Overview

• What did they evaluate? The researchers compared different feeding windows in relation to diets and their effects on weight loss, body composition, and various cardiometabolic and psychometric health indicators.

• The study results indicate significantly greater weight loss in the timed-feeding group. However, it should be noted that certain methodological features could introduce bias and, therefore, limit the generalizability of these findings.

• For many people, intermittent fasting is a way to control calories that suits their tastes and routines. While it has its merits, so far there's no solid evidence that it's better than other ways of restricting calories.

What's the problem?

Fasting, historically a religious and spiritual practice with physical implications, consists of significant caloric restriction or abstinence from food and beverages for periods ranging from hours to weeks. Although its traditional use persists, modalities such as intermittent fasting have gained popularity as dietary trends. Celebrities and certain health professionals have promoted its purported benefits, driving its widespread adoption. Despite the paucity of controlled human studies, research in this field is advancing rapidly.

Intermittent fasting involves cycles of food intake and abstinence that extend beyond the usual overnight fast. Its various modalities, such as alternate-day fasting, full-day fasting, and time-restricted feeding (TRE), share the principle of alternating periods of eating with little or no caloric intake. This time restriction is postulated to induce a reduction in total daily intake. The rise of popular diets has driven scientific interest in intermittent fasting, although more rigorous human studies are still needed. However, there has been an increase in recent reviews and meta-analyses exploring the effects of different protocols.

Time-restricted eating (TRE) is a type of intermittent fasting that alternates periods of calorie fasting with eating windows. Common variations include the "Weight Gainer Diet" (16 hours of fasting/8 hours of eating) and the "Warrior Diet" (20 hours of fasting/4 hours of eating). While specific research on TRE is limited, benefits of intermittent fasting on body composition and blood markers have been suggested. An analysis of studies in overweight individuals found that intermittent fasting promotes weight loss compared to inactivity, although it does not outperform continuous dieting. However, these studies did not focus exclusively on TRE. Preliminary research suggests that TRE might reduce weight and improve certain blood markers, but lacks comparison groups on continuous diets. Studies that did control calorie intake and compare TRE with non-time-restricted diets have yielded equivocal results. One study showed greater fat loss and metabolic improvements with TRE, while two others found no differences in body composition. A study comparing different meal frequencies (without calorie control) reported mixed results on cardiovascular markers and similar weight loss. In summary, TRE may influence weight and blood markers, but current evidence does not position it as superior to continuous dieting.

Like other fasting protocols, daily calorie restriction is a common consequence, provided it is not compensated by eating more during the permitted hours. A central question is whether the benefits of fasting are primarily due to this calorie reduction and subsequent weight loss, or whether periods of food abstinence offer intrinsic advantages. A study by Stote et al. (2007) observed a decrease of 65 kcal/day in the single-meal group, without this translating into weight loss. In contrast, Moro et al. (2016), controlling for calories and meal timing, found improvements in body composition and blood markers with time-restricted eating (TRE). A recent meta-analysis suggests a slight weight loss advantage with intermittent fasting, although it is crucial to consider that many studies compare fasting with control groups who eat without calorie restriction and often ingest more calories. Therefore, the calorie deficit, rather than the fasting protocol itself, could be the key factor in the observed weight loss. The study analyzed supports the idea of ​​eating within a specific window to optimize fat loss. To avoid misinterpretations, let's examine this study in detail.

Purpose and Hypothesis

The study compared two diets with different eating window lengths, seeking to determine whether a specific eating schedule optimized weight loss and cardiometabolic health.

What Did They Test and How?

Participants

The study included 90 participants (80% women) with obesity, with an average BMI of 39.6 and a mean age of 43 years. Attrition was comparable between the diet groups. Another study, impacted by COVID-19, had 59 participants complete all phases.

Study Procedures

Participants were randomly assigned two eating schedules, which they were required to follow for a 14-week period, which was considered long enough for this study. Researchers asked participants to maintain their assigned eating schedule at least six days a week.

• Scheduled feeding protocol: 8-hour window between 7:00 a.m. and 3:00 p.m.

• Control group: unrestricted access to food for 12 hours.

To ensure a fair comparison, both groups received the same weight-loss treatment: individualized counseling by a registered dietitian at baseline and at weeks 2, 6, and 10, a calorie deficit of 500 kcal/day relative to resting energy expenditure, and an exercise program of 75 to 150 minutes per week tailored to their baseline activity level. The crucial difference between the groups was solely in the feeding window, as detailed above.

Measurements

The primary objective was to evaluate changes in weight and body fat. Secondary objectives included measuring cardiometabolic risk factors and exploring behavioral and subjective aspects.

Participants' body weight was measured biweekly in the laboratory, always under fasting conditions. Body composition was determined using DEXA, which allowed fat loss to be assessed in two ways: as a proportion of total weight loss and as an absolute change in fat mass. It is important to note that the relationship between fat loss and weight loss was only analyzed in participants who completed the study and experienced a weight loss of at least 3.6 kg. This decision was justified to avoid unreliable data that could arise from technical errors in the DEXA scan in the face of minimal losses. However, the relevance of searching for significant differences between groups using this criterion deserves further discussion.

Energy and macronutrient intake was determined through a self-reported 3-day food record, implementing an innovative validation strategy using digital photographs of meals. A weight-loss model was applied to analyze disparities in caloric intake between groups. Daily recording of eating periods allowed for the assessment of adherence, defined as completion of the designated time window within a 30-minute tolerance. The protocol also included the administration of psychometric questionnaires to assess mood, sleep quality, and physical activity levels.

