Effects of Time-Restricted Eating (TRE) on Weight Loss and Health: A Systematic Review and Meta-Analysis

This study is a meta-analysis that systematically analyzed the effects of time-restricted eating (TRE) on weight loss and metabolic health in overweight and obese individuals. The effects of TRE are determined by two main factors -- energy restriction and eating timing -- with the conclusion that energy restriction plays a more dominant role in health improvement. The study also emphasizes that eating earlier in the day (e-TRE) is more beneficial for health, and proposes future directions for TRE research.

1. Introduction: Obesity and the Rise of TRE

Over the past 20 years, the global surge in overweight and obesity has made chronic diseases such as cardiovascular disease, diabetes, and certain cancers serious public health concerns. The most common approach to addressing these issues is daily caloric restriction (CR), sometimes combined with exercise.

However, long-term caloric restriction often encounters these difficulties:

Physiological adaptation to weight loss
Sudden hunger
Relatively low dietary adherence rates

In this context, a recently emerging dietary strategy is time-restricted eating (TRE). TRE involves eating only during a fixed window of 8--12 hours per day, and can be viewed as an intermittent caloric restriction strategy. By limiting daily eating hours, caloric intake is unintentionally reduced.

Why Is TRE Gaining Attention?

TRE is considered superior to traditional continuous caloric restriction and other intermittent fasting methods because it aligns eating times with the body's circadian rhythm, beyond simply reducing caloric intake. The advantages of reducing energy intake and maintaining dietary adherence more easily make it a promising strategy for long-term weight loss and metabolic health improvement.

Early animal studies showed that time-restricted feeding (TRF) had positive effects on metabolic health and lifespan, but whether these results apply equally to humans remained unclear.

Mixed Results in Human TRE Studies

Human TRE studies exploring potential benefits for weight loss and metabolic health improvement have yielded varying results.

Some studies showed substantial weight loss -- overweight/obese participants practicing 10-hour TRE for 8 weeks lost an average of 10.7 kg (approximately 8.5% of initial body weight).
However, other studies reported modest weight loss of less than 2--5 kg.

Controversy over metabolic improvements was even greater. Studies examining whether TRE can improve metabolic health, and whether these benefits are possible without energy restriction or are modulated by eating timing, showed diverse results.

Various Types of TRE Interventions

One important reason for these mixed results is that TRE encompasses several subtypes. Researchers classified TRE as either isocaloric (energy intake controlled) or ad libitum (energy intake uncontrolled) based on whether energy intake was restricted, and as early TRE (e-TRE) or delayed TRE (d-TRE) based on whether meals were consumed earlier or later in the day.

Generally, e-TRE is thought to help improve metabolic homeostasis by maintaining the daily rhythm of the meal-fast cycle. In contrast, d-TRE, where meals misalign with the circadian rhythm, may actually increase hunger and harm metabolic health. These differences in energy intake patterns and meal timing contributed to the high heterogeneity of TRE research.

Limitations of Previous Meta-Analyses and Study Objectives

Previous meta-analyses of TRE had several limitations. For example, some had low study quality or inconsistent inclusion criteria, included very short intervention periods (7 days or less) or religious fasting (Ramadan), or included non-randomized controlled studies (n-RCTs).

This study conducted a systematic review and meta-analysis to address these limitations and determine the practical effects of TRE on weight loss and metabolic health. In particular, it classified TRE into isocaloric vs. ad libitum and e-TRE vs. d-TRE to explore the fundamental mechanisms of TRE.

The authors hypothesized that energy restriction contributes more than eating timing to weight loss and metabolic health in TRE strategies. Specifically, they proposed that participants in ad libitum conditions with reduced energy intake would experience greater health improvements, that e-TRE would improve health outcomes beyond d-TRE only in ad libitum conditions, and that the timing advantage would disappear in isocaloric conditions where energy intake was predetermined.

2. Results: Multifaceted Analysis of TRE Effects

This meta-analysis included 19 studies in total. Three studies were three-armed trials (two intervention groups and one control group), which were each treated as separate comparisons, yielding 22 intervention groups. Participants' mean ages ranged from 22.7 to 65 years, and mean BMI ranged from 27.8 to 38.9 kg/m2. Eating windows ranged from 4 to less than 12 hours, and study durations varied from 5 to 48 weeks.

A total of 3,156 studies were screened, and after duplicate removal and full-text review, 19 studies were ultimately included. Of these, 11 were isocaloric and 8 were ad libitum. Six studies included e-TRE, 7 included d-TRE, and 8 did not specify eating timing.

