2.2. Study Protocol

A participant flowchart is shown in Figure 1. Protocols for the meal timing interventions are shown in Supplemental Figure. This 3-day randomized crossover study compared two different conditions: eating a late dinner (at 21:00) vs. eating an early dinner (at 18:00). The participants were randomly allocated to two groups of seven participants each by E.Y. and K.N. using a random number generator. On the first day, the participants were given a prescribed diet and instructed to eat at the designated time in their normal environment. On the second and third days, they were provided with a prescribed diet and ate at a set time under laboratory conditions. Participants were allowed to drink as much water as they wished. The participants were asked to dedicate 15 min to each meal, and the order in which the different foods included in each prescribed meal were consumed was not specified. All participants were asked to refrain from exercising from the day before the 3-day experimental period began until the end of the 3-day period. The participants were instructed not to eat anything after 21:15 on the day before the experiment began, and to fast from the time they woke up until they visited the laboratory for testing at 7:30 on the second and third days of the experiment. Participants were allowed to consume only water after dinner. Participants were instructed to go to bed at 24:00 and wake up at 6:30, and record their sleep status. The wash-out period lasted at least 3 days (5 ± 2 days). The experiment was conducted between March 2019 and December 2020. The protocol was registered as a randomized controlled trial entitled “Effects of meal timing and content on glycemic fluctuations and metabolic responses” under the University Hospital Medical Information Network (UMIN) registration number UMIN000042542 and was approved by the ethical committee of the Prefectural University of Kumamoto (30–22). All participants signed an informed consent form describing the purpose, methods, and significance of the study.

The flow diagram of the participant.