Effects of breakfast consumption of carbohydrates and caffeine on core body temperature profiles: a randomized controlled trial in habitual breakfast-skipping young adults

It has been suggested that breakfast influences daily physiological functions. However, evidence from human intervention studies remains limited. This study aimed to investigate the effects of breakfast composition, including carbohydrates and caffeine, on the core body temperature profiles under free-living conditions in young adults who habitually skipped breakfast. In this randomized, non-blinded, parallel-group trial, university students who habitually skipped breakfast were assigned to one of four groups: no breakfast (NB, n = 8), caffeinated drinks (CD, n = 7), carbohydrate-rich rice balls (RB, n = 9), or a combination of both (CD + RB, n = 8). After 5 d of baseline measurements from Monday to Friday, participants consumed the assigned breakfast within 1 h of waking from Monday to Friday with no other energy or caffeine intake until noon during the intervention week. Core body temperature was continuously measured using a wearable sensor. Secondary outcomes included salivary cortisol levels and subjective measures such as sleep, appetite, and concentration. All breakfast interventions significantly advanced meal timing and the eating window midpoint. Core body temperature increased during the morning in the CD and RB groups and decreased during the evening in the CD + RB group compared to the baseline. Within-group analyses revealed significant phase advances in the core body temperature midpoint in all breakfast intervention groups (CD, RB, and CD + RB), but not in the NB group; however, the between-group difference in the magnitude of phase advance did not reach statistical significance ( p = 0.338). No significant changes were observed in salivary cortisol levels. In the CD + RB group, subjective sleep initiation and maintenance worsened significantly, and appetite before dinner was significantly reduced during the intervention. Five-day breakfast interventions were associated with intervention-specific changes in core body temperature profiles under free-living conditions. Further studies with longer intervention periods are warranted to clarify the effects of breakfast consumption on physiological function. UMIN-CTR (Trial ID: UMIN000054216; registration date: 24 April, 2024).