Purpose of Review: The aim of this short review is to provide an updated commentary on the current literature examining the impact of meal timing on obesity and weight gain in adults. The potential mechanisms, including novel and emerging factors, behind timing of food intake across the 24-h period in the development of obesity, and dietary strategies manipulating meal timing to ameliorate weight gain are also explored. Recent Findings: Dietary patterns that feature meal timing outside of the regular daytime hours can contribute to circadian disruption as food is metabolised in opposition to internal daily rhythms and can feedback on the timekeeping mechanisms setting these rhythms. Epidemiological evidence examining the impact of late meal timing patterns is beginning to suggest that eating at night increases the risk of weight gain over time. Mechanisms contributing to this include changes to the efficiency of metabolism across the day, and dysregulation of appetite hormone and gut microbiota by mis-timed meals. Summary: When meals are eaten, in relation to the time of day, is increasingly considered of importance when implementing dietary change in order to address the growing burden of obesity, although further research is required in order to determine optimal patterns.

Current dietary trends show that humans consume up to 40% of their energy intake during the night. Those who habitually eat during the night are observed to have an increased risk of metabolic conditions such as type-2 diabetes and cardiovascular disease. Increasing evidence suggest that a biological consequence of eating during the night is a larger postprandial glucose response, compared to meals eaten earlier in the day. However, findings from individual acute postprandial studies have been inconsistent, due to variations in protocols. Therefore, this review aimed to systematically summarize findings from acute postprandial studies and investigate whether postprandial glucose and insulin response at night differs to during the day in healthy adults. This would indicate a possible physiological mechanism linking habitual nighttime eating and increased risk of metabolic conditions. Seven electronic databases were searched in February 2018. Included studies met the following criteria: had a day-time test between 0700 – 1600h, a nighttime test between 2000 and 0400h, the test meals were identical and consumed by the same participant at both day and night time points, preceded by a 3-h fast (minimum). Primary outcome measures were postprandial glucose and insulin incremental area under the curve (iAUC) or area under the curve (AUC). Studies that reported numerical data were included in the meta-analyses, conducted using Stata statistical software (version 13.0, StataCorp, College Station, TX, USA). For eligible studies that did not report numerical data, their authors’ conclusions on the effect of time of day on the primary outcome measures were summarized qualitatively. Full text of 172 articles were assessed for eligibility. Fifteen studies met the eligibility criteria, ten of which were included in the meta-analyses. Meta-analysis for glucose showed a lower postprandial glucose response in the day compared to during the night, after an identical meal (SMD = −1.66; 95% CI, −1.97 to −1.36; p < .001). This was supported by the findings from included studies ineligible for meta-analysis. Meta-analysis also showed a lower postprandial insulin response in the day compared to during the night (SMD = −0.35; 95% CI, −0.63 to −0.06; p = .016). However, findings from included studies ineligible for meta-analysis were inconsistent. Our results suggest poor glucose tolerance at night compared to the day. This may be a contributing factor to the increased risk of metabolic diseases observed in those who habitually eat during the night, such as shift workers.

Understanding shift workers dietary intake patterns may inform interventions targeted at lowering chronic disease risk. This study examined the temporal distribution of food intake as shift workers rotate between night shifts, day shift and/or days off to identify differences in energy intake, eating frequency, and adherence to dietary guidelines by shift type (night shift vs. day). Night shift (NS) workers completed a four-day food diary that included a minimum of two night shifts and one-day shift (DS)/day off (DO), recording all food, beverages and time of consumption. Comparisons were between shift types, using ANOVA for continuous data and generalized estimating equations for count data, data reported as mean (SE). When comparing NS and DSDO, there were no differences in energy intake (24 h) (8853 (702) vs. 9041 (605) kJ, n = 22) or adherence to dietary guidelines. There was no difference between the number of eating occasions on NS and DSDO (5.6(0.3) vs 5.1(0.6) occasions) but less energy per eating occasion at night (1661(125) vs 1933(159) kJ). When working NS compared with DSDO there was higher total sugar (%E, 19.1(2.0) vs 15.0(2.4)) and lower saturated fat (%E, 13.8(1.2) vs 15.7(1.3)). Further, DSDO were characterized by a pattern of three main meals and a prolonged fasting period. It is important to determine if reducing eating occasions and providing opportunities for fasting improves metabolic health.

Eating at night time, as is frequent in shift workers, may contribute to increased cardiovascular disease (CVD) risk through a disruption in usual lipid metabolism, resulting in repeated and sustained hyperlipidemia at night. This systematic review aimed to investigate the impact of eating a meal at night compared with the same meal eaten during the day on postprandial lipemia. Six databases were searched: CINAHL Plus, Cochrane Library, EMBASE, Ovid MEDLINE, Informit, and SCOPUS. Eligible studies were original research cross-over design with a minimum fasting period of 5 h before testing preceded by a standardized control meal; measured postprandial triacylglycerol (TAG) for 5 h or greater; had meal time between 0700 h and 1600 h for day time and between 2000 h and 0400 h for night time; and had within-study test meals (food or drink) that were identical in macronutrient composition and energy. Two authors independently completed eligibility and quality assessment using the American Dietetic Association Quality Criteria Checklist for Primary Research. After removing duplicates, 4,423 articles were screened, yielding 5 studies for qualitative synthesis. All studies identified at least one parameter of the postprandial TAG response that was different as a result of meal time (e.g., the total concentration or the time course kinetics). Two studies reported a greater total TAG concentration (area under curve) at night compared with day, and 3 studies found no difference. Four studies reported that the kinetics of the postprandial time course of TAGs was different at night compared with during the day. Inconsistent reporting in the primary studies was a limitation of the review. Night eating may negatively affect postprandial lipemia and this review shows there is a need to rigorously test this using standardized methods and analysis with larger sample sizes. This is critical for informing strategies to lower CVD risk for shift workers.

There is evidence to indicate that the central biological clock (i.e., our endogenous circadian system) plays a role in physiological processes in the body that impact energy regulation and metabolism. Cross-sectional data suggest that energy consumption later in the day and during the night is associated with weight gain. These findings have led to speculation that when, as well as what, we eat may be important for maintaining energy balance. Emerging literature suggests that prioritising energy intake to earlier during the day may help with body weight maintenance. Evidence from tightly controlled acute experimental studies indicates a disparity in the body’s ability to utilise (expend) energy equally across the day and night. Energy expenditure both at rest (resting metabolic rate) and after eating (thermic effect of food) is typically more efficient earlier during the day. In this review, we discuss the key evidence for a circadian pattern in energy utilisation and balance, which depends on meal timing. Whilst there is limited evidence that simply prioritising energy intake to earlier in the day is an effective strategy for weight loss, we highlight the potential benefits of considering the role of meal timing for improving metabolic health and energy balance. This review demonstrates that to advance our understanding of the contribution of the endogenous circadian system toward energy balance, targeted studies that utilise appropriate methodologies are required that focus on meal timing and frequency.