Adherence and Retention in Early or Late Time-Restricted Eating: A Narrative Review of Randomized Controlled Trials

Abstract

Time-restricted eating (TRE) is a form of intermittent fasting that involves reducing the time-period in which food is typically consumed daily. While TRE is known to induce health benefits, particularly for adults with obesity, there is currently debate about whether the time of day in which food is consumed also contributes to the health benefits of TRE. Early TRE (eTRE) and late TRE (lTRE) are subtypes of TRE that involve consuming food and caloric beverages either in the early or later part of the day. A growing body of literature indicates that eTRE may offer additional health benefits compared with lTRE. An important and unanswered question, however, is whether most adults can adhere to this type of eating pattern and whether adherence and retention differ between eTRE and lTRE. This narrative review compared adherence and retention in studies that implemented either eTRE or lTRE in adults for 8 weeks or longer. Five databases were searched, and 10 studies met our eligibility criteria. The key finding was that participants had high and comparable levels of adherence and retention in both eTRE and lTRE interventions. Specifically, the mean adherence rate was 81.4% for eTRE and 82.3% for lTRE, while the mean retention rate was 81% for eTRE and 85.8% for lTRE in eligible studies. Thus, the findings support the feasibility of both approaches. The lowest adherence and retention rates occurred in studies in which either eTRE or lTRE regimens were combined with other dietary interventions. Notably, the duration of the eating window did not seem to negatively affect adherence and retention rates for either eTRE or lTRE. More research is warranted to determine the influence of other factors, such as age and study location, on adherence to and retention of both eTRE and lTRE interventions.

INTRODUCTION

Time-restricted eating (TRE) is a simple eating approach in which individuals shorten the daily eating period (duration between the first and last meal) without necessarily limiting caloric intake and/or changing diet quality.¹ Because TRE does not require calorie counting or macronutrient tracking, it reduces cognitive load,² thereby alleviating some of the challenges typically associated with following traditional dietary interventions. This approach has been found to reduce body weight and improve cardiometabolic parameters in adults with overweight or obesity.^3–5 For example, a systematic review and meta-analysis that included 19 TRE studies suggested that TRE reduced body weight and fat mass in adults, and also improved blood pressure, fasting glucose concentrations, and triglyceride levels, compared with baseline.⁵ Such findings suggest that TRE can provide substantial health benefits, thus encouraging the application of TRE in the clinical field.

The relative health benefits of the “early” or “later” eating window in the TRE intervention have received increasing attention, which has led to a growing number of investigations of the effects of TRE subtypes, specifically the early TRE (eTRE) and late TRE (lTRE) approaches in the management of body weight and insulin resistance.⁶ eTRE refers to TRE in which food is primarily eaten during the morning, and the last meal of the day occurs in the mid-afternoon.⁷ In contrast, lTRE involves an eating window beginning after midday, with the last meal of the day occurring in the early or late evening.⁸ Although both TRE protocols produce weight loss and improve metabolic markers, a growing body of evidence has suggested greater beneficial effects with respect to glucose, insulin, and insulin sensitivity with eTRE, potentially due to the alignment with the body’s circadian rhythms.^9,10 In line with this, several studies have shown that insulin sensitivity and beta-cell function are better in the morning,^11,12 suggesting that the early eating window is more optimal for health than the later. Additionally, a meta-analysis comparing the effects of eTRE and lTRE suggests that having an “early eating window” that aligns with the circadian rhythm patterns may result in greater health benefits than eating later in the day.⁶ Such findings suggest that the time of day in which food is consumed may be as important as the shortened eating period. In contrast, others argue that extended daily fasting periods, rather than the time of day in which food is consumed, is the most important contributing factor to the benefits of TRE. In support of this, studies have found that lTRE produces numerous health benefits, including reduction of body weight, visceral fat, insulin resistance, and oxidative stress, compared with non-TRE controls.^13,14

The health benefits associated with both eTRE and lTRE are unlikely to be maintained, however, without sustained adherence. An important question remains as to whether individuals can adhere to both types of TRE, particularly over the long-term. There are several potential barriers to long-term adherence to TRE interventions, which can include biological, behavioral, psychological, and environmental factors.¹⁵ A few examples of potential barriers include: misalignment with daily activity patterns, challenges with self-monitoring, negative emotions, discouraging social situations, and hunger.^15,16 There are likely different challenges in following eTRE or lTRE. For example, some individuals following eTRE may find it challenging to avoid calorie intake in the early evening because of social or family get-together events that typically occur in the evening. Work commitments may also be one of the barriers to following eTRE. For instance, night shift workers could face challenges in adhering to the early eating window. Additionally, some people may experience greater levels of hunger in the early evening than in the morning due to the circadian rhythmic increases in ghrelin, which is typically higher in the afternoon than in the morning.^17,18 On the other hand, some individuals may experience higher levels of hunger in the morning and therefore find it challenging to adhere to lTRE.

