Intermittent Fasting 101

Updated: Dec 10, 2019

Intermittent fasting has recently gained a lot of momentum in the weight loss industry. Many "experts" swear by this new method of eating and all of it's benefits. Could this be the new and improved way of eating? Let's dive deep in the latest research.


Intermittent fasting (IF) is technically not a diet. It is consider an eating pattern where you cycle between fasting periods and eating periods. IF is very different from all other diets as it doesn't tell you what to eat but rather when to eat.

Although IF has become more popular in the recent years, it is not a new concept. Humans have been fasting for many reasons since the beginning of times. Our ancestors would often fast due to lack of food. The hunter and gatherers could go days before their next meal and therefore were forced to fast. There are also many religion that observe fasting periods, such as Islam, Christianity, Judaism and Buddhism. Furthermore, most of us fast every night when we sleep (unless you sleepwalk to the fridge or get a midnight snack). It might not be a long fast but technically we are still in a fasted state.

Our bodies have evolved to be able to function without food temporarily. Mechanisms are in place to make sure our specie would survive in case of famine. These involve storing energy for future use in the form of fat cells (yes, fat is useful!) . Just like in the ketogenic diet (Check out article for more info), when no carbohydrates (or food in this case) is available, our body produces ketones to use as fuel. Ketones are made in the liver from our fat stores therefore we don't need to consume any food to produce them.

There are many different ways to go about IF. The most common ones are:

  • 16-8: Also known as "time restricted eating. This consists of an 16h fasting period followed by an 8h feeding period.

  • Example: Meals are consumed between 11am to 7pm everyday

  • 5-2: This consists of 5 days of regular eating combined with 2 days of fasting (Usually not consecutive days) **

  • Example: Fasting days - Monday and Thursday.

  • 24h fast: Fasting for a period of 24h once or twice a week **

  • Example: Stop eating after dinner Monday night (6pm) and fast until 6pm Tuesday

  • Alternate day fasting: Fasting every second day**

  • Example: Fasting days: Monday-Wednesday-Friday

  • Prolonged fasts: Fasting for longer than 48h.**

  • Example: 3 days to 1 month.

**Some experts suggest that on fasting days, individuals are still able to consume 25% of energy requirements and have the same benefits while making those fasting days easier.


1. Simplicity

This method of eating is very simple. There are times when you can eat, and times when you fast. There is no food restriction on this diet so people may feel less deprive (during eating periods that is). If you are someone who works well with a black&white rigid plan, this regimen might suit your needs.

2. No Food Restrictions

That's right! When following IF you do not have to worry about what foods you eat. As long as you eat within the given time frame, you do not need to worry about the foods you eat, the calories or quantity (although I always recommend a healthy balance diet).

3. Weight Loss

Most people wanting to try IF or who are currently doing this "diet" are motivated by potential weight loss. There is a lot of evidence suggesting that fasting is an indeed an effective way to lose weight. If you think about it, it makes sense. When you are fasting, chances are you are reducing total amount of calories consumed over a certain period of time. A 2014 literature review suggests that alternate day fasting and time restricted eating are both effective eating patterns for major weight loss (average of 3-8% reduction in body weight in 3-24 weeks). Alternate day fasting resulted in more significant weight loss than time restricted eating. Again, this makes sense because when you are fasting every second day, your overall energy intake is much lower.

However you should know that there is NO SIGNIFICANT DIFFERENCE in weight loss between intermittent fasting regimens and continuous calorie deficit diets (CCDD). The greater the calorie deficit was, the greater was the weight loss. Other clinical studies comparing IF and CCDD have also come to the same conclusion: no significant difference in weight loss between groups.

Furthermore, according to a 2015 literature review, drop out rates of IF compared to continuous calorie deficit diet are not significantly different. In this review, drop outs rates ranged from 0-65% for both diets. These findings could indicate that this diet is not easier to follow than any other diets out there

The big question is: Is the weight loss sustainable? Unfortunately, like all other diets, most studies demonstrated that participants regain most of their weight once they stopped following IF.

What about obesity?