2.1. Energy Intake and Eating Timing Are Associated with Moderate Weight Loss

TRE effectively reduced body weight percentage compared to controls (mean -2.04%, 95% CI: -2.57 to -1.50). Meta-regression analysis showed that weight loss percentage was significantly associated with energy intake (Z = 5.23, p < 0.001) and eating window (Z = 2.26, p = 0.024) but not with intervention duration (Z = 0.36, p = 0.72). This means that greater energy intake reduction and shorter eating windows lead to greater weight loss. No publication bias was detected (Egger's test p = 0.93).

Meta-analysis results of weight loss Figure 2: Meta-analysis results for weight loss

2.2. TRE Helps Improve Waist Circumference and Body Composition

TRE reduced the following compared to controls regardless of caloric restriction:

Waist circumference: -2.42 cm (95% CI: -3.42 to -1.42)
BMI: -0.78 kg/m2 (95% CI: -1.01 to -0.54)
Fat mass: -1.36 kg (95% CI: -1.76 to -0.97)

However, TRE did not significantly reduce visceral fat (-0.04 kg, 95% CI: -0.10 to 0.01). Additionally, TRE also slightly reduced lean mass (muscle mass) (-0.43 kg, 95% CI: -0.77 to -0.08).

2.3. TRE Reduces Energy Intake Whether Intentionally or Not

Participants in TRE groups reduced daily energy intake by an average of 201.77 kcal compared to controls (95% CI: -304.12 to -99.43). Meta-regression showed that eating window length significantly affected energy intake (Z = 2.05, p = 0.040), while intervention duration did not (Z = 0.57, p = 0.57). This demonstrates that shorter eating windows lead to greater energy intake reduction. No publication bias was found (Egger's test p = 0.92).

Meta-analysis results of energy intake Figure 3: Meta-analysis results for energy intake

2.4. TRE Improves Systolic Blood Pressure and Blood Glucose Levels

TRE effectively improved the following metabolic risk factors compared to controls regardless of energy restriction:

Systolic blood pressure: -2.42% (95% CI: -4.34 to -0.50)
Fasting glucose: -2.57 mg/dL (95% CI: -4.73 to -0.42)
Fasting insulin: -1.81 uIU/mL (95% CI: -3.24 to -0.38)
HbA1c: -0.34 (95% CI: -0.60 to -0.07)

However, other metabolic markers (diastolic blood pressure, triglycerides, total cholesterol, HDL, LDL, etc.) were not significantly affected.

2.5. Energy Reduction Provides Greater Benefits Than Isocaloric Diets

To determine how important actual energy expenditure is in TRE diets, subgroup analysis was conducted by dividing TRE into isocaloric (energy controlled) and ad libitum (energy uncontrolled) conditions.

Weight loss percentage: The ad libitum condition showed 3.08% weight loss compared to controls (-3.08%, 95% CI: -3.42 to -2.73), while the isocaloric condition showed smaller weight loss (-1.25%, 95% CI: -1.74 to -0.75).
Energy intake: The ad libitum condition showed substantial energy reduction between TRE and control groups (-445.96 kcal/day, 95% CI: -468.58 to -423.33), while the isocaloric condition showed no significant energy reduction (-51.53 kcal/day, 95% CI: -106.62 to 3.55).

This suggests that weight loss in isocaloric conditions is due to eating timing benefits, while weight loss in ad libitum conditions is due to substantial energy deficit. However, the fact that weight loss (-3.08%) was less than expected despite a substantial energy deficit (-445.96 kcal/day) in the ad libitum condition suggests participants may have over-reported their daily energy reduction.

Other anthropometric and body composition analyses also showed that waist circumference, BMI, and fat mass reductions were greater in ad libitum than isocaloric conditions. There was no difference in visceral fat between groups. Lean mass decreased significantly in ad libitum TRE groups compared to controls (-0.68 kg, 95% CI: -1.14 to -0.23) but showed no difference between groups in isocaloric studies.

For metabolic risk factors, systolic blood pressure, triglycerides, fasting insulin, and HOMA-IR decreased more in ad libitum studies, with smaller changes when energy intake was controlled.

In conclusion, TRE showed improvements in weight loss, anthropometric indices, body composition, and metabolic health measures. Subgroup analysis based on energy restriction supports the hypothesis that energy restriction contributes to these benefits -- when energy intake was controlled, improvements in weight and metabolic markers were smaller.

2.6. Eating Timing Also Improves Health in TRE Patients

The subgroup analyses above demonstrated the role of energy deficit in improving health outcomes with TRE. However, even in isocaloric conditions, improvements in weight loss percentage, anthropometric indices, body composition, and some metabolic markers were noteworthy. Since many studies confirmed the importance of eating timing in TRE, additional analysis was conducted using four subtypes considering both energy restriction status and eating timing (e-TRE or d-TRE) to separate the effects of eating timing and energy expenditure.