To our knowledge, there has not been a comprehensive review of adherence and retention rates to eTRE and lTRE interventions tested in randomized controlled trials. A better understanding of the effects of each of these TRE interventions on adherence levels can inform the delivery of future interventions. Therefore, this narrative review aimed to explore how the type of assigned TRE (early versus late) affected adherence and retention levels in studies involving adults with overweight and obesity. Because studies with eTRE or lTRE regimens are diverse in participant selection and methodologies, a narrative review was conducted to focus on comparing the adherence and retention rates of eTRE and lTRE interventions.

METHODS

Search Strategy and Eligibility Criteria

The literature search was conducted in PubMed, Web of Science, Scopus, CENTRAL, and Google Scholar databases, focusing on studies with eTRE or lTRE applied to adults. The search terms were “intermittent fasting,” “time-restricted eating,” and “time-restricted feeding.” The literature search was completed in July 2023. The titles and abstracts of identified articles were screened for relevance, and the full text was examined for eligibility when appropriate. Studies were included if they met the following criteria: (1) randomized controlled trial with adults aged ≥18 years old and body mass index of ≥25 kg/m²; (2) inclusion of an intervention arm of eTRE (eating window within 6 am to 4 pm) or lTRE (eating window within 12 pm to 10 pm) (Figure 1); (3) intervention included ≥14 hours of the daily fasting period and a 4 to 10 hours eating window; (4) intervention duration of ≥8 weeks and ≥5 days of intervention per week; (5) adherence and retention levels reported; and (6) full text and written in English.

Figure 1.

The Concept of Early and Late Time-Restricted Eating (TRE)

Adherence

In the eligible studies, adherence to eTRE or lTRE was measured by participants’ self-reported days in which they consumed food and calorie-containing beverages within the designed eating window. In most of the clinical trials included in this review, strategies were incorporated to facilitate the accuracy of self-reported adherence. For example, participants were asked to report adherence by recording it daily in a diary, filling out an online survey, or sending text communications using mobile applications. The included studies reported adherence to eTRE and lTRE by presenting “the average number of days that participants reported adhering to the eating window” or “the number or percentage of participants who reported adhering to the eating window.” Based on the contents and provided data from each study, the number of days in which participants adhered to the intervention was converted to percentages for comparison between studies.

Retention

Participant retention was determined based on “the number of participants who completed the final assessments.” All the eligible studies provided the number of participants who withdrew during the intervention phase. Retention rates were calculated using “the number of participants who completed the intervention” divided by “the number of participants who participated in the intervention.”

RESULTS

A total of 3502 studies were identified and screened for eligibility. After the initial screening, 321 articles were chosen for further selection. Subsequently, 12 articles were screened for full text. Of these, 2 studies were excluded, because participants were allowed to start their meals at any time of the day, resulting in 10 eligible studies (Figure 2).¹⁹

Figure 2.

Flow Diagram of Literature Search. Adapted from Page and colleagues.¹⁹

Characteristics of the Eligible Studies

This narrative review included 10 highly diverse studies in methodological design and participant selection. Data related to the eTRE or lTRE adherence and retention were extracted from the included studies. In addition, the characteristics of the eligible studies, including the country where the study was conducted, intervention groups, intervention strategies, number of dropouts, and assessment of adherence, were organized in Table 1 to assist in further discussion of adherence and retention rates.^{13,14,20–26} None of the included studies had an intervention duration longer than 12 months, and the designed eating windows of the eligible studies varied from 4 to 8 hours. Individuals in the qualifying research were generally within the young or middle-aged demographic, and the studies were conducted in different countries.

Table 1.

