Counting time instead of calories also controls blood sugar and belly fat

By Sandee LaMotte, CNN

(CNN) — People with type 2 diabetes who followed a time-restricted diet lost belly fat and improved their blood sugar as much as those who counted calories, according to a new randomized clinical trial that followed a group of 75 people for six months.

“Our findings show time restricted eating is a viable alternative for people with type 2 diabetes who are sick of calorie counting for weight loss,” said lead study author Krista Varady, a professor of nutrition at the University of Illinois Chicago, in an email.

Varady was also lead author of a July study that found time-restricted eating had the same impact on weight loss as calorie counting among a racially diverse group of 77 people who were followed for a year.

In the new study, published Friday in JAMA Network Open, people who counted time instead of calories also lost a few more pounds but not much more: There was a 3.6% reduction in weight for time-restricted eating and a 1.8% loss from counting calories compared with those in the control group who were not assigned to a diet.

“We found that just by limiting the eating window to 8 hours per day, the time restricted eating group naturally cut out about 300 calories per day,” Varady said, even though they were allowed to eat whatever they wanted.

Calorie counters were asked to cut 500 calories per day and found tracking the food “too tedious, so they only ended up cutting out 200 calories per day,” Varady said, a factor she attributes to the difference in weight loss.

What was important — and surprising — was that both groups significantly improved their A1C, a test that averages blood sugar levels over the previous three months, she said.

“Both groups reduced A1C by almost a full point (0.9%). This is pretty major,” Varady said. “They started with A1C at 8, so now they are down to 7. Dropping below 6.5 would mean they achieved diabetes remission.”

Both groups also lost equal amounts of dangerous visceral belly fat, which contributes to diabetes, heart disease and stroke, Varady said.

“Controlling blood sugar is more important for those with type 2 diabetes, but losing weight (specifically visceral fat) usually leads to better blood sugar control,” she added. “Even 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.”

Debate on time-restricted eating

Do these results mean time-restricted eating is best for everyone wishing to lose weight? Not quite, said Dr. David Katz, a specialist in preventive and lifestyle medicine. Katz, president and founder of the nonprofit True Health Initiative, a global coalition of experts dedicated to evidence-based lifestyle medicine, was not involved in the study.

“Time restricted eating is a tactic, nothing more, nothing less,” Katz said in an email. “It’s a tactic to ‘punch a hole’ in daily calorie intake. It confers no metabolic magic; the benefits associated derive from caloric reduction, and thus, weight loss.”

Preventive cardiologist Dr. Ethan Weiss followed time-restricted eating in his personal life until he conducted a clinical trial that found no benefits for weight loss or cardiovascular health. In fact, people in his study who fasted from 8 p.m. until noon the next day lost lean mass, which includes muscle.

However, there was no loss of lean muscle mass in the new study, Varady said.

A 2022 study that followed 139 obese Chinese people for a year also found no differences between time-restricted eating and calorie control on body fat, metabolic risk factors or overall weight.

Weiss said he became enamored with time-restricted eating after studies found mice that ate high-fat, high-sugar foods during an eight-hour period didn’t get fat and sick like other mice eating the same diet all day. In 2014, Weiss began eating only in a noon to 8 p.m. window. He quit after the negative results of his study.

“I definitely lost weight when I did it,” said Weiss, who is affiliated with the Cardiovascular Research Institute at the University of California, San Francisco. “But my family hated it, right? If we were going out at 11 o’clock in the morning, I was always quote ‘hangry’ by noon and we’d have to stop whatever we were doing for me to get something to eat. They said I was a pain in the ass.”

Weiss was not involved in the new study, and while he feels it was “nicely done,” he said he believes the jury is still out on the benefits of counting time versus calories.

“There’s a huge debate among scientists on the topic, but to me it doesn’t feel like it’s a magic bullet,” Weiss said. “Maybe it can fit into a suite of interventions we can offer people. I think that seems reasonable.”

Everyone is different

The best route to weight loss can differ from person to person, experts say, which is one reason why there are so many dietary options. One thing is clear, however: Having support along the weight-loss journey helps.

“In our study we provided accountability by meeting with participants weekly for the first 3 months and then biweekly for the last three months,” Varady said. “Work with a dietitian or find a weight loss support group that can provide accountability, as this makes a big difference in sticking to a diet.”

If someone feels that time-restricted eating might be right for them, Varady suggested starting with a 10- to 12-hour window, gradually moving it down to eight hours over a couple of weeks.

“Anyone with type 2 diabetes should first talk with their doctor or with a dietitian before trying time-restricted eating, since certain medications can cause low blood sugar if they’re not taken with food,” she said.

In addition, staying well-hydrated during the fasting period can reduce side effects such as headaches from dehydration, she said.

Time-restricted eating is right for some, but not for others, Katz said. “Having more than one tactic is advantageous, because it empowers more people to find a way that works best and is most comfortable for them,” he said.

“Finally, calorie-control tactics can be superimposed on any dietary pattern but are no substitute for the overall quality of that diet,” he said. “What we eat when we eat is more important than when.”

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