Analysis of cardiometabolic risk factors, including: blood pressure, glucose, insulin (and its resistance through HOMA-IR), HbA1c and lipids.

What Did They Find?

Body Composition

Both groups showed significant decreases in body weight. Figure 1 details the evolution of weight loss between feeding periods throughout the study, as measured in a non-fasting state in the laboratory. The primary analysis showed a significantly greater weight loss of 2.3 kg in the timed-feeding group. However, changes in absolute fat loss and the ratio of fat loss to weight loss did not differ significantly between groups. No other significant differences were observed in total body weight or body fat. The researchers also conducted a secondary analysis, restricted to participants who completed the study. This analysis confirmed the primary result, indicating a greater weight loss of 2.3 kg in the timed-feeding group (95% CI, -3.9 to -0.7 kg; p = 0.006). Although statistical significance persisted, the p-value suggested a slight decrease in robustness (p = 0.002 vs. p = 0.006). Furthermore, this analysis in participants who completed the study identified a marginally significant difference in absolute fat mass (-1.8 kg, p = .047) and trunk fat mass (-1.2 kg, p = .03).

Figure 1 Weight Loss and Body Composition Changes

(A) Change in scale body weight readings in the lab (nonfasted).  Data shown are least-squares means and 95% CIs from linear mixed modeling with adjustment for age, sex, and race. There were no statistically significant differences between groups for body fat (B), fat-free mass (C), or the ratio of fat loss to weight loss (n = 41) (D). Data shown in panels B-D are means and 95% CIs from multiple imputation by chained equations, followed by linear regression with adjustment for age, sex, and race.

Adherence, and Physical Activity

The application of a weight-loss modeling tool to the study of dietary intake revealed a statistically significant difference in energy intake between groups. The experimental group, which received regular-interval feeding, had an energy consumption 214 kcal/day higher than the control group. Time-based analysis of intake revealed that the experimental group spent an average of 7.6 hours eating, compared to 12.3 hours for the control group (p < 0.001). Importantly, no intergroup disparities were identified in self-reported physical activity or the macronutrient composition of the diet.

Cardiometabolic Health

Scheduled feeding significantly decreased diastolic blood pressure by 4 mmHg (∆+4 mmHg) compared with the control group. An analysis of participants who completed the study revealed a slightly greater reduction of 5 mmHg (∆+5 mmHg, p = 0.01). No significant changes were found in other cardiometabolic risk factors.

What Do the Findings Mean?

While the study was generally conducted with rigor, several limitations affect the strength of its conclusions. The authors state, “The key finding of this study is that TRE was more effective for weight loss than eating for 12 hours or more.” However, after analyzing the methods section, we found two major issues that prevent us from supporting this conclusion.

The study data revealed that scheduled eating led to a significant decrease in energy intake, with an average difference of 214 kcal/day (1,284 kcal/week) compared to the control group. An intriguing finding is that, despite this considerable caloric difference, the scheduled eating group did not experience proportionally greater weight loss.

The primary outcome variable, weight loss, was rigorously measured in a laboratory setting. However, the lack of fasting during the intervention limits the strength of this evidence. Fortunately, DEXA measurements, performed in the fasting state at baseline and endpoint, provide greater reliability for fat mass data. The primary analysis of these data revealed no significant differences between groups in terms of fat mass reduction, despite a favorable trend (1.8 times greater) in the scheduled-feeding group. However, a secondary analysis of participants who completed the study did show a significant reduction in total and trunk fat mass with scheduled feeding. It should be noted that this latter analysis, by excluding those who dropped out of the study, could overestimate the true effects.

Having noted the main limitations of this study, let's examine its direct results. Scheduled feeding achieved a weight reduction of 2.6 kg, but this difference was not confirmed by DEXA, a more objective method, which only revealed a 1.8 kg difference in fat mass in a secondary analysis. Given the participant selection (>3.6 kg weight loss) and a 200 kcal lower caloric intake in the scheduled feeding group, this difference appears modest. Apart from a slight 5 mmHg reduction in diastolic blood pressure, possibly linked to the slightly greater weight loss, no other significant differences were observed in cardiometabolic variables, mood, sleep, adherence, or physical activity. Given these minor unsupported changes in secondary outcomes and the lack of strong evidence favoring intermittent fasting over calorie-balanced diets, its effectiveness remains uncertain due to the lack of precise caloric controls. The diversity of intermittent fasting strategies further complicates identifying the most effective. Therefore, the striking benefits of fasting on weight loss or metabolism require confirmation in rigorously controlled clinical trials.

How Can You Apply These Findings?

Although we disagree with the researcher's conclusion, we believe that intermittent fasting, in its various forms, represents a flexible and useful dietary strategy for achieving different goals. Its ability to facilitate calorie restriction at specific times or on days of high demand makes it a valuable tool. For example, individuals with busy schedules can opt for light meals in the morning and midday, enjoying a more complete dinner with the family. The versatility in structuring meals, adapting to habits and lifestyles, is undeniable. Contrary to some claims, intermittent fasting does not appear to increase the risk of muscle loss, and its effectiveness ultimately lies in energy balance. Therefore, there is no need to focus excessively on complex protocols like the 16:8; the key lies in using fasting as a means to control calorie intake and promote fat loss.

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