Demographics and Characteristics of the Studies

Study (country)	Study groups (N; age; % female; BMI)	Intervention duration	Intervention strategies	Dropouts during the intervention	Adherence assessments	Adherence rate	Retention rate
Queiroz et al, 202320 (Brazil)	16:8 eTRE+ CR (16; 33 ± 6a; 84.6%a; 30.8 ± 3a)	8 weeks	Eat from 8 am to 4 pm daily and reduce energy intake by 25%	N = 3, 19%	Sent photos daily through a mobile app, 3-day food records	85% of the participants were considered AEW	81%
	16:8 lTRE+ CR (16; 30 ± 7a; 81.8%a; 30.5 ± 3a)		Eat from 12 to 8 pm daily and reduce energy intake by 25%	N = 5, 31%	Sent photos daily through a mobile app, 3-day food records	73% of the participants were considered AEW	69%
Zhang et al, 202226 (China)	18:6 eTRE (21; 23.8 ± 2.7; 43%; 27.1 ± 3.2)	8 weeks	Eat ad libitum from 7 am to 1 pm daily	N = 0	AEW: daily adherence log	Reported AEW on average rate of daily compliance (89%±13.3%)	100%
	18:6 lTRE (20; 23.2 ± 2.2; 45%; 28.5 ± 3.6)		Eat ad libitum from 12 to 6 pm daily	N = 0	AEW: daily adherence log	Reported AEW on average rate of daily compliance (77.9%±6.3%)	100%
He et al, 202214 (China)	16:8 eTRE (38; 43.7 ± 9.9; 39%; 29.7 ± 3.1)	12 weeks	Eat ad libitum from 8 am to 4 pm daily, self-selected between eTRE or lTRE	N = 8, 21%	AEW: self-reported days of adherence	Reported AEW on average 61.4 ± 24.7 days/12 weeks (73%)	79%
	16:8 lTRE (17; 41.6 ± 12; 29%; 29.2 ± 3.7)		Eat ad libitum from 12 to 8 pm daily, self-selected between eTRE or lTRE	N = 3, 18%	AEW: self-reported days of adherence	Reported AEW on average 74.9 ± 11.1 days/12 weeks (89%)	82%
	16:8 eTRE+ LCD (32; 40.6 ± 9.1; 31%; 29.1 ± 3.4)		eTRE combined with LCD, self-selected between eTRE or lTRE	N = 5, 16%	FFQ and self-reported days of adherence	Reported AEW on average 57.0 ± 22.1 days/12 weeks (68%)	84%
	16:8 lTRE+ LCD (20; 36.5 ± 8; 25%; 28.8 ± 3.6)		lTRE combined with LCD, self-selected between eTRE or lTRE	N = 3, 15%	FFQ and self-reported days of adherence	Reported AEW on average 58.7 ± 23.7 days/12 weeks (70%)	85%
Lin et al, 202323 (USA)	16:8 lTRE (30; 44 ± 12; 83%; 37 ± 6)	12 months	6-month weight loss phase: Eat ad libitum from 12 pm to 8:00 pm 6-month weight maintenance phase: eat from 10 am to 8 pm	N = 4, 13%	AEW: self-reported days of adherence	Reported AEW on average 6.1 ± 0.8 days/week (87%)	87%
Wei et al, 202325 (China)	16:8 eTRE + CR (45; 32.3 ± 10.5; 47%; 32.2 ± 3.4)	12 months	Consume the prescribed calories from 8 am to 4 pm daily	N = 11, 24%	AEW and ACI: dietary log and daily food picture and mealtimes on mobile app	Month 6: 87.9% ± 10.3%; month 12: 85% ± 10.7%	76%
Liu et al, 202227 (China)	16:8 eTRE + CR (69; 31.6 ± 9.3; 47.8%; 31.8 ± 2.9)	12 months	Consume the prescribed calories from 8 am to 4 pm daily	N = 12, 17%	AEW and ACI: dietary log and daily food picture and mealtimes on mobile app	Month 6: 87.8% ± 11.8%; month 12: 84.0% ± 16.1%	83%
Jamshed et al, 202221 (USA)	16:8 eTRE + CR (45; 43 ± 10; 78%; 40.1 ± 6.6)	14 weeks	Consume the prescribed calories from 7 am to 3 pm at least 6 days/week	N = 16, 36%	AEW: surveys administered via REDCap software	Reported adhered on average 6 ± 0.8 days/week (86%)	64%
Kotarsky et al, 202122 (USA)	16:8 lTRE + EX (13; 45 ± 9.9a; 82%a; 29.8 ± 2.7a)	8 weeks	Eat ad libitum from 12 to 8 pm daily plus aerobic and resistance training	N = 2, 15%	AEW: 3-day dietary records	2 instances of dietary noncompliance were reported among 2 participants (82% adherent to the intervention plan)	85%
Lowe et al, 202024 (USA)	16:8 lTRE (59; 46.8 ± 10.8; 40.7%; 32.9 ± 4.9)	12 weeks	Eat ad libitum from 12 to 8 pm daily	N = 9, 15%	AEW: self-reported	83.5%	85%
Cienfuegos et al, 202013 (USA)	20:4 lTRE (19; 47 ± 9; 89%; 37 ± 4.4)	8 weeks	Eat ad libitum from 3 to 7 pm daily	N = 3, 16%	AEW: daily adherence log	Reported AEW on average 6.2 ± 0.8 days/week (89%)	84%
	18:6 lTRE (20; 47 ± 13; 95%; 37 ± 4.5)		Eat ad libitum from 1 to 7 pm daily	N = 1, 5%	AEW: daily adherence log	Reported AEW on average 6.2 ± 0.4 days/week (89%)	95%

