Associations between Variability in Between- and Within-Day Dietary Intake with Adiposity and Glucose Homeostasis in Adults: A Systematic Review

This systematic review aims to comprehensively evaluate the literature regarding the impact of variations in dietary intake, both between- and within-day, on adiposity and glucose metabolism. We included observational and experimental articles obtained from PubMed, Scopus, Cochrane Library, and gray literature until 9 October, 2023, evaluating the impact of between- or within-day variations in meal, energy, or macronutrient intake on these outcomes. Our focus was on adults aged >= 18 y, spanning both healthy individuals and those with type 2 diabetes mellitus (T2DM). Given the diverse range of exposures, treatments, and outcomes among the selected articles, we chose a qualitative synthesis approach to effectively analyze the data. Eighty articles from 43 observational and 37 experimental studies were included, involving 89,178 participants. Patterns of dietary intake variation were identified and systematically organized into distinct categories based on similarities. Between-day variations in dietary intake consisted of between-day variations in both the quantity consumed and meal timing. Meanwhile, within-day variations encompassed factors such as eating window, meal omission, within-day meal timing, within-day variation in dietary intake quantity, and temporal distribution. Despite mixed results, time-restricted eating was generally associated with lower adiposity. However, limited control for total daily energy intake (TDEI) suggests that the contribution of lower energy intake cannot be conclusively excluded. Conversely, the adverse effect of meal omission on glucose parameters was consistently supported by randomized trials. Interestingly, the results showed that consuming a substantial portion of TDEI in the morning may increase the likelihood of observing improvements in adiposity. Furthermore, inconsistencies in outcomes across articles examining the effects in healthy compared with T2DM populations, or in energy-sufficient compared with deficient individuals, indicate potential condition-specific effects. These fi ndings support the need for further investigation into the effects of between- and within-day variations in dietary intake to better understand their impact on adiposity and glucose homeostasis. This review was registered in PROSPERO as CRD42020214307.