The study involved more than 200 individuals randomized to one of three groups: eat only in the morning (from 8:00 AM to noon) followed by 20 hours of fasting 3 days per week and eat as desired on the other days; daily calorie restriction to 70% of requirements; or standard weight loss advice.
The IF plus early time-restricted eating intervention was associated with a significant improvement in a key measure of glucose control versus calorie restriction at 6 months, while both interventions were linked to benefits in terms of cardiovascular risk markers and body composition, compared with the standard weight loss advice.
However, the research, published in Nature Medicine, showed that the additional benefit of IF plus early time-restricted eating did not persist, and less than half of participants were still following the plan at 18 months, compared with almost 80% of those in the calorie-restriction group.
"Following a time-restricted, IF diet could help lower the chances of developing type 2 diabetes," said senior author Leonie K. Heilbronn, PhD, University of Adelaide, South Australia, in a press release.
This is "the largest study in the world to date, and the first powered to assess how the body processes and uses glucose after eating a meal," with the latter being a better indicator of diabetes risk than a fasting glucose test, added first author Xiao Tong Teong, a PhD student, also at the University of Adelaide.
"The results of this study add to the growing body of evidence to indicate that meal timing and fasting advice extends the health benefits of a restricted-calorie diet, independently from weight loss, and this may be influential in clinical practice," Teong added.
Adherence Difficult to IF Plus Early Time-Restricted Eating
Asked to comment, Krista Varady, PhD, said that the study design "would have been stronger if the time-restricted eating and IF interventions were separated" and compared.
"Time-restricted eating has been shown to naturally reduce calorie intake by 300-500 kcal/day," she told Medscape Medical News, "so I'm not sure why the investigators chose to combine [it] with IF. It...defeats the point of time-restricted eating."
Varady, who recently coauthored a review of the clinical application of intermittent fasting for weight loss, also doubted whether individuals would adhere to combined early time-restricted eating and IF. "In all honesty, I don't think anyone would follow this diet for very long," she said.
She added that the feasibility of this particular approach is "very questionable. In general, people don't like diets that require them to skip dinner with family/friends on multiple days of the week," explained Varady, professor of nutrition at the University of Illinois, Chicago. "These regimens make social eating very difficult, which results in high attrition."
"Indeed, evidence from a recent large-scale observational study of nearly 800,000 adults shows that Americans who engage in time-restricted eating placed their eating window in the afternoon or evening," she noted.
Varady therefore suggested that future trials should test "more feasible time-restricted eating approaches," such as those with later eating windows and without "vigilant calorie monitoring."
"These types of diets are much easier to follow and are more likely to produce lasting weight and glycemic control in people with obesity and prediabetes," she observed.
A Novel Way to Cut Calories?
The Australian authors say there is growing interest in extending the established health benefits of calorie restriction through new approaches such as timing of meals and prolonged fasting, with IF — defined as fasting interspersed with days of ad libitum eating — gaining in popularity as an alternative to simple calorie restriction.
Time-restricted eating, which emphasizes shorter daily eating windows in alignment with circadian rhythms, has also become popular in recent years, although the authors acknowledge that current evidence suggests any benefits over calorie restriction alone in terms of body composition, blood lipids, or glucose parameters are small.
To examine the combination of IF plus early time-restricted eating, in the direct trial, the team recruited individuals aged 35-75 years who had a score of at least 12 on the Australian Type 2 Diabetes Risk Assessment Tool but did not have a diagnosis of diabetes and had stable weight for more than 6 months prior to study entry.
The participants were randomized to one of three groups:IF plus early time-restricted eating, which allowed consumption of 30% of calculated baseline energy requirements between 8:00 AM and midday, followed by a 20-hour fast from midday on 3 nonconsecutive days per week. They consumed their regular diet on nonfasting days.Calorie restriction, where they consumed 70% of daily calculated baseline energy requirements each day and were given rotating menu plans, but no specific mealtimes.Standard care, where they were given a booklet on current guidelines, with no counseling or meal replacement.
There were clinic visits every 2 weeks for the first 6 months of follow-up, and then monthly visits for 12 months. The two intervention groups had one-on-one diet counseling for the first 6 months. All groups were instructed to maintain their usual physical activity levels.
Two hundred and nine individuals were enrolled between September 26, 2018 and May 4, 2020. Their mean age was 58 years, and 57% were women. Mean body mass index (BMI) was 34.8 kg/m2.
In all, 40.7% of participants were allocated to IF plus early time-restricted eating, 39.7% to calorie restriction, and the remaining 19.6% to standard care.
The results showed that IF plus early time-restricted eating was associated with a significantly greater improvement in the primary outcome of postprandial glucose area under the curve (AUC) at month 6 compared with calorie restriction, at –10.1 mg/dL/min versus –3.6 mg/dL/min (P = .03).
"To our knowledge, no [prior] studies have been powered for postprandial assessments of glycemia, which are better indicators of diabetes risk than fasting assessment," the authors underline.
IF plus early time-restricted eating was also associated with greater reductions in postprandial insulin AUC versus calorie restriction at 6 months (P = .04). However, the differences between the IF plus early time-restricted eating and calorie restriction groups for postmeal insulin did not remain significant at 18 months of follow-up.
Both IF plus early time-restricted eating and calorie restriction were associated with greater reductions in A1c levels at 6 months versus standard care, but there was no significant difference between the two active interventions (P = .46).
Both interventions were also associated with improvements in markers of cardiovascular risk versus standard care, such as systolic blood pressure at 2 months, diastolic blood pressure at 6 months, and fasting triglycerides at both time points, with no significant differences between the two intervention groups.
IF plus early time-restricted eating and calorie restriction were also both associated with greater reductions in BMI and fat mass in the first 6 months, as well as in waist circumference.
Calorie Restriction Easier to Stick to, Less Likely to Cause Fatigue
When offered the chance to modify their diet plan at 6 months, 46% of participants in the IF plus early time-restricted eating group said they would maintain 3 days of restrictions per week, while 51% chose to reduce the restrictions to 2 days per week.
In contrast, 97% of those who completed the calorie-restriction plan indicated they would continue with their current diet plan.
At 18 months, 42% of participants in the IF plus early time-restricted eating group said they still undertook 2 to 3 days of restrictions per week, while 78% of those assigned to calorie restriction reported that they followed a calorie-restricted diet.
Fatigue was more common with IF plus early time-restricted eating, reported by 56% of participants versus 37% of those following calorie restriction, and 35% of those in the standard care group at 6 months. Headaches and constipation were more common in the intervention groups than with standard care.
The study was supported by a National Health and Medical Research Council Project Grant, an Australian Government Research Training Program Scholarship from the University of Adelaide, and a Diabetes Australia Research Program Grant.
No relevant financial relationships were declared.
Nat Med. Published online April 6, 2023. Full text
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