^aReported data only included completers; all data were presented as mean  ±  standard deviation. Abbreviations: AEW, adherent to eating window; ACI, adherent to caloric intake; CMT, consistent meal timing group; CR, caloric restriction; eTRE, early time-restricted eating; EX, exercise training; FFQ, Food Frequency Questionnaire; LCD, low-carbohydrate diet; lTRE, late time-restricted eating; N, sample size; N/A, not applicable; non-TRE, not time-restricted eating; SD, standard deviation; SEM, standard error of the mean; USA, United States of America.

The sample sizes of the eligible studies ranged from 13 to 69 participants per intervention group, and participants’ age requirements ranged from 18 to 75 years. Target participants of the studies included adults with metabolic syndrome,¹⁴ nonalcoholic fatty liver disease,²⁵ and overweight or obesity.^{13,20–24,26,27} The percentage of female participants in the eTRE and lTRE intervention arms of the eligible studies ranged from 25% to 95%. The average body mass index of the study participants ranged from 27.1 to 40.1 kg/m². Participants withdrew for the following reasons during the intervention phase: dissatisfied with the diet schedule or difficulty following eTRE,^20,21 scheduling conflict, unrelated medical issues, pregnancy, unable to contact the participant, unexpected data loss due to the COVID-19 pandemic, and other personal reasons.

Table 1 shows the demographics and characteristics of the included studies. Half of the included studies (n = 5) were conducted in the United States,^13,21–24 4 were carried out in China,^14,25–27 and 1 in Brazil.²⁰ Of the 10 included studies, 3 included both eTRE and lTRE intervention arms,^14,20,26 3 studies used an eTRE intervention arm only,^21,25,27 and 4 studies used an lTRE intervention arm only.^13,22–24 The intervention duration in the studies varied and included 8 weeks (n = 4),^13,20,22,26 12 weeks (n = 2),^14,24 14 weeks (n = 1),²¹ and 12 months (n = 3).^23,25,27 The eating windows of eligible studies for eTRE ranged from 6 to 8 hours, and from 4 to 8 hours for studies with lTRE.

Only half of the eligible studies adopted either an eTRE or lTRE regimen alone,^{13,14,23,24,26} and the other half used an eTRE or lTRE regimen combined with other lifestyle interventions, including calorie restriction (CR),^20,21,25,27 low carbohydrate diet (LCD),¹⁴ or exercise training (EX).²² Among the eligible studies, He and colleagues implemented a unique study design that randomized participants to TRE alone or to TRE plus LCD, then allowed participants to self-select between eTRE and lTRE.¹⁴ The study reported that 69.1% of participants from the TRE alone group and 61.4% from the TRE plus LCD group self-selected to adapt to the eTRE intervention.¹⁴ In addition, 2 eligible studies instructed participants to follow exercise prescriptions as part of the intervention,^21,22 while 1 eligible study had no instruction regarding physical activity,²⁴ and others instructed participants to maintain the usual physical activity levels throughout the intervention.^{13,14,20,23,25–27}

