Cardiologist Ethan J. Weiss, MD, followed an intermittent-fasting diet for 7 years. He lost about 3.6 kg (8 lb) and began recommending the approach to friends and patients who wanted to lose weight.
“I liked the way the diet was so simple,” said Dr. Weiss, an associate professor at the Cardiovascular Research Institute, University of California, San Francisco. But he also felt “it was too good to be true because you can eat what you want as long as it’s within a narrow window.”
So when, last year, he conducted a randomized, controlled trial, TREAT, testing such an approach – eating during just 8 hours a day, fasting for the remaining 16 hours – versus an eating plan of three meals a day without restrictions, he was somewhat dismayed to find the group of people who fasted didn’t lose any more weight than the other group.
The approach used in this study is known as time-restricted eating. It involves designating periods of time within the day when people can consume whatever they want; they then “fast” at times outside those eating windows. Other methods include alternate-day fasting, or the well-known 5:2 diet. In the latter, people eat a “normal” amount of around 2,000 calories per day on 5 days of the week, but for the other 2 days, they restrict caloric intake to 500 calories per day.
Intermittent fasting is an umbrella term encompassing all of these different approaches.
Dr. Weiss’s work builds on more than a decade of research into this type of eating plan by scientists, including Krista Varady, PhD, professor of nutrition at the University of Illinois at Chicago, who presented an overview of her own studies last fall at the virtual annual meeting of the European Association for the Study of Diabetes.
Although much of the work has suggested that the shorter duration of eating period in this type of diet leads to lower calorie intake and weight loss while avoiding the need for the tedious calorie-counting of conventional diets, Dr. Weiss’s data – published last year – throws a spanner in the works and now complicates the evidence base.
A promise of simplicity: ‘All you have to do is watch the clock’
Dr. Varady said she, too, is intrigued by the simplicity of intermittent-fasting diets.
In 2018, Dr. Varady and colleagues tested the weight-loss efficacy of 12 weeks of time-restricted feeding in a pilot study of 23 people with obesity.
Participants were permitted an 8-hour eating window (10 a.m. to 6 p.m.) followed by water-only fasting of 16 hours (6 p.m. to 10 a.m.) the next day (sometimes referred to as the 16:8 diet). Researchers measured weight loss and fat mass, as well as metabolic parameters, and compared the active group with 23 matched-control participants who ate freely.
There were no restrictions on type or quantity of food consumed by the control group during the 8-hour period, but individuals in the time-restricted feeding group consumed around 350 calories less than the comparator group.
Dr. Varady thinks this is most likely because of the fact that people normally eat during a 14-hour window and time-restricted feeding cuts that down by 6 hours.
“One of the most beautiful things about time-restricted feeding is that it doesn’t require calorie monitoring,” she explained. “People get burnt out with having to constantly monitor calories. All you have to do is watch the clock.”
Adherence was quite high, she reported, although most people skipped 1 day, often a Saturday, likely because of social engagements.
Weight loss in the time-restricted feeding group was mild to moderate. After 3 months, mean body weight decreased by 2.6%, or approximately 3 kg (7-8 lb), relative to those who ate freely, but this was a significant difference (P < .05).
But the researchers observed little change in metabolic disease risk factors between the groups.
In the time-restricted feeding group, systolic blood pressure dropped from 128 mm Hg to 121 mm Hg over the 12-week period, which was significant relative to the control group (P < .05) but there were no significant changes in fasting glucose, fasting lipids, fasting insulin, or insulin resistance relative to the comparator group.
In contrast to Dr. Varady’s findings, Dr. Weiss’s randomized TREAT trial, which used a similar 16:8 period of time-restricted versus unrestricted eating in 116 individuals with overweight or obesity, did not find greater weight loss in the group restricted to eating within the 8-hour window.
As previously reported by this news organization, those who fasted for 16 hours of each day (n = 59) did lose some weight, compared with the control group (n = 57) over 12 weeks, but the difference in weight loss between the groups was not significant (−0.26 kg; P = .63).
And there were no significant differences in any of the secondary outcomes of fat mass, fasting insulin, fasting glucose, hemoglobin A1c levels, estimated energy intake, total energy expenditure, and resting energy expenditure between the time-restricted eating and regular feeding groups.
“I don’t claim time-restricted eating is dead,” Dr. Weiss said, “but the hope that you can eat for a limited time each day and solve metabolic disease is not there.”