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Intermittent fasting: Something for You to "Chew On"

By Jessica Kelner, D.O. - March 11, 2019

Throughout human history, there have been periods of time where people have not eaten. This wasn't a diet, a fad, or even a choice at times.   Not eating for a period of time may seem like a punishment of sort.  However, historically, humans experienced periods of fasting that  may have been over  hours, a day, or sometimes days. Fasting is normal part of human survival and written into our genetic code for periods of time that we didn't have access to food or sustenance.  Our bodies are built to deal with this type of "intermittent fasting". Intermittent fasting referring to periods of time in which we would not consume useful calories.  

There are also known voluntary abstinence from food and drink (intermittent fasting) that has been practiced around the world for thousands of years.  Books on anthropology and religious practices describe a vast variety of fasting forms and practices. People may have not known the scietific effects of what they were doing to their bodies, but for some it allowed them to achieve other goals.  

In today’s modern world, we are fortunate enough to  have constant access to food.  You just go to your local grocery store and there is the food waiting for you to purchase it.  You know,  those perfectly ripe bananas and pre-packaged meats.  You can buy these items 365 days a year, even when things are out of season! Amazing!   We are incredibly fortunate not to have to go and hunt and gather our meals every day.  Although doing so would give us plenty of exercise that most American's aren't getting.  That would lead down another tangent on the lack of physical activity in our country and the constant calorie consumption. Especially high carbohydrate and sugar calorie consumption. Doesn't that sound like a health crisis just waiting to happen? 

Back to intermittent fasting... 

There has been a recent renewed interest in types of fasting regimens as scientist and physicians search for “cures” to the major health issues facing the  United States and other developing countries.   

You may be asking yourself, "what does a "fast" look like?"

Here are some Different Types of Fasting Regimens: 
  1. Complete Alternating Day Fasting: Involves alternating fasting days (no energy containing foods or beverages with eating days (food and beverages consumed ad-libitum)
  1. Modified Fasting Regimens: Consumption of 20-25% of the energy needs on scheduled fasting days.  
  1. Time Restricted Feeding: Protocols allow individuals to consume ad-libitum energy intake within specific windows of time, which induces fasting periods on a routine basis.
  1. Religious Fasting: A variety of fasting regimens for religious or spiritual purposes

Alternate Day Fasting
Involves fasting days, in which no calories or beverages are consumed, alternating with days of ad libitum food and beverage consumption. This was found to be as effective as caloric restriction in decreasing fasting insulin and glucose concentrations. It also had beneficial effects for cancer risks.  The studies the have been done aon this show weight loss, decrease of regulatory glucose markers like insulin, potential for improvement in lipid profiles, and improvement in inflammatory markers. 

Modified Fasting Regimens
These type of fasts allow for consumption of 20-25% of the caloric energy requirements of the individual on regularly scheduled “fast” days.  The term “fast” here is referring to “severely limited energy intake” and not “no energy intake”.  This is the popular 5:2 diet, which involves severe energy restriction for 2 non-consecutive days/week and ad-libitum eating the other 5 days/week.  Studies of this fasting regimen have showed decreased visceral fat, decreases in insulin like growth factor, and adipocyte size (fat cells).  Previous research on this diet has shown weight loss, improvement of inflammatory markers, improvement of lipid profiles, improvement of fatigue, increases of self confidence and improvement of mood. 

Time Restricted Feedings
This seems to be one of the more popular methods of intermittent fasting.  There are many different protocols that allow the individual to eat and drink ad-libitum for a certain period of time during the day, and “fast” for a certain period of the day on a regular basis.  Daily fasting intervals range from 12-20 hours. With this method of intermittent fasting, it is important to synchronize intermittent fasting regimens with daily circadian rhythms (sleep/wake cycles). Studies show even just an 11 hour fasting interval produced 1.3% of weight loss.  They also found that eating one meal per day reduced fasting glucose levels (sugar). However, there were no changes in mood that they could find.

Religious Fasts
In the Islamic religion,  there is a fast that occurs from dawn to sunset during the entire month of Ramadan. They are also forbidden from consuming fluids and medications. Depending on the location the person is in, their fast can carry from 11-22 hours.  Research on this type of fasting has found  weight loss during this period of time of 2.7 pounds during the month of Ramadan, with an average weight regain of 1.5 lbs 2 weeks after Ramadan. They also found that fasting blood glucose levels and LDL levels decreased during this period.  This fasting pattern is opposite of normal human circadian rhythm patterns and is not ideal for a weight loss intervention.

In the Jewish religion, there are 6 fast days, with the most well known fast day being Yom Kippur, the Day of Atonement. Two of the fast days are “major fast days” which are a period of 25 hours (sunset to nightfall the next day). On the major fast days, they are not allowed to eat or drink, work, bathe, or experience other life pleasures.  During the minor fast days, the fast lasts from before sunrise to nightfall on the same day. Only food and water is restricted during this time.  The length of time of the minor fasts can vary depending on the season of the year.

How does intermittent fasting improve health? 
By limiting food consumption to the daytime, we are able to harness the natural circadian rhythm to improve metabolism.  Animals that are restricted to activity during a certain period of time develop an internal clock (circadian rhythm) that allows for physiological processes to be optimized during the time the animal is awake.  Wake/sleep cycles (circadian rhythm) affect the integration of the body's metabolism, hormone production, physical coordination, and sleep.  

The circadian rhythm has an impact on the GI (gastrointestinal) tract as well.  For example, gastric emptying and blood flow are greatest during the day time and slower during the evening time. This is also when humans are most active.  Intermittent fasting may also directly effect the microbiome in the GI tract. The microbes in our GI tract help us to process and break down our food.  The gut microbiome (bacteria in our GI tract) influence energy absorption and storage. They actually help us break down food.   Certain microbes that are increased with obesity can directly alter the permeability (holes in the membranes) of the GI tract membrane, leading to increase gut permeability and promoting systemic inflammation. 

Lastly, there have been many studies showing that night eating is associated with reduced sleep and poor quality sleep. This can lead to increased risks of obesity, diabetes, cardiovascular disease, and cancer.    Eating at night time can de-synchronize the natural circadian rhythm and disrupt normal sleep cycles. 

One additonal thing that none of the studies I referenced is exercise.  With any dietary plan, I would highly encourage an exercise program to be used along with it to achieve optimal health benefits and results.  If you are going to try an intermittent fasting diet, make sure to drink plenty of water on your fast days. 

Is intermittent fasting for everyone?  Certainly not.  Can it be utilized for certain people to help with medical conditions, weight loss, or mood? Yes, it can.  Please consult your personal physician before starting any specialized diet. 

If you have questions about intermittent fasting or would like to learn more about how your diet is influencing your health, you can visit for more information.  If you would like to schedule an appointment to come in to discuss these things in person, please call 720-370-9559. 

R. Patterson, G. Laughlin, D. Sears, A. LaCroix, C. Marinac, L. Gallow, S. Hartman, L. Natarajan, C. Senger, M. Martinez, A. Villasenor. "Intermittent Fasting and Human Metabolic Health." J Acad Nutr Diet. 2015 Aug; 115(8): 1203–1212.