Although the study is modest in scale and not without flaws, it contributes additional data on the effectiveness of time-restricted diets.
Intermittent fasting, also known as time-restricted eating, has been shown to aid weight loss. However, the mechanism behind this effect may not be as complex as previously theorized, involving changes in fasting metabolism or circadian rhythms. Instead, the primary reason could be straightforward: restricting the time available for eating leads to a reduction in overall calorie intake.
According to a recent study, individuals who adhered to a time-restricted diet lost a similar amount of weight compared to those who followed the same diet without time restrictions in a randomized controlled trial.
This finding addresses a long standing query in time-restricted eating (TRE) research, which has primarily relied on small feeding studies involving 15 participants or fewer, yielding mixed results and having imperfect methodologies.
The current study, led by Johns Hopkins internal medicine specialist Nisa Marisa Maruthur, has its own set of limitations and does not represent the definitive opinion on the matter. But as nutrition specialists Krista Varady and Vanessa Oddo from the University of Illinois point out in a companion editorial, it advances our knowledge of the fundamental mechanisms underpinning TRE. Maruthur and colleagues show that TRE works to reduce calorie consumption alone, which is why it is beneficial for weight loss when used in conjunction with a controlled feeding scheme.
Of the forty-one participants in the study, twenty had a regular eating pattern (UEP) and twenty had followed a time-restricted diet for 12 weeks. The majority of participants (93%) were Black women who had either pre-diabetes or diabetes under diet management. This limited the generalizability of the results. However, the study meticulously regulated the individuals’ dietary intake and timing; every participant had identical macro- and micronutrient-rich controlled meals (breakfast, lunch, dinner, and snack). A calorie amount was provided to each participant for their meals using a standardized, established method that calculates their baseline caloric needs. They were instructed to keep up their present exercise regimen, which was tracked by an accelerometer worn on their wrist.
Eighty percent of the total daily calories in the time-restricted group were taken before 1 pm, with eating taking place within a 10-hour window from 8 am to 6 pm. Within the typical eating group, individuals ate from 8 a.m. to midnight, with supper and a late-night snack accounting for 55% of total calories consumed. Participants in each dining group were given a window of time within which to consume each prepared meal. Dietitians addressed adherence concerns and the participants’ eating was closely monitored using food diaries and urine testing during their three meals a week at the trial site. In both groups, about 96% of participants adhered to the timetables for the full thirty minutes.
With 93 percent in the time-restricted group and 95 percent in the regular eating group, diet adherence—eating all the food allotted and not consuming outside food—was likewise very good. Both groups dropped almost the same amount of weight—2.4 kg, or 5.3 pounds—over the course of the 12-week study, with no statistically significant difference between them. Additionally, the researchers did not find any variations in the two groups’ blood pressure, cholesterol levels, waist circumference, or glucose homeostasis.
“Our results indicate that when food intake is matched across groups and calories are held constant, TRE, as operationalized in our study, does not enhance weight loss,” Maruthur and her colleagues concluded.
The study’s limitations are acknowledged by the authors, who state that different groups of people and possibly shorter time constraints—e.g., eight hours rather than ten—may have produced different results. They demanded more investigation into those issues.
Experts from outside the field praised the study and said it’s not surprising. Adam Collins, a nutritionist at the University of Surrey, commented, “The headline finding that TRE does not magically lead to more weight loss sounds sensational but is also obvious.”
The University of Glasgow’s Naveed Sattar, a professor of cardiometabolic medicine, praised the work as “well done.”
It “tells us what we expected—that there is nothing magical about time-restricted eating on weight change other than effects to reduce caloric intake,” he said. “If time-restricted eating helps some people eat less calories than they would otherwise, great.”
Experts Varady and Oddo, on the other hand, believe it is beneficial for anyone attempting to reduce weight. “Many patients stop following standard-care diets (such as daily calorie restriction) because they become frustrated with having to monitor food intake vigilantly each day,” their remark stated. “Thus, TRE can bypass this requirement simply by allowing participants to ‘watch the clock’ instead of monitoring calories while still producing weight loss.” The researchers described it as a “simplified” and “accessible” nutritional approach that those with less means can adopt.
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