Adherence to eTRE

A total of 6 studies contained an eTRE intervention arm (Table 1; average adherence rate = 81.4%).^{14,20,21,25–27} Two studies applied an eTRE regimen alone (adherence rates ranged from 73% to 89%; average = 81%).^14,26 Five studies applied an eTRE plus dietary modification (CR or LCD), showing adherence rates ranging from 68% to 86% (average = 81.6%).^{14,20,21,25,27} Among these studies, He and colleagues included an eTRE alone group (adherence rate = 73%) and an eTRE combined with LCD group (adherence rate = 68%).¹⁴ For all eTRE studies, the highest adherence rate (89%) was reported by Zhang and colleagues, who tested 6-h eTRE over 8 weeks.²⁶ The lowest adherence rate (68%) of eTRE was reported by He and colleagues, who tested 8-h eTRE combined with LCD for 12 weeks.¹⁴

A downward trend in adherence was reported by Jamshed and colleagues (adherence declined by 0.4 days/week over the intervention period) and Zhang and colleagues (did not provide an adherence decline rate); the former applied 14-week eTRE (8-hour eating window) combined with energy restriction among adults aged between 25 and 75 years, while the latter used 8-week eTRE and lTRE (6-hour eating window) alone among younger adults with overweight and obesity and observed downward trends in adherence in both groups.^21,26

Among the 6 eligible studies, the intervention duration ranged from 8 weeks to 12 months, and the eating window varied from 6 to 8 hours. The 8-week eTRE intervention studies showed 85% and 89% adherence rates (average = 87%),^20,26 and the 12-week and 14-week intervention studies reported adherence to eTRE ranging from 68% to 86% (average = 75.7%).^14,21 The 12-month intervention studies with eTRE showed 84% and 85% adherence rates (average = 84.5%).^25,27 Regarding the target daily fasting goal, a study with eTRE and a 6-hour eating window (6-h eTRE) reported 89% adherence,²⁶ and studies with 8-h eTRE reported adherence rates ranging from 68% to 86% (average = 80.2%).^{14,20,21,25,27}

Retention in eTRE

The average retention rate of the 6 studies that contained the eTRE intervention arm was 81% (Table 1).^{14,20,21,25–27} Two studies that applied an eTRE regimen alone reported retention rates of 79% and 100% (average = 89.5%),^14,26 and 5 studies that applied eTRE plus dietary modification showed retention rates ranging from 64% to 84% (average = 77.6%).^{14,20,21,25,27} Among these studies, He and colleagues included an eTRE alone group (retention rate = 79%) and an eTRE combined with LCD group (retention rate = 84%).¹⁴ Two studies that applied eTRE combined with CR recorded dropout reasons as participants had difficulty following eTRE or were dissatisfied with the diet schedule.^20,21

Retention rates varied with different intervention durations and eating windows. The 8-week eTRE intervention showed 81% and 100% retention rates (average = 90.5%).^20,26 The 12-week and 14-week intervention studies reported retention rates ranging from 64% to 84% (average = 75.7%).^14,21 The 12-month intervention studies with eTRE showed 83% and 76% retention rates (average = 79.5%).^25,27 Regarding the eating window, the study with 6-h eTRE reported a 100% retention rate,²⁶ and studies with 8-h eTRE reported retention rates ranging from 64% to 84% (average = 77.8%).^{14,20,21,25,27} Additionally, the highest retention rate (100%) of eTRE was reported by Zhang and colleagues.²⁶ The lowest retention rate (64%) of eTRE was reported by Jamshed and colleagues, who tested 8-h eTRE combined with CR for 14 weeks. It is noteworthy that the retention rate was likely impacted by multiple reasons, including unexpected data loss during the COVID-19 pndemic.²¹

Adherence to lTRE

A total of 7 studies contained a lTRE intervention arm (Table 1, average adherence rate = 82.3%).^{13,14,20,22–24,26} Among the 7 eligible studies that contained a lTRE intervention arm, the intervention duration ranged from 8 weeks to 12 months. Five studies applied a lTRE regimen alone (adherence rates ranged from 77.9% to 89%; average = 85.9%),^{13,14,23,24,26} and 3 studies used lTRE plus dietary modification (CR, LCD) or EX as a study intervention (adherence rates ranged from 70% to 82%; average = 75%).^14,20,22 Among these studies, He and colleagues included a lTRE alone group (adherence rate = 89%) and a lTRE combined with LCD group (adherence rate = 70%).¹⁴ Furthermore, the highest adherence rate (89%) of lTRE was reported by Cienfuegos and colleagues, who tested 8 weeks of 4-h and 6-h lTRE,¹³ and by He and colleagues, who tested 12 weeks of 8-h lTRE.¹⁴ The lowest adherence rate (70%) of lTRE was reported by He and colleagues, who tested 8-h lTRE combined with LCD for 12 weeks.¹⁴ Notably, a downward trend in adherence was observed by Zhang and colleagues,²⁶ who used 8-week lTRE among younger adults with overweight and obesity.

