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Last updated: Jan 24, 2020
4 min read

Intermittent fasting: understanding the potential benefits and side effects

Tzvi Doronchimene richa

Medically Reviewed by Tzvi Doron, DO

Written by Chimene Richa, MD


If you have any medical questions or concerns, please talk to your healthcare provider. The articles on Health Guide are underpinned by peer-reviewed research and information drawn from medical societies and governmental agencies. However, they are not a substitute for professional medical advice, diagnosis, or treatment.

Most diets emphasize what you should be eating; intermittent fasting focuses on when you should be eating.

Intermittent fasting builds on the idea that if you limit the time that you spend eating, there is a good chance that you are going to eat less. In other words, you decrease your overall calorie intake (Zubryzcki, 2018).

In intermittent fasting, your eating pattern cycles between eating and not eating over a specific time period. All of your calories are consumed during a defined time frame. Intermittent fasting does not specify which types of foods are allowed.

Many cultures and religions have used fasting for both spiritual and health reasons and recognize the health benefits of fasting. Fasting dates back to not only ancient civilizations, like the ancient Greeks but also our hunter-gatherer ancestors.


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How is intermittent fasting done?

One benefit of intermittent fasting is that you can adjust it based on your routine. Several fasting protocols exist to choose from. The three most popular forms of intermittent fasting are alternate-day fasting, periodic fasting, and time-restricted fasting.

  • Alternate-day fasting: This involves alternating between 24-hour normal eating days and 24-hour fasting days. On fasting days, you can eat nothing at all or eat very little, around 500 calories. Some people refer to this as the eat-stop-eat method.
  • Periodic fasting: This uses periods of whole-day fasting. A popular example is the 5:2 diet where you fast for two days per week (usually not consecutive days) and eat normally the remaining five days. Again on the fasting days, you can fast completely, or you can have a very low-calorie intake of approximately 500 calories.
  • Time-restricted feeding: This restricts your calorie intake to a specific eating window. The most common way to do this is to only eat during an 8-hour window. For example, you may have your first meal at 11 am and finish your last meal at 7 pm. This is also called 16:8 intermittent fasting (or Leangains method), where you have a 16-hour fast followed by all meals within an 8-hour eating window. 

Whether you choose alternate-day fasting, the 5:2 fasting plan, or the Leangains method, the important thing is to stick with it. Find a plan that works with your lifestyle to increase your chances of losing weight.

What are the health benefits of intermittent fasting?

Intermittent fasting is mainly used to promote weight loss, and there are several reasons why it can work for you.

First, by decreasing the amount of time you have to eat, it reduces the number of calories you end up consuming––assuming that you are eating healthy foods during the non-fast periods. Decreasing caloric intake, regardless of the method, has been shown to help people lose weight, both short-term and long-term.

Another theory behind why intermittent fasting works is that fasting triggers changes in your metabolism. People who fast for 12-24 hours may see a >20% decrease in blood glucose––the primary source of energy for most of the body. This decrease causes your body to shift to breaking down fat to get the energy it needs (Freire, 2020).

Studies show that the average weight loss in adults classified as overweight or obese ranges between approximately 4-10% over intermittent fasting periods of 4-24 weeks. However, intermittent fasting promotes about the same amount of weight loss in the long term as regular calorie restriction diets or conventional weight loss diets (Rynders, 2019).

Other reported health benefits of intermittent fasting include decreased blood pressure, loss of body fat, lower blood sugar and insulin levels, reduced cholesterol, and decreased inflammation. However, there are more studies in rodents than in humans showing these benefits.

In human studies, intermittent fasting does not decrease these risk factors significantly more than calorie-restriction weight-loss diets. The most important thing with regards to all of these risk factors is that you lose weight, and not necessarily how you do it.

Lastly, some rodent studies indicate that intermittent fasting may help us live longer and fight cancer, but there is limited research in these areas. More long-term human studies are needed to have a better understanding of intermittent fasting and its potential benefits.

What are the potential risks of intermittent fasting?

Overall, intermittent fasting appears to be a safe option for weight loss. However, you should talk to your healthcare provider before you begin any weight management plan.

Some side effects, which are usually temporary, include fatigue, headaches, constipation, bad breath, difficulty concentrating, and irritability. These occurred in less than 20% of people in clinical trials (Cioffi, 2018).

Intermittent fasting seems to be safe for adults but could be harmful to children, the elderly, or underweight people. Also pregnant women, those who are breastfeeding, or people with a history of eating disorders should not engage in intermittent fasting without consulting a healthcare provider (Freire, 2020).

Losing weight is one of the most important things you can do to improve your health, especially if you are currently diagnosed as overweight or having obesity. Even a 5-10% weight loss can decrease your risk of heart disease, high blood pressure, type 2 diabetes, high cholesterol. Talk to your healthcare provider or nutritionist to see if intermittent fasting is right for you.


  1. Cioffi, I., Evangelista, A., Ponzo, V., Ciccone, G., Soldati, L., Santarpia, L., et al. (2018). Intermittent versus continuous energy restriction on weight loss and cardiometabolic outcomes: a systematic review and meta-analysis of randomized controlled trials. Journal of Translational Medicine, 16(1), 371–386. doi: 10.1186/s12967-018-1748-4. Retrieved from https://www.ncbi.nlm.nih.gov/pubmed/30583725
  2. Freire, R. (2020). Scientific evidence of diets for weight loss: Different macronutrient composition, intermittent fasting, and popular diets. Nutrition, 69, 110549. doi: 10.1016/j.nut.2019.07.001. Retrieved from https://www.ncbi.nlm.nih.gov/pubmed/31525701
  3. Rynders, C. A., Thomas, E. A., Zaman, A., Pan, Z., Catenacci, V. A., & Melanson, E. L. (2019). Effectiveness of Intermittent Fasting and Time-Restricted Feeding Compared to Continuous Energy Restriction for Weight Loss. Nutrients, 11(10), 2442. doi: 10.3390/nu11102442. Retrieved from https://www.ncbi.nlm.nih.gov/pubmed/31614992
  4. Zubrzycki, A., Cierpka-Kmiec, K., Kmiec, Z., & Wronska, A. (2018). The role of low-calorie diets and intermittent fasting in the treatment of obesity and type 2 diabetes. Journal of Physiology and Pharmacology, 69(5), 663–683. doi: 10.26402/jpp.2018.5.02. Retrieved from https://www.ncbi.nlm.nih.gov/pubmed/30683819

Dr. Tzvi Doron is Board Certified in Family Medicine by the American Board of Family Medicine and is Ro's Chief Clinical Officer.