Weight loss: In ad libitum conditions, e-TRE produced greater weight loss than d-TRE. In isocaloric conditions, weight decreased with e-TRE but not with d-TRE.
Energy intake: In ad libitum conditions, e-TRE produced greater energy intake reduction than d-TRE, but there was no difference in isocaloric conditions.
Waist circumference, fat mass, lean mass, fasting insulin, HbA1c: In ad libitum conditions without energy restriction, e-TRE showed greater improvements than d-TRE, but there was no difference when energy intake was controlled.
HOMA-IR: Improved only in e-TRE without energy restriction.

These results demonstrate that eating timing can enhance TRE's effects to some degree for improved health outcomes in ad libitum conditions.

3. Discussion: Synergy Between Energy Restriction and Circadian Rhythm

This systematic review and meta-analysis explored the effects of TRE on weight loss and metabolic health based on 19 randomized controlled trials involving 1,201 overweight and obese participants. We found that TRE is effective for weight loss and metabolic health improvement, and that these benefits result from the combined effects of two factors: energy restriction and eating timing. Subgroup analysis further confirmed that energy restriction plays a more dominant role than eating timing.

In isocaloric conditions, only e-TRE improved health outcomes, and while ad libitum d-TRE was also effective with substantial energy reduction, it was not as beneficial as e-TRE.

3.1. Weight Loss and Energy Intake in TRE Strategies

TRE reduced weight and actual energy expenditure but did not reach the clinically significant threshold of 5% weight loss. This is consistent with previous meta-analyses. Meta-regression showed that weight loss was significantly associated with energy expenditure (p < 0.001) and eating window (p = 0.024) but not intervention duration (p = 0.72). This shows that greater caloric deficit and shorter eating windows increase TRE's weight loss potential within safe ranges.

Since most people eat over more than 12 hours daily, restricting eating windows can reduce meal quantity, whether intentionally or not, to achieve weight loss. An 8-hour eating window has been reported as a safe and sustainable duration, while excessively restricted eating windows may increase binge eating risk. The lack of correlation between weight loss and intervention duration suggests the need for studies with longer and more diverse intervention periods. Some studies have suggested that the threshold for TRE effects is approximately 12 weeks, with highest dietary adherence and best effects at this point, after which adherence declines and effects diminish.

Although the magnitude of weight loss was modest, subgroup analysis clearly showed the benefits of eating timing in TRE. For people with metabolic symptoms, TRE can be used as a strategy to improve metabolic markers.

Subgroup analysis showed more weight loss in ad libitum than isocaloric conditions. Both e-TRE and d-TRE reduced weight in ad libitum conditions with substantial energy deficit (approximately 450 kcal/day), but e-TRE was more effective. The weight loss benefit of d-TRE disappeared in isocaloric conditions where energy intake was practically identical. Weight loss with e-TRE under prescribed conditions suggests that appropriate eating timing can actually improve health outcomes in overweight and obese individuals even without energy reduction. In conclusion, weight loss in TRE strategies was likely driven by the combination of energy deficit and eating timing, but energy reduction showed a stronger effect.

3.2. Effects of TRE on Anthropometric Indices, Body Composition, and Some Metabolic Outcomes

In anthropometric and body composition analysis, TRE reduced waist circumference, BMI, fat mass, and lean mass but did not affect visceral fat. From this perspective, TRE may not be more effective than traditional caloric restriction diets at improving fat mass reduction while maintaining lean mass. In traditional caloric restriction, weight loss always accompanies lean mass loss. The lean mass reduction may be because overweight and obese individuals were included in this meta-analysis, and physical exercise combined with adequate protein intake can mitigate lean mass loss. The differing lean mass changes between isocaloric and ad libitum conditions in subgroup analysis shows that energy restriction matters for lean mass maintenance.

For metabolic risk factors, TRE improved systolic blood pressure and glycemic markers (fasting glucose, fasting insulin, HbA1c) but did not change other markers (diastolic blood pressure, triglycerides, total cholesterol, HDL, LDL, HOMA-IR, or resting metabolic rate) between TRE and control groups.

Blood pressure is associated with metabolic disease and heart failure, but research on TRE's effects on blood pressure has yielded mixed results. This meta-analysis showed systolic blood pressure decreased in ad libitum TRE groups with substantial energy reduction, while diastolic blood pressure remained unchanged. A study of shift workers found significant blood pressure reductions in participants with elevated systolic (>= 130 mmHg) or diastolic (>= 85 mmHg) blood pressure, suggesting TRE's importance for those at higher baseline cardiometabolic risk. Subgroup analysis showed systolic blood pressure decreased in ad libitum conditions, with e-TRE being more effective than d-TRE only at relatively lower energy intake levels.

Plasma lipids are another factor affecting metabolic and cardiovascular disease. However, TRE's benefits are more pronounced in people with metabolic syndrome, and since most participants in this meta-analysis did not have metabolic symptoms, no significant improvements in triglycerides, HDL, or LDL were observed.

