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What is Intermittent Fasting?

Contributors: Barbara Gordon, RDN, LD

Published: May 07, 2019

Reviewed: April 13, 2021

What is Intermittent Fasting?
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Put “intermittent fasting” into a search engine and you’ll pull up millions of hits! Web sites tout the value of intermittent fasting for managing weight loss, heart disease, Type 2 diabetes and sleep problems. Some even suggest that intermittent fasting may slow the aging process. Unfortunately, the science at this time is lacking. Plus, much of the research comes from studies on small groups of people or is limited to animals. Thus, it cannot be determined whether the findings of these studies apply to the general population. And, some of the studies were conducted for very short periods. The long-term effects of intermittent fasting are not yet known.

So, why is this pattern of rotating periods of eating with periods of fasting garnering so much interest? Is it the magic bullet or a current fad? Three patterns of intermittent fasting were examined and what the research found about this potential weight loss strategy reviewed. Read on to find out what was learned.

Alternate Day Fasting

What is it?

Alternate-day fasting refers to rotating days of eating and days of fasting. On fasting days no foods or beverages with calories are consumed. Calorie-free drinks, such as water, black coffee and tea are permitted. On non-fasting days, you can eat whatever you want – although following healthful eating guidelines is recommended. Theoretically, over the course of a week, you would have cut the total number of calories that you typically consume. And, over time, by eating fewer calories the pounds would be expected to drop off.

Is it effective for weight loss?

Studies found that the amount of weight lost following an alternate day fast was equal to that lost via a typical low calorie weight loss plan. However, as the description reveals, this is a challenging fasting pattern, and side effects reported included serious hunger pangs and decreased concentration on the fasting days. Also, some studies revealed that people ate more than usual on non-fasting days. Thus, they did not restrict enough calories to promote weight loss.  Plus, the long-term effects on your health of this eating pattern are not yet known. More research is needed on alternative day fasting as an approach to weight loss.

 Modified Fasting

What is it?

Modified fasting involves eating very little amounts of food on fasting days. Some modified fasts restrict intake on fasting days to 20% to 25% of needed calories. Others advocate limiting intake on fasting days to only 500 calories. Another version, referred to as the 5:2 fast, calls for fasting two days per week. Participants on this fast follow their usual eating patterns on the other five days of the week. 

Is it effective for weight loss?

The results of modified fasting for weight loss are contradictory. Participants in some modified fasting weight loss studies lost more weight than those on traditional weight loss diets, although the difference was not significant. Other studies found no difference in weight loss between modified fasting and a reduced calorie diet.  At this time, there is not enough research to know if this dietary pattern is a safe and effective way to lose and keep off those extra pounds.

Time-restricted Fasting

What is it?

Time-restricted fasting limits the intake of calories primarily to waking hours. The goal is to fast eight to 12 hours per day with the bulk of the fast occurring while you are sleeping. This is an easier fasting pattern for people who do not snack after dinner or eat breakfast immediately upon rising.

Is it effective for weight loss?

The jury is still out about the effectiveness of time-restricted fasting for weight loss. Most studies have been done with lab animals and research findings of studies conducted with people are sparse.

The Bottom Line

Intermittent fasting is not currently a recommended treatment for weight loss or any other health condition. There needs to be more research, especially regarding any negative side effects of fasting, such as the risk for nutrient deficiencies. Intermittent fasting may pose a health risk for certain individuals, so it is not recommended for people with diabetes, women who are pregnant or breastfeeding and individuals with a history of eating disorders or disordered eating. Anyone considering fasting should first discuss it with a doctor, especially if taking medications that may need to be adjusted.

For a personalized weight management plan that meets your individual needs, consult a registered dietitian nutritionist. An RDN can create a weight loss program based on your unique health and nutrition needs and goals. To find a dietitian in your area, search the Academy's Find a Nutrition Expert database.

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