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Most diets focus on what you should eat–intermittent fasting focuses on when you should eat. The diet is based on the idea that if you limit the time that you spend eating, you eat less. In other words, you decrease your overall calorie intake (Zubryzcki, 2018).
While some studies show intermittent fasting has potential to aid in weight loss, the diet is not without risk, and more research is needed to understand the diet. Continue reading to learn more about intermittent fasting, the potential benefits, and side effects.
What is intermittent fasting?
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.
How to do intermittent fasting?
The three most popular methods of intermittent fasting are alternate-day fasting, periodic fasting, and time-restricted fasting.
- Alternate-day fasting alternates between 24-hour normal eating days and 24-hour fasting days. On fasting days, you eat nothing at all or eat very little (around 500 calories). Some people refer to this as the eat-stop-eat method.
- Periodic fasting uses periods of whole-day fasting. A popular example is the 5:2 diet–fast for two days per week (usually not consecutive days) and eat normally the remaining five days. On fasting days, you can fast completely or maintain a very low-calorie intake of approximately 500 calories.
- Time-restricted fasting restricts your calorie intake to a specific “eating window”–most commonly 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, it’s important to be consistent. Find the plan that works best with your lifestyle so you’re more likely to stick with it and see results.
What are the benefits of intermittent fasting?
Intermittent fasting is mainly used to promote weight loss, and there are several reasons why it might work for you.
First, decreasing the amount of time you have to eat reduces the number of calories you consume (in theory)-assuming you eat healthy foods during 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 intermittent fasting is that fasting triggers changes in your metabolism. People who fast for 12-24 hours may see up to 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. 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 (Freire, 2020). 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, more studies on rodents than humans show these benefits (Rynder, 2019). In human studies, intermittent fasting does not decrease these risk factors significantly more than calorie-restriction weight-loss diets (Freire, 2020). Some rodent studies indicate that intermittent fasting may help us live longer and fight cancer, but more research is needed to better understand intermittent fasting.
What are the risks of intermittent fasting?
Intermittent fasting appears to be a safe option for supporting weight loss. However, you should consult your healthcare provider before you begin any weight management plan. Some side effects of intermittent fasting include fatigue, headaches, constipation, bad breath, difficulty concentrating, and irritability. These side effects are usually temporary, and occurred in less than 20% of participants in clinical trials (Cioffi, 2018).
Intermittent fasting may be harmful to children, the elderly, or people who are underweight. Pregnant people, people who are breastfeeding, or people with a history of eating disorders should not engage in intermittent fasting without consulting a healthcare provider (Freire, 2020).
Weight loss is a personal journey, and you can never tell if a person needs to lose weight simply by looking at them. If you’re concerned that your weight may be negatively impacting your health, make an appointment with a healthcare provider. They can help you determine if weight loss is in your best interest, and what weight loss method (if any) is safe and effective for you.
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.
- Cioffi, I., Evangelista, A., Ponzo, V., 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
- 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
- Rynders, C. A., Thomas, E. A., Zaman, A., et al. (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
- Zubrzycki, A., Cierpka-Kmiec, K., Kmiec, Z., et al. (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.