Updated: Apr 26,2019
Intermittent fasting is gaining popularity in fitness and health circles as the latest dietary trend. Intermittent fasting refers to alternating times of fasting with times of feasting. Intermittent fasting protocols can be grouped into 3 categories:
- Alternate-Day Fasting. These protocols designate 1-3 days per week (or every other day) to fast. Non-fasting days have no restriction on caloric intake. For example: a person might choose to finish dinner by 8pm on Sunday and not eat again until 8pm on Monday (a 24-hour fast) every week.
- Alternate-Day Energy Restriction. These protocols designate 1-3 days per week (or every other day) to limit caloric intake. Other days have no restriction on caloric intake. For example: a person who normally requires 2200 calories per day might restrict caloric intake to 1600 calories on Mondays and Thursdays.
- Time-Restricted Eating. These protocols restrict eating to specific hours each day. Essentially, these protocols prolong the normal overnight fast. The most common restriction is to fast for 16 hours per day (8pm until noon) and to restrict eating to 8 hours of the day (noon until 8pm). This is the most popular type of intermittent fasting because it is the easiest to do.
You may already be scratching your head. Haven’t we been told for years that the best way to boost our metabolism and lose weight is to eat small, frequent meals? Haven’t we been told that the most important meal of the day to support weight loss is breakfast? Doesn’t this new idea of intermittent fasting fly in the face of everything we have been taught until now?
The answer is: maybe yes and maybe no. Science is never black and white. Let’s take a look at why intermittent fasting might work, what research supports intermittent fasting, and who is most likely to benefit.
Why might intermittent fasting work?
Fasting is nothing new to humans. Whether out of necessity or because of religious rituals, humans have been fasting for thousands of years. The human body is equipped to handle extensive periods of time with no food.
One explanation for why intermittent fasting promotes weight loss is simply because the total caloric intake is lower than usual. But caloric intake cannot explain all of the observed benefits. It is more likely that intermittent fasting works because of how it affects metabolism—or the way we burn and use energy.
During periods of fasting, the body’s metabolism shifts in multiple ways. The body is forced into accessing sources of energy besides food, meaning you are more likely to start burning off stored fat for energy. During fasting, blood sugar levels drop, insulin levels drop, and growth hormone increases. After periods of fasting, body cells are more sensitive to insulin, meaning that you will have better control over blood sugar levels and you will be less likely to store the foods you eat as fat.
What research supports intermittent fasting?
One of the largest randomized controlled trials on intermittent fasting in humans involved 106 overweight or obese women, between the ages of 30 and 45 years. [i] The women were assigned to one of 2 treatment groups: one group was put on a diet that restricted total caloric intake to 25% below their estimated requirements every day for 6 months (continuous caloric restriction); the other group was put on a diet that restricted caloric intake in the same exact way but only for 2 days per week for 6 months (intermittent caloric restriction). The results will probably catch you by surprise: both diets were equally effective for achieving weight loss. Both diets were also equally effective for improving a variety of blood tests related to the risks of diabetes and heart disease.
A 2014 comprehensive review of human studies concluded that restricting energy intake on 1-3 days per week is equally effective as continuous caloric restriction for improving blood sugar balance and reducing the risk of diabetes in overweight or obese people. [ii] And a 2015 review of human studies concluded that alternate-day fasting appears to be effective for weight loss and improving cholesterol levels in people regardless of their original weight.[iii]
Overall, there is a growing body of evidence to show that restricting caloric intake on 1-3 days per week is useful for weight loss and for reducing the risk factors for diabetes. In contrast, there is very little research on the most popular form of intermittent fasting—time-restricted eating. This makes it challenging to draw any definite conclusions as to the effects of restricting eating time to 8 hours of each day.
Who is most likely to benefit from intermittent fasting?
Most studies on intermittent fasting have been conducted with individuals who are overweight or obese, but there is some evidence that intermittent fasting also improves metabolic biomarkers in normal-weight individuals. We must remember that every person is unique, with biochemical individuality. Intermittent fasting is not necessarily a good idea for everyone. For someone who is thin, with a high metabolic rate, skipping breakfast might create metabolic havoc.
At the Care Group, we look at each person as an individual and assess their unique needs. Just as small, frequent meals are not the best idea for every person, intermittent fasting is not either. If you would like to explore your various dietary options, schedule an appointment with one of our holistic nutritionists. We are here to help make sense of the seemingly constant changes in science and nutrition.
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[i] Harvie MN, Pegington M, Mattson MP, et al. The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women. Int J Obes (Lond). 2011;35(5):714-727.
[ii] Barnosky AR, Hoddy KK, Unterman TG, Varady KA. Intermittent fasting vs daily calorie restriction for type 2 diabetes prevention: a review of human findings. Transl Res. 2014;164(4):302-311.
[iii] Tinsley GM, La Bounty PM. Effects of intermittent fasting on body composition and clinical health markers in humans. Nutr Rev. 2015;73(10):661-674.
About the Author: Dr. Gerard Guillory, MD is Board Certified in Internal Medicine and has published two books on Irritable Bowel Syndrome (IBS). In 1985, he opened The Care Group, PC. Today, his clinic is a Primary Care facility that is a hybrid of functional and traditional medicine treating patients with digestive disorders, autoimmune disease, and other conditions. You can learn more about Dr. Guillory here.