Intermittent fasting has become a popular approach for weight control. Some people see this as fasting for a few days a week and eating on the others. Another approach, however, is to practice time-restricted eating. You would limit meals to certain hours of the day and make sure that you spend at least 12 hours and preferably longer consuming no calories. Several trials show that this approach works just as well as cutting calories throughout the day, if not better.
Time-Restricted Eating for People with Type 2 Diabetes:
In the most recent research on this topic, investigators compared two ways of eating (JAMA Network Open, Oct. 27, 2023). They hoped to learn which one worked best to help people with type 2 diabetes lose weight.
To begin, they assigned 75 volunteers into one of three groups. In one group, people ate only between noon and 8 pm. The second group paid close attention to counting calories and reduced their usual intake by 25 percent. A third group did not change their eating behavior and served as controls. The researchers monitored blood sugar levels as well as weight and waist circumference during the six months of the study.
Both of the groups that changed their eating patterns lowered their HbA1c levels during the study. That is a way of measuring blood sugar over a number of weeks. Those practicing time-restricted eating found it easier to follow the rules. Their weight loss of 3.6% was significant compared to the control group. The calorie counters, on the other hand, lost a modest amount of weight (1.8%) that was not significant. According to the authors, these encouraging results should be further tested in a larger, longer-lasting study.
Counting Calories Compared to Restricted Eating Period:
Which works better to lose weight—counting calories or restricting eating times? Scientists at the University of Illinois, Chicago, recruited 90 obese adults and assigned them randomly to an 8-hour restricted eating period, caloric restriction or no change (Annals of Internal Medicine, June 27, 2023). Both the people who cut their calories by 25% and those who ate only between noon and 8:00 pm consumed fewer calories and lost more pounds than those in the control (no-change) group. Both time-restricted eating and deliberate calorie reduction resulted in similar weight loss, on average. Because individuals vary in their preferences, this study supports the use of either approach. Another strength of this particular study is that the participants came from diverse ethnic groups.
Early Time-Restricted Eating Helps Weight Loss:
Previous studies have also noted a number of potential benefits from limiting eating time (Annual Review of Nutrition, Aug. 21, 2019). The only problem is that there have not been many well-controlled trials of this eating strategy for weight loss. An earlier study suggests that eating only in the first part of the day can be helpful (JAMA Internal Medicine, Aug. 8, 2022).
The study entailed a randomized clinical weight-loss program involving 90 overweight adults. According to random assignment, they ate their meals either between 7 am and 3 pm or within a 12-hour window. All participants had a limited-calorie diet.
After 14 weeks, those who followed the early time-restricted eating plan had lost more weight compared to those who spread their meals out over 12 hours or longer. Early eaters lost nearly 14 pounds, while those who ate throughout the day lost almost 9 pounds during the same time. That difference is equivalent to 214 fewer calories a day.
Although those in the early time-restricted eating group also lost a little bit more body fat, it wasn’t statistically significant. However, they improved their diastolic blood pressure and some measures of mood, including fatigue.
Review of Time-Restricted Eating Studies:
An earlier review also concluded that limiting eating to 8 to 10 hours of the day can help people lose weight. In addition, this tactic can improve their metabolic health (Endocrine Reviews, Sep. 22, 2021). The authors suggested that paying attention to the clock is easier (and more effective) than counting calories. They recommended, however, that the hours for eating be consistent day after day. What follows automatically from such a regimen is intermittent fasting for at least 14 hours out of 24.
One of the scientists noted:
“Intermittent fasting can improve sleep and a person’s quality of life as well as reduce the risk of obesity, diabetes and heart disease.”
Can Time-Restricted Eating Make Weight Loss Easier?
However, a study published in JAMA Internal Medicine threw the idea of time-restricted eating into question (JAMA Internal Medicine, Nov. 2020). Investigators at the University of California, San Francisco, recruited 141 overweight people to participate in a controlled trial. Out of those, 105 completed the three-month study.
How the Study Worked:
The researchers randomly assigned the volunteers to time-restricted eating or continuous meal timing. In other words, one group could eat as they normally did (CMT), while the intervention group ate nothing before noon or after 8:00 o’clock in the evening. Consequently, the intervention group fasted for 16 hours a day.
Sadly, the results were underwhelming. The time-restricted eating group lost roughly 2 to 3 pounds by the end of 12 weeks. (That was statistically significant, though not impressive.) The control group shed about one and a half pounds during the same period of time. That difference is not statistically significant. Nor could the researchers detect differences between the two groups in estimated energy intake.
Interestingly, the weight that was shed in the intermittent fasting group was more muscle than fat. The lead author was surprised by these results, which confirm the need for placebo-controlled trials. He had anticipated that intermittent fasting would give people an advantage in their weight-loss efforts.
Would an Earlier Eating Period Perform Better?
Critics complained that the time frame this study used may have contributed to the disappointing results. They suggested that time-restricted eating might be more effective earlier in the day. That hypothesis was supported (though not proved) by the new early time-restricted eating study reported above.
Time-Restricted Eating for Elite Cyclists:
One other study of time-restricted eating is worth noting, although it is quite small (Journal of the International Society of Sports Nutrition, Dec. 11, 2020). Sixteen elite young cyclists participated in a time-restricted eating trial during a month when they were training hard. The group randomly assigned to restricted hours consumed all their meals between 10 am and 6 pm. The other group ate any time between 7 am and 9 pm. Remember, all of these volunteers were exercising like crazy the whole time. The scientists found that the dietary intervention promoted weight loss (fat only) and improved some metabolic markers. There were no noticeable advantages in cycling performance, but the restrictions on meals appear to reduce inflammation.
We have interviewed experts on chronobiology and time-restricted eating. If the topic interests you, you might listen to Show 1143: Can You Control Your Blood Sugar by Fasting? with Dr. Jason Fung. Dr. Marie-Pierre St. Onge also discussed this idea in Show 1094: Does It Matter When You Eat?