Among the 7 eligible studies, the intervention duration ranged from 8 weeks to 12 months, and the eating window varied from 4 to 8 hours. Eligible studies with 8 weeks of lTRE intervention reported adherence rates ranging from 73% to 89% (average = 82.2%).^13,20,22,26 The 12-week studies reported adherence to lTRE ranging from 70% to 89% (average = 80.8%).^14,24 A 12-month study reported 87% adherence to lTRE.²³ Regarding the target daily fasting goal, a study with lTRE and a 4-hour eating window (4-h lTRE) showed 89% adherence,¹³ whereas studies with 6-h lTRE showed 77.9% and 89% adherence rates (average = 83.5%),^13,26 and studies with 8-h lTRE reported adherence ranging from 70% to 89% (average = 80.8%).^{14,20,22–24}

Retention in lTRE

The average retention rate of the 7 studies that contained the lTRE intervention arm was 85.8% (Table 1).^{13,14,20,22–24,26} Five studies that applied a lTRE regimen alone reported retention rates ranging from 82% to 100% (average = 88.8%),^{13,14,23,24,26} and 3 studies that used lTRE plus dietary modification reported retention rates ranging from 69% to 85% (average = 79.7%).^14,20,22 Among these studies, He and colleagues included a lTRE alone group (retention rate = 82%) and a lTRE combined with LCD (retention rate = 85%).¹⁴

Eligible studies with 8 weeks of lTRE intervention reported retention rates ranging from 69% to 100% (average = 86.6%).^13,20,22,26 The 12-week studies showed 82% and 85% retention rates (average = 84%).^14,24 A 12-month study reported an 87% retention rate.²³ Regarding the eating window, the study with 4-h lTRE showed an 84% retention rate,¹³ studies with 6-h lTRE showed 100% and 95% retention rates (average = 97.5%),^13,26 and studies with 8-h lTRE reported retention rates ranging from 69% to 87% (average = 82.2%).^{14,20,22–24} Additionally, the highest retention rate (100%) of lTRE was reported by Zhang and colleagues, who tested 6-h lTRE for 8 weeks.²⁶ The lowest retention rate (69%) of lTRE was reported by Queiroz and colleagues, who tested 8-h lTRE combined with CR for 8 weeks, and the retention rate was impacted by multiple factors, including scheduling conflicts, loss of contact, and personal reasons.²⁰

Studies That Applied Both eTRE and lTRE Interventions

Three of the included studies contained both eTRE and lTRE intervention arms.^14,20,26 One of these studies (by He and colleagues) included 4 groups: ETRE alone, lTRE alone, eTRE combined with LCD, and lTRE combined with LCD.¹⁴ Another of these studies tested eTRE and lTRE interventions alone,²⁶ and the third study tested TRE interventions combined with CR.²⁰

Zhang and colleagues applied 8-week eTRE and lTRE in younger adults (aged between 18 and 30 years); they showed higher adherence to eTRE (89%) than to lTRE (77.9%), with a 100% retention rate for both groups.²⁶ Similarly, Queiroz and colleagues applied 8-week eTRE and lTRE combined with CR in adults (aged between 20 and 40 years); they showed higher adherence and retention rates in eTRE combined with CR (adherence = 85%; retention = 81%) than in lTRE combined with CR (adherence = 73%; retention = 69%).²⁰

On the other hand, He and colleagues randomized adults (aged between 18 and 65 years) to 12-week TRE, then showed lower adherence and retention rates in eTRE (adherence = 73%; retention = 79%) compared with lTRE (adherence = 89%; retention = 82%).¹⁴ Additionally, He and colleagues applied 12-week eTRE and lTRE combined with LCD in adults, and found lower adherence and retention rates in eTRE combined with LCD (adherence = 68%; retention = 84%) than in lTRE combined with LCD (adherence = 70%; retention = 85%).¹⁴