Some "experts" suggest that IF could be a solution to the obesity epidemic in North America. This grand statement full of promises is however not backed up by science. A 2017 clinical study, examined 100 obese individuals over the period of 1 year following either alternate day fasting or continuous calorie deficit diet. The results showed no significant difference in weight loss, body composition, blood pressure, heart rate, fasting glucose and fasting insulin. On top of not demonstrating any additional benefits, participants in the alternate day fasting group had a 38% drop out compared to 29% in the other group.

Takeaway: Weight loss occurring during IF is due to overall calorie deficit. When you eat fewer meals or fast for the whole day, you are bound to reduce calorie intake. This method is "easier" in the sense that you do not have to count calories in order to be in a deficit. Fasting will most likely put you in a calorie deficit. Intermittent fasting is a tool but not the solution to our obesity epidemics. There are other ways to lose significant weight WITHOUT fasting.

4. Heart health & Diabetes

Weight loss in general has demonstrate positive health outcomes relating to diabetes and heart health. Some animal studies have demonstrated that IF could have a a lowering effect on fasting glucose and fasting insulin. However these results have yet to be demonstrated in a long term human study. Many studies who compared intermittent fasting and continuous calorie deficit have not been able to support that intermittent fasting can affect health markers (such as blood lipids, blood pressure, fasting glucose and insulin resistance) at a greater extent than CCDD.

Takeaway: Currently there is not enough evidence to demonstrate that IF is an efficient way to positively influence health outcomes relating to diabetes and heart health.

5. Inflammation

Inflammation is our body's natural response to perceived harm. It can be acute (i.e. you bang your knee or resistant training sessions) or chronic (arthritis). Although inflammation is a normal response, high levels of inflammation can lead to other diseases such as heart disease. Strong evidences suggest that intermittent fasting may reduce overall inflammation when followed for a prolonged period of time (<30 days).

Takeaway: There is significant evidence that IF can reduce inflammation markers

6. Metabolic Health and the Circadian Cycle

All animals have evolved to perform different activities based on their circadian cycle to optimize performance. As humans, we are no different (see image below to see how we regulate activities around our circadian cycle). Some animals study demonstrate that IF (with eating period during the day) can improve metabolism. However, not enough evidence has been establish in human model. Furthermore, studies demonstrate that eating patterns that align with our circadian cycle can decrease risk of cardiovascular and metabolic issues.

Takeaways: Intermittent fasting or not, scheduling our feeding window according to our circadian cycle seems to have many health benefits. IF can make this easier as you set distinctive times to eat.

7. Body Composition

Weight loss is usually the marker we use to test if a diet is working or not. However the problem with that is that weight loss has many roots : water loss, fast loss and muscle loss. Ideally, we want to lose fat mass while conserving our muscles. We already discovered that IF as well as continuous calorie deficit diets are efficient ways to lose weight. However, it is important to know where is this weight loss coming from. A few studies demonstrated that, at equal energy intake, men who followed IF had a greater fat loss while retaining muscle mass than men following a calorie deficit diet. Although this is interesting data, more studies are needed as the studies to confirm this hypothesis (I personally have found 2). Also, in these studies, all participants were men who engaged in resistant training 3-5x/week which might have a role to play in body composition.

Takeaway: IF might be an effective way to lose weight while maintaining muscle mass in men who engage in resistance training 3-5x/week. However, although there is two strong studies supporting this, more are needed to confirm hypothesis. Also there is no evidence this applies to women.

8. Budget Friendly

This is a no brainer. When are fasting, you eat less food. When you eat less food, your groceries are cheaper therefore your wallet is happier :).


1. It's simple, but NOT easy!

Like I said previously, this diet is simple to follow. There are times when you eat and times where you do not. Black and White. However, depriving yourself of food at times when you are hungry is HARD and counter intuitive. You will have to rely on will power which is a difficult thing to do when you feel hungry.

2. Restriction ==> Binge eating!

Restriction perpetuates overeating and/or binge eating episodes. When we feel deprived/restricted it can lead us to swing to the other extreme and overindulge in ALL the food. If we think about it from an evolutionary perspective it makes sense that the thought of food deprivation and restriction triggers binge eating episodes. Back then, starvation was a real issue that endangered our specie. Therefore, when food was present, humans would eat as much as they possibly could because who knew when the next rations would be available. Depriving ourselves from food can trigger our primal brain to want to eat all the food when present. Emotional starvation can also be the trigger. Just telling yourself that you CAN'T eat now can be triggering. Intermittent fasting is a type of deprivation that can lead to overeating and binge episodes. If you have a history of eating disorder or are easily triggered by restriction, IF might not be a great idea for you.

