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Choosing Your Fasting Diet Time Limit

Timothy Boyer Ph.D.'s picture
Fasting is a proven method of weight loss.

Here’s the latest about what works when choosing a fasting diet time limit that could make your dieting a little easier for you.


Have you thought about trying fasting for weight loss, but were unsure what time limits are needed between your meal time and your fasting time? According to a new study that compared two different time limits, settling on a time-restricted fasting diet style is not as hard as you might think.

Does Intermittent Fasting Really Work for Weight Loss?

Often referred to as intermittent fasting, a typical fasting style diet entails skipping meals 12 to 24 hours at a time up two to three times per week punctuated with days of normal eating of just about whatever you want—as long as you don’t binge or fill up on junk food calories. Some of the more popular styles of fasting fall into two types—24-hour and 16/8 fasts.

The 24-hour fast is where you choose two 24-hour periods during the week that are non-consecutive, avoid all food, and stick to just water or some cleansing tonic recommended for weight loss.

The 16/8 fast is one in which you gradually shorten your eating period to an 8-hour time slot while leaving the remaining consecutive 16 hours as a time for fasting. This is then repeated every day until you’ve reached your weight loss goal.

However, a recent study tested the limits of the 16/8 fasting style a little bit by trying a 20/4 and 18/6 fasting split to see if pushing the envelope can make a difference.

According to a news release from the University of Chicago Illinois:

Participants in the 4-hour time-restricted feeding diet group were asked to eat only between the hours of 1 p.m. and 5 p.m. Participants in the 6-hour time-restricted feeding diet group were asked to eat only between the hours of 1 p.m. and 7 p.m.

In both the study groups, patients were allowed to eat whatever they wanted during the 4-hour or 6-hour eating period. During the fasting hours, participants were directed to only drink water or calorie-free beverages. In the control group, participants were directed to maintain their weight and not change their diet or physical activity levels.

“This is the first human clinical trial to compare the effects of two popular forms of time-restricted feeding on body weight and cardiometabolic risk factors,” said Krista Varady, professor of nutrition at the UIC College of Applied Health Sciences and the principal investigator leading the study.

What the researchers found after the 10-week long study during which weight, insulin resistance, oxidative stress, blood pressure, LDL cholesterol, HDL cholesterol, triglycerides and inflammatory markers were tracked, was that:

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• The participants in both daily fasting groups reduced calorie intake by about 550 calories each day.
• Members of both groups lost approximately 3% of their body weight.
• Insulin resistance and oxidative stress levels were reduced among participants in the study groups when compared with the control group.
• There was no effect on blood pressure, LDL cholesterol, HDL cholesterol or triglycerides.

But most importantly, there was no significant difference in weight loss or benefit between undergoing a 20/4 or an 18/6 fasting split. Dr. Varady stated for the news release that:

“The findings of this study are promising and reinforce what we’ve seen in other studies—fasting diets are a viable option for people who want to lose weight, especially for people who do not want to count calories or find other diets to be fatiguing,” Varady said.

“It’s also telling that there was no added weight loss benefit for people who sustained a longer fast—until we have further studies that directly compare the two diets or seek to study the optimal time for fasting, these results suggest that the 6-hour fast might make sense for most people who want to pursue a daily fasting diet.”

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The study and its results can be reviewed in the journal Cell Metabolism referenced below.

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Timothy Boyer has a Ph.D. in Molecular and Cellular Biology from the University of Arizona. For 20+ years he has been employed as a freelance health and science writer. Today, with an eye on the latest news, Timothy continues writing about science with a focus on what you need to know for healthier living. For continual updates about health, you can also follow Timothy on Twitter at TimBoyerWrites.

Image Source: Courtesy of Susann Mielke from Pixabay.


How long should you fast for weight loss?” University of Chicago Illinois news release 2020.

Effects of 4- and 6-h Time-Restricted Feeding on Weight and Cardiometabolic Health: A Randomized Controlled Trial in Adults with Obesity" Sofia Cienfuegos, et al. Cell Metabolism, 2020.