▎ WuXi AppTec content team editor
Obesity has become a major global public health challenge. According to the recent Report on the Nutrition and Chronic Disease Status of Chinese Residents, more than half of Chinese adults are overweight or obese.
"Keep your mouth shut" is recognized as an important weight loss strategy. Because it is easy to follow, time-limited eating is a popular method. However, based on the current limited evidence, the long-term efficacy and safety of time-limited eating as a weight loss strategy remain uncertain.
截图来源：The New England Journal of Medicine
Recently, the results of the clinical trial led by Professor Zhang Huijie of the Southern Hospital of Southern Medical University were published in the New England Journal of Medicine (NEJM), which provides important evidence for the effect of time-limited eating. This study showed that for obese patients, time-limited eating regimens are no more beneficial than calorie restriction alone in terms of weight loss, improvement of body fat, or metabolic risk factors. This suggests that caloric intake restriction explains much of the effect of a time-limited eating regimen.
The research team suggests that, nonetheless, a time-limited eating regimen can still be an alternative to weight management, but it is also important to limit calorie intake.
Time-limited eating: Is it eating less, or is it an added effect?
Time-limited daily eating is a type of intermittent fasting that requires eating only within a certain time window each day (such as within 4-8 hours).
Usually, this strategy does not limit calories and the foods eaten during eating. However, shortening the eating window may be accompanied by a decrease in food intake, resulting in a decrease in calorie intake; on the other hand, there are also studies showing that time-limited eating can affect the body's circadian rhythm, thereby improving metabolism and obesity.
However, can time-limited eating have an additional long-term effect compared to calorie restriction alone? This issue has not yet been fully explored.
This study answers this question positively.
Practical design of dietary interventions
In a single-center clinical trial, a total of 139 obese patients aged 18 to 75 years (average 31.9 ± 9.1 years old) (body mass index [BMI] 28 to 45, average body weight 88.2 ± 11.6 kg) were randomized to compare the weight loss effects of time-limited eating combined with calorie restriction (n=69) versus calorie restriction alone (n=70). People who are participating in other weight loss programs, taking weight-affecting drugs, or have a variety of chronic diseases such as diabetes and severe cardiovascular and cerebrovascular diseases do not meet the admission criteria.
Specifically, the time-limited feeding group is required to eat only between 8:00 a.m. and 4:00 p.m. Outside of these 8 hours a day, only non-calorie beverages are allowed. This window of time takes into account the habits of Chinese group – usually eating more hearty at noon and eating a large meal in the evening is more likely to increase fat storage. In contrast, before the study began, they had an average of 10 hours and 23 minutes of eating time per day, so the reduction in eating time was not too harsh.
Image credit: 123RF
During the 12 months of the study, all participants were asked to limit calorie intake, 1500 kcal to 1800 kcal per day for men and 1200 kcal to 1500 kcal per day for women, equivalent to 75% of the participants' previous daily caloric intake. The research team also controlled the diet structure, 40% to 55% of the calories in the diet before the start came from carbohydrates, 15% to 20% of the calories came from protein, and 20% to 30% of the calories came from fat.
In order to improve compliance, all participants received dietary counseling during the trial, and further strengthened diet management by distributing diet manuals, encouraging weighing of food, taking photos and punch cards every day to record diets, and regular feedback guidance. As many as 97.1% and 84.9% of the participants completed the 6-month and 12-month interventions, respectively.
These measures also guaranteed the advantages of the study in design and implementation— prescription-based interventions, similar calorie restrictions and concerns about dietary quality in both groups, relatively long trial durations, and a high proportion of participants adhering to the specified protocol.
Both dietary interventions were effective and there were no significant differences
At 12 months, the weight loss effect of both groups of participants was similar from multiple indicators:
Patients in the time-limited eating combined calorie restriction group lost an average of 8.0 kg of body weight, and the average weight loss of the calorie restriction group alone was 6.3 kg, and there was no statistically significant difference between the groups (net difference -1.8 kg; 95% CI -4.0 kg to 0.4 kg; P=0.11).
The proportions of patients with weight loss of more than 5%, 10%, and 15% were similar in both groups.
In different subgroups, the weight changes in both groups remained similar.
In terms of secondary outcomes, similar improvements were seen in both groups in terms of waist circumference (8.8 cm vs 7.0 cm), BMI (2.9 vs 2.3), body fat (-5.9 kg vs -4.5 kg), and lean body mass (1.7 kg vs-1.4 kg). The paper notes that there were no group-to-group differences in lean body mass, suggesting the importance of a balanced diet and adequate protein intake in a time-limited eating regimen.
▲The effect of time-limited eating combined with calorie restriction and simple calorie restriction on weight and body composition. (Image source: References[1])
In terms of metabolic risk factors, there were no statistical differences between the two groups, such as blood pressure, blood lipids, blood glucose, and insulin resistance.
In addition, there was no significant difference in the number of adverse events between the two groups. No deaths or serious adverse events were reported during the trial. The two groups had similar rates of mild adverse events, such as fatigue, dizziness, headache, decreased appetite, epigastric pain, indigestion, and constipation.
Future questions to be answered
The research team objectively pointed out that the study has some limitations. For example, the results did not generalize to people with diabetes or cardiovascular disease, or to other ethnic groups, nor did they represent the effects of time-limited eating over different periods of time; the trials did not assess overall calorie expenditure and did not control the consumption of physical activity. Since weight loss results may vary by sex, further research is needed in the future into the different effects of time-limited eating on men and women.
The NEJM concurrent distribution society article praised the trial as providing an important benchmark for dietary lifestyle interventions that took into account the quality, quantity and timing of nutrition and applied the convenience of digital platform development to facilitate self-monitoring and feedback interventions.
Looking to the future, the editorial proposes that "the concept of time-limited eating is evolving ... Without deliberately restricting calories, eating for a limited time may also help achieve calorie restriction and improve metabolic health." The article further points out that future research needs to identify more specific questions that can guide practice, such as the most appropriate time window for eating, the impact of the same time window but starting sooner or later, and who is most likely to benefit from it, practical implementations, and so on.