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Pros and cons of intermittent fasting

By Darcie Fisher, Tufts Medical Center Correspondent

Say the word “fasting” in relation to weight loss and chances are the thought of being REALLY HUNGRY crosses your mind!  Intermittent fasting has become very trendy. We asked Tufts Medical Center Registered Dietitian Kelsey Kalenderian, RDN, LDN from the Weight and Wellness Center at Tufts Medical Center to “weigh in” on the pros and cons of this popular way to lose weight.

1.  What is intermittent fasting and how does it work?

Intermittent fasting (IF) has become a very popular diet for weight loss or weight management. Simply put, IF is an eating pattern that cycles between periods of fasting and eating. It does not specify which foods you should eat but rather when you should eat them. There are currently two main types: 16:8 and 5:2. The 16:8 is a time restricted feeding where you fast for 16 hours and eat your regular intake for 8 hours. The 5:2 is a more extreme approach, where individuals eat their regular diet for 5 days of the week and fast or eat few hundred calories for two days of the week. 

“The 16:8 approach may be a more realistic option for those who want to try it,” says Kalenderian. “The 5:2 approach may be less sustainable over the long-term. One potential drawback of the 5:2 eating pattern is that individuals may consider their regular eating days “feast days” since they eliminate a significant amount of calories on the fasting days.” 

Another option that may be more sustainable to consider is an overnight fast of at least 12 hours. An example of this would be not eating after 8 pm and then having breakfast at 8 am. This eating pattern reduces time of eating each day and may prevent unwanted snacking on less nutrient dense foods in the evening, such as sweets or desserts.

 2. Can you dive right in, or should you ease in to intermittent fasting?

Individuals who are interested in IF can absolutely dive in and start with whatever plan works best for them: overnight fast (12 hours), 16:8, or 5:2. There is no research to suggest that people need to ease themselves into IF. However, an individual may want to trial the overnight fast or 16:8 approach first, as they tend to be the more sustainable options. Some side effects to be aware of for those who do start IF are persistent hunger, cold, irritability, low energy, distraction and lower work performance.  

3. Would you consider this a diet or a lifestyle?

IF is considered a popular diet and a lifestyle, it all depends on the individual’s motive and intentions. For instance, if an individual is doing an overnight fast (12 hour fast, 12 hour eating window) or the 16:8 approach, then that may be considered a lifestyle. These individuals may continue to do IF because it requires less time with meal prep/planning, cooking, etc., since they are likely having only 2 meals per day.

If an individual is intentionally reducing their eating time to a certain number of hours per day or a certain number of days per week to induce weight loss for a short period of time, then that may be considered a diet. 

4.  Is it true the quality of the food you eat doesn't matter as much as the timing? 

It’s important to focus on food quality during eating times, especially if the goal is to facilitate weight loss. It is not possible, or it’s highly unlikely, to eat fast food or hyper-processed food during the eating periods and expect to lose weight and improve overall health. “Eating plenty of minimally processed foods like nuts, seeds, beans, fruits, non-starchy vegetables, lean proteins and heart healthy fats may help promote weight loss and reduce risk of type 2 diabetes, heart disease and cancer,” said Kalenderian.

5. Is there anyone who shouldn't try intermittent fasting?

“The 5:2 approach is potentially hazardous, especially to those with diabetes,” warns Kalenderian. “The safety of intermittent fasting for someone with diabetes depends on their medications and risk for hypoglycemia. If someone with type 1 or type 2 diabetes is considering IF, they should consult their doctor first.” 

Pregnant and/or breastfeeding women and those who have a history of an eating disorder should not attempt fasting. Another population of concern is athletes.  

To learn more, request an appointment with our specialists in the Weight and Wellness Center.


Published January 2020

The above content is provided for educational purposes by Tufts Medical Center. It is free for educational use. For information about your own health, contact your physician.