the internet, you will probably come across multiple articles associating it with weight loss. That’s because weight loss is the number one reason why people are interested in it. But does IF really help with weight loss and is it sustainable? 

In a systematic review of 27 research trials looking at IF and weight loss in obese individuals, it was found that IF resulted in a weight loss of 0.8% - 13% of baseline body weight, regardless of changes in overall calorie intake. However, all studies were short in duration. Therefore it can be concluded that while IF is a successful weightloss intervention for obese individuals, more studies need to be done to further assess the long-term sustainability and health effects of IF.

IF for Diabetes Management:

Type II diabetes mellitus (T2DM)  is a disease which results in the inability for the body to properly use insulin. Individuals who are overweight and inactive are at higher risk for developing diabetes. As discussed above, research is promising for showing that IF may result in weight loss, which would also help reduce the risk of developing type II diabetes. 

Several studies have been done to study the effects of IF on T2DM. A literature review done on 17 studies found that IF may be an effective non-medicinal treatment option for those with T2DM. Several studies reported that IF improved body composition by resulting in weight loss and a reduction of body fat. An improvement in overall body composition also resulted in better regulation of the hormone leptin, which helps in weight control by suppressing appetite and increasing energy expenditure. Additionally, it improved insulin sensitivity. Since obesity, excess calorie intake and poor blood sugar control are risk factors for developing T2DM, an improvement in these factors are beneficial for both at-risk individuals, and as a form of management for those who are diabetic.

While IF can help to reduce insulin resistance and better control blood sugars, patients taking medications that help to control blood sugar should be closely monitored by their physician to avoid any hypoglycemic events. Additionally, physicians may have to make appropriate adjustments for their medications, especially on fasting days. 

Can Anyone do IF?

While IF could be a good option to restrict calories or help with blood sugar control, it may not be suitable for everyone depending on one’s medical history and lifestyle. Examples of people who may not be a good match for IF could be people with poorly controlled blood sugar levels such as diabetics, pregnant or breastfeeding women, people with history or tendencies of disordered eating, athletes who are looking to focus on performance and/or muscle gain.

A Dietitian’s Thoughts on IF:

As with every other diet trend on the market, there are pros and cons. What might work for someone, may not work for someone else.

Compared to the other fad diets, IF does not require a severe restriction of calories or eliminate any food groups, which can be detrimental to health and is often impossible to sustain long-term. It also does not have rules about what someone can or cannot eat.

However, IF is time-restricted and limits food intake to certain hours each day which can make it difficult to follow. 

If weight loss is a goal and a barrier to meeting this is eating excess calories late at night, then IF may not be the worst idea. Alternatively, if you aren’t hungry for breakfast or eat dinner pretty early, then IF may work. 

However, if you have a history of an eating disorder, are pregnant, diagnosed with a specific medical condition, currently taking any medications, or are an athlete, then IF is probably not the best route to follow. 

IF does show some benefit when it comes to weight loss and body composition changes BUT so did restricting calories 7 days a week (aka a typical caloric deficit). The question then would be: which style of eating best works for YOU with your lifestyle and goals? 

The most effective way to achieve your weight loss goals would be to follow an overall healthy eating pattern and be physically active throughout the week. This does not mean any specific fad diet or trend, like IF, needs to be followed.

If you are interested in starting IF, make sure to consult with a registered dietitian first to make sure it is right for you. Additionally, it will be important to figure out the most effective form that properly fits your lifestyle, and to make sure you are meeting all your nutritional needs.

Bottom Line: IF is NOT for everyone and more research is needed to further evaluate long-term benefits of IF! As with every other diet trend, it may be difficult to sustain and can lead to distorted eating and unhealthy lifestyle habits. To achieve your goals, follow a non-restrictive healthy eating pattern that you will enjoy and be able to maintain!

If you would like to speak to our team of Registered Dietitians to see if this is something that would work for you, please contact us! We’d love to start working with you and help you come up with the right plan for your specific goals and needs! 

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Albosta, M., & Bakke, J. (2021). Intermittent fasting: is there a role in the treatment of diabetes? A review of the literature and guide for primary care physicians. Clinical Diabetes and Endocrinology, 7(1), 3. https://doi.org/10.1186/s40842-020-00116-1


Cleveland Clinic (2019, April). Intermittent fasting: 4 different types explained. https://health.clevelandclinic.org/intermittent-fasting-4-different-types-explained/


Kerndt, P. R., Naughton, J. L., Driscoll, C. E., & Loxterkamp, D. A. (1982). Fasting: the history, pathophysiology and complications. The Western Journal of Medicine, 137(5), 379–399. Retrieved from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1274154/pdf/westjmed00207-0055.pdf


Welton, S., Minty, R., O'Driscoll, T., Willms, H., Poirier, D., Madden, S., & Kelly, L. (2020). Intermittent fasting and weight loss: Systematic review. Canadian Family Physician, 66(2), 117–125. Retrieved from:  https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7021351/pdf/0660117.pdf


Zouhal, H., Saeidi, A., Salhi, A., Li, H., Essop, M. F., Laher, I., Rhibi, F., Amani-Shalamzari, S., & Ben Abderrahman, A. (2020). Exercise training and fasting: current insights. Open Access Journal of Sports Medicine, 11, 1–28. https://doi.org/10.2147/OAJSM.S224919

IF Intermittent Fasting
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