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  • J. Kehler

The Facts on Fasting

Intermittent Fasting; Fact or Fantasy?

More and more people are asking me questions about fasting. What is it? Is it safe? Is it hard? Does it work? Is it a fad? Fasting is simply restricting when you are allowed to eat. Fasting has been a part of cultural and religious rituals since the dawn of time, but recently “Intermittent Fasting” has come into the health and fitness spotlight with claims that range anywhere from easy weight loss without calorie counting to providing added cardiovascular protection.



I decided it was time I did another deep dig into the science to separate what we know from what we think we know, and what we know only sells magazines. I sifted through a lot of data from several studies and review journals in an attempt to get to the bottom of this hot topic, but the truth is much research is still needed in this area and many of the studies that have been performed on fasting have specifically been on male rodents in the lab, with very few studies being performed on humans to date. Of the studies on humans several were dealing with trained athletes and metabolically fit individuals. I was able to find only 10 human fasting studies specifically dealing with overweight and obese people, involving anywhere from 10 - 107 individuals (so statistically small numbers), study durations ran anywhere from 1 day (again not very useful) to just over a year. Here’s what I learned.


Intermittent Fasting defined. Intermittent Fasting is restricting when you are allowed to eat, with the intention of forcing your body to burn stored body fat for energy instead of food, and reduce calorie intake.

There are traditionally 2 approaches taken:

  1. The use of “Fasting Days” within the week. An example would be; Fasting/Not eating for a given period (16 hours, between 8pm and Noon the next day as an example) 1-3 times per week, or even not eating at all, every other day.

  2. The use of “Eating Windows” each day. The most common examples being either 8 hour, or 10 hour eating windows, where one must consume all their daily calories and not eat outside that timeframe.


Is it safe?

There is a wide consensus that people with advanced diabetes or who are on medications for diabetes, people with a history of eating disorders like anorexia and bulimia, and pregnant or breastfeeding women should not attempt intermittent fasting unless under the close supervision of a physician who can monitor them. Beyond that it appears relatively safe, though again many more studies are needed.


Is it hard?

This one is relative. Each of us has our own daily rhythm and flow. For example I know many people who naturally eat within a 8 or 10 hour window each day. It’s just how they’ve always operated, but these people typically don’t get hungry until later in the day. For those of us who don’t naturally eat in windows, or for those considering fasting days... What does the data say? If we look at the compliance rate/drop off rate from the 10 studies we have to read, depending on the fasting style used the drop off rate runs between 25% and 40% which is quite high for any study, so we can extrapolate that for many people fasting can be tough to stick to, and that many study participants who did not drop out complained of constant hunger which did not ease as the studies continued.



Does it work? What does science say?

In July 2017, one of my favorite studies was published. It was conducted on 100 people between the ages of 18 and 65 years old, who’s BMI was between 25.0 and 39.9 and who live a sedentary lifestyle (less than 60 minutes of medium to intense exercise per week, for at least 3 months). I like this study because it took baseline data for a full month before commencing 6 months of weight loss through intermittent fasting, and then continued for 6 months of weight loss maintenance to measure the longer term effects. All in all a well structured study that also split participants into 3 equal groups (1. traditional caloric restriction, 2. alternate day fasting, and 3. no-intervention).


What they found:

Intermittent fasting has not been found to provide improved weight loss as compared to caloric restriction, and in several studies underperformed comparatively. More so, participants in the fasting groups found hunger to be a significant challenge comparatively.

As for cardiovascular protections and other health benefits: Blood pressure, heart rate, cholesterol, triglycerides, fasting plasma glucose, fasting insulin, insulin resistance, high-sensitivity C-reactive protein and homocysteine levels were not significantly different between intervention and control groups at 6 or 12 months.


In conclusion,

There is some evidence that people can successfully lose weight through intermittent fasting, although it appears to be less successful than nutritional changes and caloric restriction. There is also some evidence to suggest that heavily restricting when you can eat can cause unwanted changes in metabolism that may result in the burning of fewer calories.

All the experts seem to agree that we simply don’t have enough information from enough sources to recommend fasting as a weight loss strategy compared to existing interventions, but that there are relatively few safety concerns as long as you’re not part of an at risk group (people with advanced diabetes or who are on medications for diabetes, people with a history of eating disorders like anorexia and bulimia, and pregnant or breastfeeding women).


https://www.sciencedirect.com/science/article/pii/S1550413118302535

https://jamanetwork.com/journals/jamainternalmedicine/fullarticle/2623528

https://www.annualreviews.org/doi/full/10.1146/annurev-nutr-071816-064634?url_ver=Z39.88-2003&rfr_id=ori%3Arid%3Acrossref.org&rfr_dat=cr_pub%3Dpubmed&

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7021351/

https://centerfordiscovery.com/blog/the-dangers-of-intermittent-fasting/


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