Takeaway: Intermittent fasting is a type a restriction and restriction can lead to overeating and/or binge eating. If you have an eating disorder, recovered from an eating disorder or tend to have disordered thoughts around food, this might not be for you.

3. Potential Health Issues (especially with women!)

There is some evidence that IF does not affect men and women the same way and can actually have some negative side effects and hormonal disturbance in women. Some studies demonstrated that after following IF for 3 weeks blood sugar control worsened in women. Furthermore, IF may interfere with hormone regulation in women and can lead to amenorrhea (a state in which a woman no longer has her period). An animal studies comparing male and female rats on intermittent fasting found that after 2 weeks of intermittent fasting, female rats lost their period and their ovaries had shrunken. Of course, humans are not the same as rats, however it is important to consider this as a possible risk in women. Women are more sensitive to calorie restriction than men. Again, in an evolutionary perspective it makes sense. When a female's body senses that there is not enough food available, it will not want to conceive. Therefore, a woman who is on a low calorie diet can easily lose her period. It's your body's way of protecting your unborn child from an unsafe environment. Women might benefit from shorter fasting windows or less fasting days than men to prevent hormonal imbalance.

Takeaway: As women, we are more sensitive to calorie restrictions therefore if you are thinking about doing IF, please consult a registered dietitian to make sure this diet can help you and will not negatively impact your health.

4. Digestion Issues

One of the benefits of IF is you can get away with eating larger meals. This is great for many people (like myself) who have a big appetite. However, these larger meals can definitely increase stress on your gastrointestinal tract which leads to digestion issues such as: indigestion, bloating, refluxes, cramps and abdominal pain. For people who already have digestion issues, larger meals might not be a great idea as it may increase those uncomfortable symptoms.

Takeaway: If you suffer from any gastrointestinal disorder, please talk to a registered dietitian prior to trying IF. It might not be suitable for you or you might have to modify your approach in order to avoid exacerbating your current symptoms.

5. Sports Performance

There are a lot of studies on physical activity and fasting, most of them on Muslim athletes observing Ramadan (where individuals fast during sun light and eat at dusk until dawn. In these studies, researchers notice that a one day fast did not impact sport performance however fasting for a long period of time (>30 days) reduced athletic performance as well as impaired cognition functions during physical activities. This is definitely important to take into account if you are an athlete.

Takeaway: Prolonged IF may interfere with your athletic performance. Your body needs fuel to perform at it's best. Fasted workouts and activity may interfere with performance and cognitive functions.

6. Hangry, Tired and Less Productive.

Lack of food makes us tired, hangry and less productive! If you are not eating, yet hungry, it is hard to focus on anything else. Our primal brain will focus on food and how to get that food. A 2016 review reported that in some studies participants following IF reported feeling more tired, less productive, less focused, increased mood swings, cold-like symptoms and suffered from headaches. Furthermore, having a balanced breakfast has shown to increase concentration and improve cognitive performance as well as help with weight maintenance.

Takeaway: Depriving yourself of food may lead to feeling hangry, tired and decrease productivity. Eating balanced meals throughout the day can help us feel energize, focused and productive!

7. Interferes with Social life

We don't solely eat for fuel. We eat for many reasons and one of them is SOCIAL. Food is often part of social events and a way to bond with others. IF can interfere with your social life as most people do not follow this type of eating and you could find yourself not able to enjoy meals with friends and family. It is definitely something to take into account when your eating habits and lifestyle interferes with your everyday life.


Fasting is NOT for everyone and I strongly suggest you speak to a registered dietitian prior to initiating any type of diets. Individuals who should not attempt fasting include(not limited to):

❌Pregnant and breastfeeding women

❌Individuals with a history of eating disorder (It can be very triggering)

❌Individuals with diabetes or chronic low blood sugar

❌Individuals who require any medication that should be taken with food


Intermittent fasting has definitely many interesting benefits. I definitely believe that it can be very useful tool in my toolbox as a registered dietitian. However, I tend to be on the careful side when a "diet" requires you to ignore your hunger cues as I believe this makes it hard to be sustainable. Furthermore, many of the benefits related to IF are mostly due to reduced overall calorie intake which can be done in a less restrictive way. The great thing about IF is that it can be flexible and personalized to you and your own habits.

