Intermittent fasting diets popular with celebrities are no better than eating throughout the day

Diets made famous by the likes of Jennifer Aniston and Reese Witherspoon that require you to limit when you eat are ‘no better for weight loss’ than eating normally. 

Researchers from the University of California, San Francisco had a group of 116 volunteers eat a celebrity inspired fasting diet and others eat three meals a day.

The fasting group could only eat between 12pm and 8pm and the other could eat when they wanted – no restrictions were placed on what food people could eat.  

While both saw a modest drop in their weight, neither was significantly different from the other group – suggesting a daily ‘fasting time’ makes no difference. 

Time restricted eating, or intermittent fasting, involves limiting when you eat to a smaller part of the day, and is favoured by celebrities as a weight loss solution. 

In October 2019 Jennifer Aniston said she doesn’t eat breakfast and only consumes liquids in the mornings – saving her eating until the final half of the day. 

The University of California team set out to find out if this form of diet control can actually help to reduce weight when compared to eating throughout the day. 

It was hoped that if it did prove to help with weigh loss it could be a relatively easy lifestyle change for obese people to implement and still lose weight. 

‘Even modest weight reduction can improve cardiovascular disease risk,’ they wrote.

‘However, long-term adherence to lifestyle changes is difficult. Therefore, it is important to find novel lifestyle-modification interventions that are effective in reducing weight and accessible and straightforward to enhance adherence.’

They looked at intermittent fasting, the process of eating separated by defined periods of fasting – usually for 12 hours in every 48 hours.

Unfortunately most of the benefits claimed for this method of dieting are either untested or under tested in humans, the authors claim.

To test their theory that it could improve weight loss, they narrowed in on the time restricted eating diet – also known as 16:8 – which has been shown to work in mice.

This involves a consistent fasting and eating period over the course of a day – usually eight hours eating for 16 hours fasting.

Previous small-scale studies in overweight people found time restricted eating can result in reduced calorie intake and a decrease in body weight. 

To see if this holds up with a larger group, the team designed a randomised clinical trial to determine the effect on overweight and obese patients. 

Volunteers stuck to the new diet for 12 weeks and completed a survey throughout the process, recording their weight on university supplied bluetooth scales.

The study only limited the time food was consumed, not what people eat or any other physical activity during the study – it was just a test of eating windows. 

They found that people were able to stick to the time restricted eating diet successfully, giving credence to the idea it would be a viable weight loss solution.

Unfortunately, there was no greater weight loss with that method than among those who eat three meals a day with no restrictions on timing.  

Researchers say there was also some evidence that following a time restricted eating diet could exacerbate muscle loss – particularly  appendicular lean mass (ALM).

The team say future studies should be aimed at understanding the effects of early versus late restricted eating, as the time set aside to eat may have an impact.

They also want to look at whether an increase in protein intake, or different times of the day could limit the loss of ALM. 

The findings have been published in the journal JAMA Internal Medicine.