Life Wellbeing Fasting diets: Study proves restricting eating to six-hour daily window really works

Fasting diets: Study proves restricting eating to six-hour daily window really works

Restricting your eating to four hours a day gets no better outcomes than eating six hours day, suggesting a fasting diet sweet spot. Photo: Getty
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A new study from the University of Illinois appears to have found the manageable sweet spot in daily fasting diets – also known as time-restricted diets – where eating is limited to a window period of some hours per day.

The researchers found that limiting your eating to four hours a day, over a 10-week period, leads to a three per cent weight loss – and that’s eating whatever you want in that period.

But the same result was found in a group that ate six hours a day, suggesting there is a limit on how restrictive these diets need to be to get the maximum weight-loss benefit.

The real value in the study was the discovery that following either of these diets for 10 weeks led to a drop in insulin resistance and oxidative stress levels.

“This is the first human clinical trial to compare the effects of two popular forms of time-restricted feeding on body weight and cardio-metabolic risk factors,” said Krista Varady, professor of nutrition at the university’s College of Applied Health Sciences and corresponding author of the story.

It is “telling that there was no added weight loss benefit for people who sustained a longer fast”, Dr Varady said.

“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 six-hour fast might make sense for most people who want to pursue a daily fasting diet.”

How the study was carried out

According to a statement from the University of Illinois, participants in the four-hour time-restricted feeding diet group were asked to eat only between the hours of 1pm and 5pm. Participants in the six-hour time-restricted feeding diet group were asked to eat only between the hours of 1pm and 7pm.

In both groups, patients were allowed to eat whatever they wanted during the eating period. During the fasting hours, participants were directed to only drink water or calorie-free beverages.

All participants were adults with obesity.

There was a control in which participants were directed to maintain their weight and not change their diet or physical activity levels.

The participants were followed for 10 weeks as weight, insulin resistance, oxidative stress, blood pressure, LDL cholesterol, HDL cholesterol, triglycerides and inflammatory markers were tracked.

Both fasting groups reduced calorie intake by about 550 calories each day simply by adhering to the schedule and lost about three per cent of their body weight.

This was regardless of whether participants ate tuna, tofu or toffee apples and with no calorie counting. In other words, the benefits were related to when participants ate and not what they ate.

Reduced risk of diabetes and cell and tissue damage

The researchers also found that 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.

There also was no significant difference in weight loss or cardio-metabolic risk factors between the four-hour and six-hour diet groups.

“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,” said Dr Varady said.

Two years ago, as fasting diets hit their popular stride, and all manner of health claims were being made, the white-coat naysayers were pushing back.

Typical was a piece at Medpage Today, ‘Intermittent Fasting: Another Fad Diet with Little Supporting Evidence.’

The main complaints were that fasting diets led to no greater weight loss than any other diet, the drop-out rate was high, and the purported impact on cardio-metabolic risk factors might be enjoyed by mice but there was no convincing evidence from human trials.

This new study doesn’t address the issue of time-restriction versus calorie-restriction, or the problem of dropping out (although a six-hour fast is not particularly arduous), but it does indicate that healthy weight loss over two and a half months is possible following a time-restricted diet.

It also appears to afford some protection from diabetes type-2 and oxidative stress, an imbalance of free radicals and antioxidants in the body, which can lead to cell and tissue damage.

Previous studies had shown that intermittent fasting is known to improve sensitivity to blood glucose-lowering hormone insulin and to protect against fatty liver.

A 2019 study discovered that mice on an intermittent fasting regimen also exhibited lower pancreatic fat.The researchers showed the mechanism by which pancreatic fat could contribute to the development of type 2 diabetes.