If you are interested about trying this, please work with a registered dietitian who will be able to guide you and support you. Ultimately, we want to make sure that you are nourishing your body and becoming healthier with no adverse effect.

Thank you for taking the time to read this! If you have any questions- do not hesitate to ask!

Marie-Pier Pitre-D'Iorio, RD


Aksungar, F.B., Topkaya, A.E. & Akyildiz, M. (2007)Interleukin-6, C-reactive protein and biochemical parameters during prolonged intermittent fasting. Ann Nutr Metab. 51(1):88-95

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. 164(4) 302-311

Hall, H. (2015) Intermittent Fasting.

Harris, L. Hamilton, J., Azevedo, L.B., Olajide, J. De Brún, C., Waller, G. Whittaker, V. Sharp, T., Lean, M., Hankey, C. & Ells, L. (2007) Intermittent fasting interventions for treatment of overweight and obesity in adults: a systematic review and meta-analysis. JBI Database of Systematic Reviews and Implementation Reports. 16(2):507–547.

Harvie, M. & Howell, A. (2017). Potential Benefits and Harms of Intermittent Energy Restriction and Intermittent Fasting Amongst Obese, Overweight and Normal Weight Subjects—A Narrative Review of Human and Animal Evidence. Behav. Sci. .7(1).

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;8(6)

Horne, B.D., Muhlestein, J.B. & Anderson, J.L. (2015) Health effects of intermittent fasting: hormesis or harm? A systematic review. Am J Clin Nutr. 102(2):464-70.

Javad, F.S. (2010) Ramadan fasting and exercise performance. Asian J Sports Med. 1:130

Maughan, R., Fallah, J. & Coyle, E. (2010) The effects of fasting on metabolism and performance. Br J Sports Med. 44:490.

Moro, T., Tinsley, G., Bianco, A., Marcolin, G., Pacelli, F., Battaglia, G., Palma, A. , Gentil, P., Neri, M. & Paoli, A. (2016) Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males. J Transl Med.14(1):290

Panda S, Hogenesch JB, Kay SA. Circadian rhythms from flies to human. Nature. 2002;417:329-335.

Patterson, R. E. & al. (2015) Intermittent Fasting and Human Metabolic Health. Journal of the Academy of Nutrition and Dietetics. 115(8): 1203-1212.

Seimon, R.V., Roekenes, J.A., Zibellini, J., Zhu, B., Gibson, A.A., Hills, A.P., Wood, R.E., King, N.A., Byrne, N.M. & Sainsbury, A. (2015) Do intermittent diets provide physiological benefits over continuous diets for weight loss? A systematic review of clinical trials. Mol Cell Endocrinol. 418:153-7

Scheer FA, Hilton MF, Mantzoros CS, Shea SA. Adverse metabolic and cardiovascular consequences of circadian misalignment. Proc Natl Acad Sci USA. 2009;106:4453-4458.

Seimona, R.V, Roekenesa J.A., Zibellinia, J., Zhua, B., Gibson, A., Hills, P.A., Wood, R.E., King, N.A., Byrne, N.M. & 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(2):153-172

Tinsley, G.M. & La Bounty, P.M. (2015) Effects of intermittent fasting on body composition and clinical health markers in humans. Nutrition reviews. Oct 1;73(10):661-74.

Tinsley, G.M., Forsse, J.S., Butler, N.K., Paoli, A., Bane, A.A., La Bounty, P.M., Morgan, G.B. & Grandjean, P.W. (2016). Time-restricted feeding in young men performing resistance training: A randomized controlled trial. Eur J Sport Sci. 2016:1–8.

Varady K.A. (2011) Intermittent versus daily calorie restriction: which diet regimen is more effective for weight loss? Obes Rev. 12:593–601.

#diet #intermittentfasting #dietreview

264 views0 comments

The Balanced Practice

founded by The Balanced Dietitian (Marie-Pier Pitre-D'Iorio)

  • White LinkedIn Icon
  • White Facebook Icon
  • White YouTube Icon
  • White Instagram Icon