DISCUSSION

The purpose of this narrative review was to report and compare the adherence and retention rates in randomized controlled trials that applied an eTRE or lTRE intervention in adults with overweight and obesity. The key finding of this review was that adherence and retention rates were high (>80%) and comparable across studies that tested eTRE and lTRE, which suggests both eTRE and lTRE are feasible options for most individuals interested in adopting a TRE eating pattern. Adherence and retention rates were generally lower in interventions in which lTRE was combined with other dietary or exercise regimens, compared with lTRE alone. It is noteworthy that the daily fasting duration did not seem to negatively affect adherence levels and retention rates for either eTRE or lTRE. Additionally, studies with a shorter intervention duration (8 weeks) showed higher retention rates than studies with longer intervention durations (12 months) for eTRE (8-week average retention = 90.5%; 12-month average retention = 79.5%).

The high average adherence rates (>80%) observed in both eTRE and lTRE interventions support the feasibility of both approaches, especially within intervention periods lasting equal to or less than 12 months. All eligible studies utilized one or more strategies to maintain participants’ adherence to eTRE or ITRE. These strategies included weekly or bi-weekly phone or online interviews with the research staff,^{13,14,23,25–27} self-selection between eTRE and lTRE,¹⁴ allowing participants to change their eating times by 1 hour without influencing the length of their eating window,²⁰ reminder calls and messages via mobile app,^24,25,27 providing protein shakes every day,^25,27 and regular group classes.^21,22 These findings highlight the importance of implementing various support mechanisms to sustain adherence to eTRE and lTRE.

While this review draws no conclusion as to whether extended intervention periods would yield lower adherence rates compared with shorter intervention durations, interestingly, 4 studies provided additional information beyond the final adherence rate, shedding light on adherence changes throughout the intervention period. Two studies, with 8-week and 14-week eTRE intervention durations, reported a downward trend in adherence.^21,26 Similarly, 2 studies with 12-month eTRE interventions showed a lower adherence rate at the 12-month follow-up compared with the 6-month follow-up.^25,27 Additionally, a downward trend in adherence was observed in an 8-week lTRE intervention,²⁶ although other lTRE studies did not provide data on adherence changes throughout the intervention. These findings suggest the necessity of maintaining or increasing the adherence rate during the second half of the eTRE and lTRE intervention period. Future studies may benefit by increasing participant interaction frequency with research staff or adopting different strategies in the middle and later intervention phases to maintain adherence.

Although combining eTRE with other dietary interventions (average adherence = 81.6%)^{14,20,21,25,27} showed similar adherence compared with interventions with eTRE alone (average adherence = 81%),^14,26 combining lTRE with other dietary regimens appeared to result in lower adherence rates (average adherence = 75%)^14,20,22 than applying lTRE alone (average adherence = 85.9%).^{13,14,23,24,26} Such findings suggest the addition of a dietary intervention has less effect on adherence in eTRE interventions compared with lTRE interentions. However, the highest adherence rates for both eTRE (89%) and lTRE (89%) were reported in 3 studies that employed eTRE and lTRE intervention arms without other dietary or exercise interventions.^13,14,26 Such findings indicate that combining eTRE or lTRE with other dietary or exercise interventions may increase challenges and burdens for participants, thus resulting in lower adherence and retention in those intervention groups.

The daily eating windows of the studies included in this review varied from 4 to 8 hours. A noteworthy finding was that average adherence rates for each duration of the eating window exceeded 80% for both eTRE and lTRE. This suggests that individuals are able to adhere to shorter daily eating windows, and the duration of the eating window did not appear to negatively impact adherence levels for either eTRE or lTRE. To our knowledge, the effects of daily eating windows on adherence levels have not been systematically investigated to date.

Comparable retention rates of eTRE (average retention = 81%) and lTRE (average retention = 85.8%) were observed from the included studies, which further supports the feasibility of both interventions. Participants’ dropout reasons varied in the eligible studies, with only 2 studies recording dropout reasons being related to an inability to comply with the intervention.^20,21 Notably, both of these studies applied eTRE plus other dietary interventions. Combining eTRE or lTRE with other dietary interventions resulted in lower average retention rates (77.6% for eTRE and 79.7% for lTRE) compared with eTRE alone (89.5%) and lTRE alone (88.8%). In line with this, the highest retention rates of eTRE (100%) and lTRE (100%) among the eligible studies were presented by the study that employed eTRE and lTRE intervention arms without other dietary or exercise interventions.²⁶ These findings again suggest that the increased challenges involved in complying with more than one dietary intervention can increase the burden on participants.