Blood glucose levels and insulin resistance contribute to both metabolic and cardiovascular disease. Previous clinical data and this meta-analysis demonstrate strong effects of TRE on fasting glucose and fasting insulin levels, presenting TRE as a useful therapy for type 2 diabetes, where glycemic control is crucial for minimizing diabetes-related complications and improving health and quality of life.

Consistent with some previous research concluding that e-TRE is superior to d-TRE for metabolic improvement, this meta-analysis also found that body composition and metabolic markers were better with e-TRE than d-TRE in ad libitum conditions. However, when energy intake was controlled, d-TRE showed no benefits. In other words, in TRE strategies, the benefits of energy deficit for weight loss and metabolism outweighed the benefits of eating timing. People consuming more energy than baseline may gain weight even with TRE strategies that adjust eating timing. Low dietary quality with low-nutritional, high-fat foods can also limit TRE's effectiveness. Most outcomes in this meta-analysis did not reach clinical significance. However, the intervention effect of TRE on health improvement was greatest in the e-TRE subgroup with substantial energy deficit.

In conclusion, TRE could lead to moderate weight loss, reductions in waist circumference, BMI, fat mass, and lean mass, and improvements in systolic blood pressure, fasting glucose, fasting insulin, and HbA1c compared to controls. However, TRE did not affect visceral fat, diastolic blood pressure, triglycerides, total cholesterol, HDL, LDL, HOMA-IR, or resting metabolic rate. Subgroup analyses based on energy intake and eating timing suggested that energy restriction and eating timing jointly contribute to weight loss and metabolic health improvement in TRE strategies, but the effect of energy reduction is more important.

3.3. Study Limitations and Future TRE Research Directions

The limitations of this systematic review and meta-analysis include:

First, the small number of trials prevented review of several relevant outcomes, and intervention strategies were relatively simple. Except for one study lasting one year (48 weeks), most study durations were relatively short (8--12 weeks), with insufficient follow-up on TRE's long-term effects.
Second, subgroup analyses and meta-regression using group-level data carry the possibility of ecological fallacy. Therefore, these conclusions do not indicate causation and cannot be generalized to the individual level.
Third, since only one study met the criteria for e-TRE without energy restriction in subgroup analysis, great caution is needed when explaining the interaction between energy expenditure and eating timing on TRE's weight loss and metabolic improvements.
Fourth, patients with metabolic symptoms were not distinguished from other participants; previous research has reported TRE as a more efficient dietary strategy for people with metabolic syndrome.
Fifth, Ramadan fasting was not included in this meta-analysis because the specific pattern of eating only after sunset and fasting during the day does not apply to the majority of non-Muslim overweight and obese populations.
Sixth, the appropriateness of energy intake measurement was not considered in exclusion criteria during paper screening.

Although most clinical RCTs measured adherence during TRE, only four studies quantitatively reported adherence as participation day percentage or weekly adherence days. Also, self-reported adherence measurements via remote video or daily adherence logs depend on participant honesty and may be subject to social desirability bias. Beyond using adherence for predicting long-term weight maintenance, the distribution of protein and carbohydrate intake is important for understanding TRE's full benefits since these macronutrients are significant in modifying body composition and blood glucose levels. Biological hunger and satiety can drive appetite and are related to TRE eating times, yet few studies measured leptin levels or reported subjective hunger.

Sex is another physiological factor that may influence TRE outcomes. Most participants in this meta-analysis were female (65.07%). Therefore, more research is needed to investigate the impact of sex differences on TRE strategies between men and women with identical baseline body composition and metabolic risk factors.

When promoting TRE strategies to a wider population, several public social issues must be considered. Activity-rest cycle disruptions from artificial lighting and industrialization indirectly interfere with the natural daily meal-fast cycle and promote excessive caloric intake. Additionally, shift workers, people of lower economic status, and those lacking social support may find it difficult to restrict daily eating to less than 12 hours. Future research should expand to understand TRE feasibility in these populations.

Conclusion: TRE Opens the Path to Health Improvement

Through this systematic review and meta-analysis, we confirmed that time-restricted eating (TRE) has a positive effect on weight loss and metabolic health improvement in overweight and obese individuals. Notably, reduced energy intake and appropriate eating timing together produce these benefits. Although the magnitude of weight loss was not very large, the reductions in waist circumference, BMI, fat mass, and lean mass, along with improvements in systolic blood pressure, fasting glucose, fasting insulin, and HbA1c, suggest that TRE can be a promising strategy for a healthier lifestyle.

However, remember that TRE's effectiveness goes beyond simply restricting eating times -- it requires comprehensive consideration of what, when, and how much you eat! We look forward to future research exploring TRE's long-term effects and applicability across diverse populations, and to developing personalized TRE strategies.