Relevance of High Glycemic Index Breakfast for Heart Rate Variability among Young Students with Early and Late Chronotypes ^†

Abstract

Background and Objectives: Previous reports suggest that spikes in plasma glucose affect cardiac autonomic modulation, reflected by a decrease in heart rate variability (HRV). Hence, the hypothesis of this analysis is that cardiac autonomic changes following an early high-glycemic-index (GI) breakfast are more prominent in individuals with later chronotypes than in those with earlier chronotypes because of their unfavorable metabolic situation at this time of the day. Similarly, chronotype-specific morning plasma melatonin levels could be mechanistically relevant for HRV changes following early high-GI breakfasts, as melatonin seems to influence glucose tolerance. Methods: Subjects with earlier (N = 22) and later chronotypes (N = 23) were asked to consume an intervention meal with a GI = 72 in the morning (7 a.m.). Chronotype was assessed by the Munich Chronotype Questionnaire. Plasma melatonin level was determined at approx. 8 a.m. Glucose data were collected by continuous glucose measurement. Blood volume pulses derived by wrist-worn wireless multisensors were used to assess successive interbeat intervals (IBIs). Time domain HRV parameters RMSSD (root mean square of successive differences of normal IBI), mean IBI and SDNN (standard deviation of normal-to-normal IBI) were calculated. Data from 36 participants (n = 21 early and n = 15 late chronotypes) met the following criteria for analysis: beats corrected <10% and effective sample rate > 60 s. HRV differences before vs. after breakfast were calculated and association with chronotype was analyzed by multivariable linear regression. Results: RMSSD, mean IBI and SDNN were higher in both chronotypes before a high-GI breakfast. Changes in the analyzed HRV parameters after a high-GI breakfast did not differ between persons with an earlier or later chronotype (all p > 0.2). Pooling the data from both chronotypes, a smaller change in mean IBI following a high-GI breakfast was associated with higher morning plasma melatonin levels (p = 0.0232). Neither age, sex nor BMI account for this association. Conclusions: These data suggest that in our very small cohort of young healthy adults, morning plasma melatonin levels, but not chronotype, are associated with parasympathetic HRV activity after an early high-GI breakfast.