What the Science Says About IF

By Brandi Givens, RD

When it comes to weight-loss plans, it seems that intermittent fasting (IF) is one of the most popular diets around. But what is intermittent fasting, how much weight loss can you expect from this method and what other benefits can you expect from trying it?

While there is an abundance of animal studies involving intermittent fasting, human studies are still somewhat limited. Because IF is not clearly defined and exists in several forms, the research varies in method and length of time. 

Weight-Loss Results From a 2020 Review

Intermittent fasting entails refraining from eating for a period of time each day or week. To help bring some clarity to weight-loss success using various intermittent fasting methods, in 2020 the College of Family Physicians of Canada published a review of available research. The review summarized studies involving several forms of fasting, including:

• Time-Restricted Eating or Feeding (TRE/TRF): This form of fasting is close to the way our ancestors ate before electricity came along. Dieters eat normally during a window of about eight to 12 hours, avoiding everything besides water the rest of the time.
• The 5/2 diet: On two non-consecutive days each week, dieters severely cut calories back from recommended intake, eating normally on the other 5 days. A typical calorie restriction in this method is 25%, or 400-600 calories on fasting days.
• Alternate-Day Fasting: This is like the 5/2 method, except calories are heavily restricted every other day instead of just twice weekly.

Weight Loss and Time-Restricted Eating

This review covered four human studies that used the time-restricted eating method in their research. The length of studies varied from two to 12 weeks, and they averaged 16 hours per day of fasting, which left an eight-hour eating window. The studies were small, with from 10 to 54 people in each group.

The resulting average weight loss for the studies was 2.7% of body weight from the baseline. The two longer, 12-week studies resulted in greater weight loss of average of 3.9% and 3.2% loss from the participants’ baseline starting weights.

This means that a 200-pound person who practices 16 hours/day of fasting might expect to lose about 7.5 pounds, or 1.5 pounds each week over 12 weeks.

Weight Loss and the 5/2 Method:

A total of eight studies used the 5/2 method of intermittent fasting in the review. These studies varied from eight to -52 weeks, and varied in calorie restrictions on fasting days. Most researchers used 20 to 25% of normal recommended intake, or 400 to 600 calories. 

The sizes of some of these studies were larger than those in the time-restricted eating studies, with from 41 to 244 people in each group. The resulting weight loss from these studies was higher, averaging 5.9% weight loss.

The two largest and longest studies were both 52 weeks long, with an average of 6.2% weight loss. The results suggest that over the course of a year, a 200-pound person might expect to maintain about 12.4 pounds of weight loss using a 5/2 intermittent fasting-plan.

Weight-Loss Results from Alternate Day Fasting:

There were nine studies which used the alternate-day fasting method, varying in length from four to 24 weeks. All of them were small studies numbering from 16 to 79 people, and most of the studies restricted calories anywhere from 25% to 40% of normal intake, three days a week. One of the alternate-day fasting studies used zero calories every other day.

The average weight loss for this method was 6.2% weight loss, or 12.4 pounds in a 200-pound person. These results closely compare to the longer-term 52-week studies that used the 5/2 plan.

Weight Loss Compared to Traditional Calorie Restriction

In the studies reviewed, intermittent fasting groups were compared to randomized control participants who followed traditional calorie restriction diets. While fasting was effective, the results were no better than simple calorie restriction.

Other Possible Health Results

Intermittent-fasting studies are showing that there may be additional health benefits as well. It’s not clear if the benefits are a result of the weight loss, or if the improved health is physiologically linked to fasting itself.

Better Blood Sugar Control

Because of the higher risk for dangerously low blood sugar intermittent fasting is not currently recommended for people with unhealthy blood sugar levels without strict monitoring by a doctor. However, some intermittent fasting research that was well-supervised for safety has shown improved blood sugar control in patients.

For example in 2010, 10 people with unhealthy blood sugar levels who were taking metformin participated in a 6-week pilot study to see if time-restricted eating was safe and effective to improve blood glucose. The participants fasted for an average of 16.8 hours daily, and results showed improved numbers for both fasting and post-meal blood sugar levels.

Another study in 2016 resulted in improved hemoglobin A1C after 12 weeks of intermittent fasting. 63 men with unhealthy blood sugar levels were involved in the study, which compared the glycemic numbers of an alternate-day fasting group to a group that just restricted calories daily. Both groups had improved A1Cs.

Improved Cardiovascular Health

Some studies indicate that intermittent fasting may result in greater heart health to potentially reduce cholesterol and support healthy blood pressure numbers.

In 2019, the American Heart Association published a journal article that reported greater longevity of cardiac patients who had followed intermittent-fasting diets for two-thirds of their lifespan. Even including fasting during the time before they were diagnosed with poor heart health. The study followed more than 2,000 patients.

Researchers believe that the healthier heart results are related to a combination of factors, including reduced oxidative stress, improved circadian rhythm and the body’s act of switching into fat-burning mode, called the ketogenic state.

Intermittent Fasting Results: What to Expect

Dieters who try intermittent fasting for weight loss and other health benefits can expect results similar to simple calorie restriction diets. This means that intermittent fasting is a potential alternative for dieters who have not had success with traditional reduced-calorie plans.

Recent human studies offer a good starting point for learning about the benefits of IF. But long-term studies are still needed to look at the risks of fasting, and to tease out which forms of fasting are best. Always talk to your doctor before starting a new diet plan to make sure it’s safe for you.

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