Both groups had similar reductions in long-term blood sugar levels, as measured by a test of hemoglobin A1C, which shows blood sugar levels over the past three months.
“Our findings show that time-restricted eating [also called intermittent fasting] is a good alternative to calorie counting for people who can’t do the traditional diet or are burned out on it,” says the study’s senior author, Krista Varady, PhD, a professor of nutrition at the University of Illinois in Chicago. For many people trying to lose weight, counting time is easier than counting calories, she says.
This study demonstrates that time-restricted eating can lead to modest weight loss in individuals with type 2 diabetes, says Sun Kim, MD, an associate professor of medicine and an endocrinologist who specializes in the treatment of T2D at Stanford University Medical Center in California. “Given that the instructions for time-restricted eating are fairly simple to give and follow, this is a nice option to discuss with patients,” says Dr. Kim, who was not involved with the research.
New Innovative Weight Loss Strategies Are ‘Critically Needed’
Weight loss — achieving a healthy weight through dietary changes, physical activity, weight loss medications or procedures like bariatric surgery — can reduce blood glucose levels and in some cases even lead to T2D remission. That’s why innovative strategies to help people achieve and maintain a healthy weight are critically needed, according to the authors.
Over 90 Percent of Participants Were People of Color
To explore whether time-restricted eating is a more effective way to lose weight and improve blood sugar than calorie restriction, investigators enrolled 75 participants with an average age of 55 years old, an average BMI of 39, and an HbA1C level of 8.1 percent.
As the name suggests, time-restricted eating is when food and liquids containing calories are consumed only during certain hours of the day. Most time-restricted eating studies to date have looked at eating “windows” between 4 and 12 hours long.
A total of 53 subjects were women, 40 percent were Hispanic, 53 percent were Black, 5 percent were white, and 1 participant was Asian. The higher-than-normal percentage of Hispanic and Black participants in the study is notable because diabetes is prevalent among those groups, so having studies that document the success of time-restricted eating for them is particularly useful, the researchers said.
5 percent of Native Americans/Alaskan Natives
1 of Blacks
8 of Hispanics
5 percent of Asian Americans
4 percent of white people
Time-Restricted Eating Group Lost Twice as Much Weight as Calorie Counters
Participants were split into three groups: those who followed the time-restricted eating rules, those who restricted calories, and a control group who didn’t change how they ate.
The time-restricted eating group ate whatever they wanted between 12 p.m. and 8 p.m. each day — they weren’t asked to count calories or eat certain types of food. During the 16-hour window of fasting, they were encouraged to drink lots of water and were allowed to drink calorie-free beverages. They kept a log and recorded the times when they started and stopped eating each day.
The people in the calorie restriction group were instructed to reduce their calorie intake by 25 percent according to their baseline energy need, which was calculated at the start of the trial. That means if a person needed 2,000 calories to perform basic life-sustaining function, their goal was to cut that by 500 calories and consume only 1,500 calories per day. People in this group met with a dietitian at the start of the study who helped them develop individualized weight loss meal plans and log their food intake into an app every day.
Participants’ weight, waist circumference, blood sugar levels, and other health indicators were measured throughout the trial.
In six months, the time-restricted eating group lost 3.6 percent of their body weight, and the calorie-counting group lost 1.8 percent of the body weight compared with the control group. Both groups reduced their HbA1C by 0.9 percent.
“We were surprised that the calorie restriction group did not lose more weight. In most studies looking at people with obesity, the time-restricted eating and calorie restriction groups lose the same amount of weight,” says Dr. Varady.
Time-Restricted Eating May Be Easier to Follow Than Traditional Dieting
Varady suspects it boils down to three reasons. “Looking at the adherence data, the time-restricted eating group was more adherent to their diet than the calorie restriction group,” she says. At least part of that may be because patients with diabetes are generally told to cut back on calories by their doctors as a first line of defense, so many of these participants likely had already tried — and struggled with — that form of dieting, according to the authors.
“Also, most participants in the time-restricted eating group reported that the diet was easy to follow, and at least half the participants in the calorie-restriction group reported the diet was difficult to follow,” says Varady.
And finally, it could come down to numbers: The calorie-restriction group reduced their intake by an average of only 197 calories per day, while the time-restricted eating group reduced their intake by 313 calories per day.
Weight Loss Was Relatively Modest When Compared With New Weight Loss Drugs
“Intermittent fasting is trendy, and people are eager to try it,” says Dr. Kim. “I think there is better data for earlier time-restricted eating (eating between 8 a.m. to 4 p.m.). However, this time period is harder to adapt, as dinner is a very social meal in our society,” she says.
In Kim’s experience, the success of intermittent fasting (as with many other weight loss interventions) is variable. “Many patients do find it difficult to maintain long term. In the study, people were adherent about six days per week,” she says.
The findings also highlight that losing weight isn’t easy, says Kim. The weight loss for the time-restricted eating group was modest (less than 4 percent) compared with effects from currently available GLP-1 receptor agonist medications like Ozempic, Wegovy, and Mounjaro, approved for diabetes and weight loss, she says.
Both Groups Had the Same Average Blood Sugar Reductions
Varady and her team were also surprised that even though weight loss was different, the HbA1C reduction and mean glucose reduction was the same. “This might be because though the two groups lost different amounts of weight, they had similar reductions in their visceral fat and their waist circumference. We speculate that this might be why their blood sugar improved similarly, even though the weight loss was different on the scale,” she says.
Many experts believe that belly fat is just as important as BMI. Too much belly fat not only increases T2D risk, it’s also associated with a higher risk of heart disease and stroke, per the CDC.
Kim points out that there are many variables that impact blood sugar, including the tweaks made to diabetes medications, which seemed to be greater in the time-restricted eating group. “Perhaps the greater surprise is that patients in calorie restriction had a meaningful decline in A1C. This may be a tribute to working with dietitians to reduce carbohydrate intake,” she says.
Interested in Intermittent Fasting? Talk With Your Doctor First
The study was small and should be followed up with larger ones, says Varady. While this acts as a “proof of concept” to show that time-restricted eating is safe for people with T2D, they should consult their doctors before starting this sort of diet, she adds.