Additionally, studies with shorter intervention duration (8 weeks) showed higher retention rates than studies with longer intervention durations (12 months) for eTRE (8-week average retention = 90.5%; 12-month average retention = 79.5%). However, similar retention rates were found in lTRE (8-week average retention = 86.6%; 12-month retention = 87%). With only 2 studies applying 12-month eTRE and 1 study applying 12-month lTRE,^23,25,27 this review could not draw definitive conclusions regarding the influence of intervention duration on retention rates for either eTRE or lTRE. Even so, participants may have faced additional obstacles, such as scheduling conflicts or medical concerns, which may have affected their retention in the study when participating in longer interventions. More studies are needed to examine the retention rates in studies involving eTRE and lTRE with longer intervention periods. Finally, when evaluating the 3 eligible studies featuring both eTRE and lTRE intervention arms, the comparison of adherence and retention rates between eTRE and lTRE yielded inconclusive findings, underscoring the need for more comprehensive investigation and analyses in future studies.

Strengths and Limitations

There are limitations to this review. First, only 10 studies met the eligibility criteria. The included studies were heterogeneous in methodologies and participant selection, and half of the studies combined eTRE or lTRE with dietary modification or exercise interventions, which could influence adherence and retention in eTRE and lTRE interventions. There may also be a potential for bias in the results, due to cultural differences, because most eTRE studies were conducted in China, and most lTRE studies were conducted in the United States. Furthermore, despite efforts to control for confounding factors, the possibility of unmeasured or inadequately controlled variables in the reviewed studies cannot be ruled out. Additionally, most eligible studies had a short intervention duration (≤14 weeks), with only 3 studies applying 12 months of intervention; thus, this review lacks the ability to report long-term (>12 months) adherence and retention rates. Moreover, the eligible studies relied on self-report, which is known to be biased;²⁸ no objective adherence data could be extracted to compare eTRE and lTRE. Lastly, the absence of reasons for participant dropout in some of the included studies may have limited our understanding of potential factors influencing attrition rates.

This review also had several strengths. First, a comprehensive literature search was conducted in 5 databases to include as many potential studies as possible. Second, this review only included randomized controlled trials to ensure the study quality, and the definitions of eTRE and lTRE were applied strictly to the study screening phase to increase the possibility of detecting differences in participants’ adherence to and retention of eTRE and lTRE. Moreover, strategies for increasing adherence to and retention of eTRE and lTRE obtained from each study were organized, and suggestions have been provided to help with future study designs.

CONCLUSION

This review suggests that both eTRE and lTRE interventions have produced high and similar levels of adherence and retention in randomized controlled trials to date. Combining eTRE or lTRE with other interventions, however, may result in lower retention rates, potentially because of the increased challenges in complying with more than one lifestyle intervention. Also it is noteworthy that daily fasting duration did not seem to affect adherence and retention rates, suggesting that individuals can adhere to TRE interventions that involve short eating windows. Finally, the existing data does not allow us to definitively conclude whether longer intervention periods result in lower adherence and retention rates compared with shorter ones. However, a decrease in adherence rates over time was noted in several studies employing eTRE interventions and in 1 study employing lTRE intervention, highlighting the need for further research to fully understand the change in adherence level throughout the eTRE and lTRE interventions.

Further exploration of factors that affect adherence and retention rates across diverse populations, including age differences, cultural differences, socioeconomic status and biological chronotypes, will aid in refining eTRE and lTRE intervention strategies for specific groups. Ultimately, gaining a more comprehensive understanding of adherence patterns, and the long-term effects of eTRE and lTRE could have significant implications for clinical practice, and future research.

Author Contributions

Y.L. undertook literature screening, data extraction, and data integration, wrote the initial draft of the review, and was the primary author revising the review. A.E. and C.M. undertook literature searching and literature screening, created the flow diagram, and were involved in revising the review. R.S.Z. created the figure describing the concept of this review and was involved in revising the review. S.D.A. designed the review, provided substantial feedback, and edited the review for critical intellectual content.

Funding

This work was supported by the National Institute on Aging (training grants T32 AG020499 for the work performed by C.M. and T32 AG062728 for the work performed by R.S.Z.).

Conflict of Interest

None declared.