Adding a fasting period to cut calories did not improve results, but experts say the common approach requires more study.
Intermittent fasting as a method for weight loss has grown in popularity in recent years, but no definitive trials support this approach. Now, a small new randomized trial may only muddy the waters, which experts say has failed to provide definitive answers about time-restricted eating as a way to lose weight.
The study, led by Deying Liu, MD (Nanfang Hospital, Southern Medical University, Guangzhou, China), showed that obese subjects who reduced their daily calorie intake by 25% did not enhance weight loss by also adhering to intermittent fasting. That you are limited to eating for 8 hours every day.
Mean weight loss at one year was 8.0 kg (17.6 lb) among those who restricted their food intake and 6.3 kg (13.9 lb) among those who ate all day, a non-significant difference (s = 0.11). The researchers reported in the April 21, 2022 issue that the percentages of participants who lost weight by more than 5%, 10%, or 15% did not differ between groups. New England Journal of Medicine.
Furthermore, intermittent fasting did not provide any benefits in terms of other measures of body composition and cardiac metabolic risk.
“Our data indicate that calorie restriction explained most of the beneficial effects of a time-restricted eating regimen,” the authors wrote. “However, our results support a time-restricted eating strategy combined with caloric restriction (prescribed in accordance with current dietary guidelines) as a viable and sustainable approach to obesity management.”
But Dariush Mozaffarian, MD, PhD (Friedman School of Nutrition Science and Policy, Tufts University, Boston, MA), told TCTMD that the trial is not helpful to address the feasibility of intermittent fasting for weight loss, citing three major shortcomings. First, the recommended combination of diets consumed in both groups — low in fat and high in carbohydrates — contrasts with research over the past 20 years that has shown high-fat, low-carb diets to be better. Second, the main analysis focused only on the 85% of participants who completed the study, rather than assuming that those who left the study did not lose any weight, which is the standard approach. This overestimates the weight loss in the study, Mozaffarian said.
Finally, he said, the trial did not address the fundamental question of interest in this area – whether intermittent fasting alone (without calorie restriction) leads to more weight loss than caloric restriction alone. “The primary goal of intermittent fasting is instead of restricting calories to fast and then eat as you please the rest of the time,” he said, adding that one would not expect to see a difference in weight loss if calories were controlled in both groups, as happened here.
“I think it was not a useful study,” Mozaffarian stated.
Dear from the guide
The authors note that some experimental studies have suggested that intermittent fasting leads to a loss of body weight and fat mass in obese people, although one previous experience They showed similar amounts of weight loss when comparing time-restricted food intake with three structured meals per day.
“However, these studies did not provide enough conclusive information to support evidence-based clinical guidelines for obesity,” wrote Liu et al. “In addition, the efficacy and safety of time-restricted eating as a long-term weight loss strategy remains uncertain, and the long-term effects on weight loss of time-restricted eating compared to daily calorie restriction alone have not been fully explored.”
For the current trial, the investigators enrolled 139 individuals (median age 32 years; 51% men) with obesity (body mass index). [BMI] 28 to 45 kg/m2) Living in Guangzhou, China, randomly choosing them to eat time-restricted food (between 8 a.m. and 4 p.m. only, with non-calorie drinks allowed outside that window) in addition to calorie restriction or calorie restriction alone.
Participants in both groups were told to eat 1,500 to 1,800 calories per day for men and 1,200 to 1,500 calories per day for women, which is about a 25% deficit compared to their normal consumption. In terms of diet composition, participants aim to get 40% to 55% of their calories from carbohydrates, 20% to 30% from fat, and 15% to 20% from protein. All were given one protein drink per day for the first six months, as well as nutritional advice for the duration of the study.
The study lasted one year, and during that time, the average percentage of days during which participants adhered to the set calories and/or eating period was 84% in both arms. The average decrease in calories and consumption of fats, carbohydrates, and protein was similar in the two groups.
Mean body weight at baseline was 88.2 kg (194.4 lb), and this decreased during the study, with no significant difference between groups. Furthermore, adherence to intermittent fasting did not provide additional benefits in terms of changes in additional measures of adiposity, cardiac metabolic risk, or quality of life. There were no deaths or serious adverse events during the trial.
Better studies are needed
Mozaffarian said the potential benefits of intermittent fasting are less about adherence, and more about the potential biological changes caused by eating over a limited period of time. He explained that fasting creates periods of potential ketogenesis and may alter adipocyte metabolism, liver metabolism, brain metabolism, and the microbiome in ways that may have long-term weight loss benefits.
“Weight loss is not just about commitment. Weight loss is about biology. The types of foods you eat, and perhaps when you eat them, change biology in different ways,” he said.
Sai Das also commented on TCTMD (Friedman School of Nutrition Science and Policy, Tufts University), and noted that the concept of time-restricted eating, or intermittent fasting, arose from a desire to find an easier and less stressful approach to lowering the amount of calories consumed.
And although this trial failed to demonstrate the advantage of intermittent fasting in the context of daily calorie restriction, the message shouldn’t be that there is no benefit, Das said, noting a “very consistent pattern” of weight gain and slight fat loss, As well as metabolic benefits, in that group. She said the trial suffered from insufficient statistical power, and the potential impact of intermittent fasting should be explored further.
Das highlighted some of the other shortcomings of this trial. For example, the 8-hour restricted eating window used in this study has not been shown to be feasible in other countries, including the United States, where the average eating window typically ranges from 12 to 14 hours. Additionally, she said she would have liked to have noticed a 20% to 25% calorie deficit tailored to each participant, rather than using the general intake ranges for men and women in general. Finally, the loss of lean mass was relatively high, accounting for about 30% of the weight loss in each group. “I think it is important that the type of calorie restriction that is being implemented is defined or established…it reduces fat loss because in the long run this is not going to be the best way to change any type of weight,” she said.
In an accompanying editorial, Blandin Lavery, MD, PhD (Columbia University Irving Medical Center, NY, NY), and Satchidananda Panda, PhD (Salk Institute for Biological Studies, La Jolla, CA), also detail some of the limitations of the current analysis, and point out That further research is warranted.
“The concept of time-restricted eating is evolving. Future studies will determine the appropriate duration of the time window for eating, which is likely to benefit from this approach, how time-restricted eating is implemented and potential mechanisms for doing so, and the effects of time-restricted eating earlier in the day compared to late of today,” they write. “From a public health standpoint, time-restricted eating may turn out to be an approach to accomplishing calorie restriction and improving metabolic health without a resource-intensive approach to intentional calorie restriction.”
When asked why it is difficult to get definitive answers to optimal eating patterns from clinical trials, Mozaffarian noted a lack of funding for these types of studies. his group issue a call To strengthen national nutrition research, and advocate in 2020 for the creation of a new National Institute of Nutrition within the National Institutes of Health (NIH) and for the reinstatement of the Office of Nutrition Research in the Office of the Director of the National Institutes of Health. The latter came to fruition last year, but, as Mozaffarian said, “until we have a National Institute of Nutrition, I think we’ll continue to have small, underfunded, and equivocal science.”