Abstract P196: The Effects of Initiating a 24-hour Fast With a Low versus a High Carbohydrate Protein Shake on Heart Rate Variability in the Elderly

Introduction: While multiple studies demonstrate the potential cardiovascular benefits of fasting, the effects of the fast-initiating meal have not been evaluated. The aim of this secondary analysis was to examine the effects of the macronutrient composition of a fast-initiating meal on the autonomic nervous system through measurements of heart rate variability (HRV) in older, sedentary adults with abdominal obesity. Methods: Using a crossover design, each of the 24 participants (13 male, 11 female) completed two 24-hour fasts (8:00 am to 8:00 am) separated by 6-10 days. Participants were 59.54 ± 6.67 years old and had a body mass index of 31.59 ± 5.59 kg/m 2 . Each fast was initiated with either a low carbohydrate protein shake or an isocaloric and isovolumetric high carbohydrate protein shake (592.13 ± 104.57 kcal). Heart rate variability was measured for 10 minutes immediately prior to, one hour after, and then 24 hours after shake consumption. Results: A mixed effects model with repeated measures was used to assess the effects of the fasting conditions on various HRV outcomes. The sympathetic (SNS) index increased significantly over the time of each fast (p = 0.0033), but was not different between conditions (p = 0.7229). The parasympathetic PNS index experienced no change over time (p = 0.1521) and no change between conditions (p = 0.8961). The stress index increased over time (p = 0.0021), but was not different between conditions (p = 0.7308). There was no difference in standard deviation of the normal-to-normal interval (SDNN) or root mean square of successive difference (RMSSD) over time (p = 0.2022 and p = 0.2770, respectively) or between condition (p = 0.9054 and p = 0.9857, respectively). The High Frequency (HF) reading was elevated in the high-carbohydrate condition compared to the low-carbohydrate condition (p < 0.0001), and the low frequency to high frequency (LF/HF) ratio showed no difference between conditions (p = 0.3460), but increased over time (p = 0.0044) and demonstrated a time by condition interaction (p = 0.0089). Conclusions: Acute fasting places strain on the cardiometabolic systems as evident by the increased SNS index and stress index as well as the increase in LF/HF ratio over the course of both fasts. A low carbohydrate shake reduced HF readings, indicating more anxiety or panic in this condition. These results agree with previous works and are unique in demonstrating that most HRV parameters are neither strengthened nor weakened by the composition of the pre-fast meal.