Intermittent fasting is effective for weight loss, but for how many hours per day?

  • Many studies support the claim that intermittent fasting is an effective tool for losing weight
  • Researchers tested two time-restricted feeding diets
  • Participants in both groups showed a reduction of about 550 calories per day

Fasting diets, also known as time-restricted feeding diets or intermittent fasting, have become increasingly popular over the years because of their flexibility. 

In a new clinical trial by University of Illinois at Chicago researchers, people on a 4-hour time-restricted feeding diet and a 6-hour time-restricted feeding diet were compared to a control group. The control group is the group in a study that does not receive treatment, and is used as a benchmark. 

The study was published in Cell Metabolism

How the study was done

Participants in the 4-hour time-restricted feeding diet group were asked to eat only between 1 pm and 5 pm, whereas participants in the 6-hour time-restricted feeding diet group were asked to eat only between 1 pm and 7 pm.

Participants in both groups, however, were allowed to eat whatever they wanted during these periods and were instructed to only drink water or calorie-free beverages. In the control group, participants were directed to not change their diet or physical activity levels, and to maintain their weight.

Both groups were followed for 10 weeks. Their weight, insulin resistance, oxidative stress, blood pressure, LDL (low-density lipoprotein) cholesterol, HDL (high-density lipoprotein) cholesterol, triglycerides (a type of fat in your blood) and inflammatory markers were tracked.

Some weight loss

In both daily fasting groups’ participants, results showed a reduction of about 550 calories each day, simply by adhering to the schedule. Participants also lost about 3% of their body weight, but no significant difference was found in cardiometabolic risk factors (your chances of having diabetes, heart disease or stroke).

Insulin resistance and oxidative stress levels were also found to be reduced among participants in the study groups when compared with the control group. However, no difference was found in LDL cholesterol (known as the "bad" cholesterol), HDL cholesterol (known as the "good" cholesterol) or triglycerides.

First human clinical trial

"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 co-author of the study.

Varady further commented: "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.”

Do longer fasts yield better results?

Varady added that the results also tell us 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 feeding diet might make sense for most people who want to pursue a daily fasting diet," she said.

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