The Cooper Institute

Founded in 1970 by the "Father of Aerobics"
Kenneth H. Cooper MD, MPH


Intermittent Fasting: What Does the Research Say?

Posted in

Wednesday, Oct 16, 2019

Unless you’ve been living under a rock, you probably have heard of intermittent fasting as the ‘new and revolutionary’ approach for successful weight loss. A quick Google search on intermittent fasting will return millions of results claiming a host of other health benefits. Is intermittent fasting really an effective and healthy plan for weight control, and is there research to support these claims?

Over 70% of the U.S. adult population is either overweight or obese and rates of prediabetes and type 2 diabetes are at an all-time high, so there is no shortage of interest and hype about what works best for weight loss. Some diets encourage us to eat like a caveman while others tell us to eat fewer acid-forming foods or to significantly reduce or eliminate carbohydrates.

There is no quick fix or gimmick that will ever live up to the hype. A healthy diet is one that helps us get to (or maintain) long-term reasonable body weight and improves key health markers. These include blood insulin, glucose, cholesterol, and triglyceride levels as well as resting blood pressure and inflammatory markers such as C-reactive protein.

Intermittent fasting is the most recent hot topic among popular diet trends, but what is it and does it work?

Fasting is defined as a period of voluntary abstinence from food and drink. People fast for a variety of reasons such as health, weight loss, or even spiritual reasons. This approach doesn’t change what you eat, rather it changes when you eat. Intermittent fasting isn’t a diet plan so much as it is a pattern of eating. There are several different approaches to intermittent fasting:

Time-restricted fasting

Followers of this approach only eat during a certain time-frame, which induces a prolonged fasting period. For example, the eating window may be between noon and 8:00 p.m. followed by fasting until noon the next day. This time pattern is known as the 16:8 fast, meaning that you are fasting for 16 hours with an 8-hour eating window. People who are just starting out may choose a 12:12 pattern such as 7 a.m. to 7 p.m. or follow at 14:10 pattern from 10:00 a.m. to 8:00 p.m. to ease into it. More advanced proponents may lean towards an 18:6 pattern (2:00 p.m. to 8:00 p.m.). The time period set for the eating window can be tailored to what works best for your schedule. Many people consider this to be the easiest and most sustainable approach to fasting. 

Modified fasting

The 5:2 diet involves normal eating for five days and fasting for two days. On the two fasting days, you are allowed to eat a total of 500 calories either as a single meal or as smaller meals (like 100 calories eaten five times). During the other 5 days, you may eat and drink whatever you wish. This diet was popularized by ‘The Fast Diet’ book by Dr. Michael Mosely. Another variation of this suggests fasting on two nonconsecutive days per week with only a 25% reduction in daily calories consumed.

Alternate-day fasting

This approach is the simplest to understand, and involves alternating fasting days with eating days. Sometimes referred to as the 24-hour fast, you simply fast for a 24-hour period. This could mean eating breakfast and lunch one day and then fasting until lunch the next day. It could also be a normal day of eating until dinnertime followed by a 24 hour fast until dinner the next day. Some people may push this to a 36-hour fast that starts after dinner on day 1 and doesn’t end until breakfast on day 3.

Prolonged fasting

Some fasts can extend into 48 hours or even a week or more. Often times, these are for religious or spiritual purposes. These types of fasts can come with significant health risks and should not be attempted without first discussing it with your health care provider to understand the risks and potential complications. 

What does the research show?

Surprisingly, most of the research on intermittent fasting has been done with male mice. However, a handful of studies with human subjects give us a clearer picture of what works, what doesn’t work, and what is safe.

Time-restricted fasting

The vast majority of available data on individuals who are awake during the day and sleep at night shows that consuming most of the daily calories earlier in the day is related to lower body weight and improved health. Time-restricted fasting prolongs the nightly fasting period, which may reduce the risk of chronic conditions such as cardiovascular disease, type 2 diabetes, and some cancers. This demonstrates that time-restricted fasting is an effective method to reduce obesity and associated metabolic dysfunction.

On the other hand, there is abundant evidence that working at night (shift work) is related to substantially increased nighttime eating as well as an increased risk of these same health conditions.   

Modified fasting

In a review of nine available studies that lasted for an average of 8-12 weeks, seven studies reported significant weight loss, while three found decreased fasting insulin levels. Of the six studies that reported on inflammatory markers, three found a significant reduction. One study showed significantly superior weight loss results, but only because it was combined with exercise. The overall conclusion is that modified fasting is not superior to the most common weight loss approach of simply reducing your daily caloric intake.      

Alternate day fasting

Human research using this approach is very sparse. The three available studies all used a small sample size with no reported information regarding physical activity. To further cloud the issue, the studies mostly used subjects who were normal-weight or only slightly overweight and the longest study only lasted 22 days. Two of the three studies reported a significant decrease in blood insulin levels and very modest weight loss. Even though there appeared to be a 30% reduction in caloric intake for the three days following a one-day fast, the subjects reported extreme hunger on fasting days. The conclusion is that this may be a very poor choice as a long-term approach to healthful eating for most people.

Prolonged fasting

Because many of these fasts are practiced for spiritual or religious purposes, studies are purely observational and inconclusive.


Even a single bout of fasting can have positive effects, but temporary behavior changes only produce temporary results. It is more important to focus on finding a healthy diet plan that you can sustain long-term. You should also look for an eating pattern that reduces or eliminates nighttime eating. Some people may find this method easier to follow than a standard “don’t eat this or that” diet because it doesn’t restrict the type of food eaten. Others may choose to layer in a food-specific diet into their fasting plan for greater success. The conclusion is that while more human studies are needed, time-restricted intermittent fasting as a lifestyle may be the best approach for long-term weight control and health.


Patterson, R.D., and Sears, D. (2017). Metabolic effects of intermittent fasting. Annual Review of Nutrition. 37:371-393.