A Breakdown of Intermittent Fasting


Fasting is by no means a new concept and has been utilized throughout history. In fact today we fast during sleeping hours only to break the fast at our first meal of the day- break-fast. Fasting is most commonly performed for spiritual or health reasons. It is also not new concept if you consider our ancestors that who endured times of food scarcity and famine and who did not have the luxury of having a 7-Eleven available on every corner as we do today.

Intermittent fasting is recently all the craze and touted for its supposed benefits of decreasing body fat while maintaining lean muscle mass. The optimal window has been established as 14-16 hours of fasting and 8-10 hours of eating. So does this mean that after reading this you up and decide to start this way of eating this week? Seems pretty easy right? Eat from 9am – 6pm and fast the remaining hours. It’s a bit more complicated than that. I recommend you take into consideration the following topics before diving in.


I. What is it?

Intermittent fasting is a way to manipulate the timing of eating within two clearly defined windows…a longer window of fasting/food restriction and a shorter eating window. There are different forms of intermittent fasting, but the most common and most popular or main stream is time restricted feeding (TRF)…that is a period of fasting time greater than 12 hours followed by a smaller, defined window of eating. An example would be meals consumed between 9am-7pm, fasting from 7pm until 9am the next morning (10-hour eating window, 14-hour fasting window).

Other forms of fasting are alternate day fasting and whole day fasting and should be supervised by a doctor or nutrition professional. Alternate day fasting involves alternating days of calorie restriction (up to 25% of daily calories) with days of no food restriction while whole day fasting involves 1-2 days of fasting (or up to 25% of daily recommended calories) with no calorie restriction the remaining days of the week.1 The purpose of this post, however, is to look at the more trendy form of intermittent fasting which is time-restricted feeding (TRF).


II. What to eat?

During the eating window I recommend sticking to a low-fat, plant based diet with plenty of fiber (AT LEAST 35g) to keep you feeling full and to maintain blood sugar control. Also drink plenty of water. In the TRF form of intermittent fasting it is suggested to maintain your normal caloric intake in your eating window. The nutrient timing is more important here rather than the overall calorie amount. TRF is not a tool to reduce calories. Other forms of fasting are however. You will still lose fat if you choose a higher fat diet over a low fat, plant based…however you will not experience the other benefits such as reduced inflammation, decreased cholesterol, etc.

Is anything allowed during the fast? First and foremost…water!!! Stay hydrated. Other non-caloric beverages (I’m not talking artificially sweetened beverages here) such as coffee or tea are allowed.


III. When to eat?

So when do you establish your eating window and fasting window? Studies suggest it may be best to time your eating window in the morning until late afternoon (i.e. eat from 8am-5pm and fast the remainder of  the day through the next morning). Why? This pattern goes in favor of our circadian rhythm and metabolic pathways. In the morning our body anticipates activity and so it wants to burn glucose and fat for energy. Conversely in the evening our body anticipates rest and so our pathways are up regulated for protein building and recovery. At night those pathways for burning fuel for energy are not up regulated as they are in the morning.2

IV. Who is intermittent fasting not for?

People with adrenal issues should not fast since fasting places further stress on the adrenals. Individuals with a past history of an eating disorder or those who do not have a healthy relationship with food should not fast. Fasting should be avoided in the presence of chronic diseases, diabetes and metabolic syndrome. It also is not recommended for adolescents who are in an active growth phase.  Pregnant or breastfeeding mothers also should not practice intermittent fasting.

V. Potential benefits:

  • Reduction in fat/adipose mass while preserving lean muscle mass

  • Increased cognitive performance

  • Improved health markers, i.e. decreased cholesterol, improved blood pressure, decreased risk for CVD

  • Autophagy – process of cleaning up damaged cells so they can be replaced by new ones thereby conferring benefits of anti-aging

  • Gut rest through a break in digestive processes

  • Decreased inflammation

*Studies have shown that when IMF is combined with resistance training fat loss and weight loss occur. It is inconclusive whether IMF alone confers the same benefit.3

VI. Possible pitfalls:

  • Overeating on non-fasting days.

  • Development of unhealthy relationship with food (i.e. becoming overly fixated with food or overly restrictive)

  • Hypoglycemia

  • Not done properly, i.e. eating the wrong foods, or at less than optimal hours. If hunger is too intense during a 14-16 hour fast bump it back to a 12-hour fast and increase from there.

Bottom Line:

Notice that I used the terms “possible” benefits and “potential” pitfalls. Why? A multitude of benefits of TRF have been seen in animal studies, however there just are not enough human studies to be conclusive. 4


1. https://www.hsph.harvard.edu/nutritionsource/healthy-weight/diet-reviews/intermittent-fasting/

2. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4109003/

3.  https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4271606/

 4.  https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5064803/

Jaime Shelbert