The Effects Of A High-Carbohydrate Versus A High-Fat Shake On Heart Rate Variability When Used To Interrupt A 38-Hour Fast

As intermittent fasting has become more popular as a potential tool to improve metabolic health, questions have arisen with regards to optimizing fasting protocols to achieve maximum benefits as well as the effect of fasting on different physiological systems of the body. To date little research has been conducted on effects of the post-fast meal macronutrient content or how that may influence cardiovascular factors such as heart rate variability. PURPOSE: To determine the effect of shakes with different macronutrient consumptions on heart rate variability when used to interrupt a 38-hr fast. METHODS: A randomized-crossover study design was used to compare three conditions. Twenty-seven healthy participants completed a 38-hr fast interrupted at the 24-hr mark with either a high-carbohydrate shake, a low-carbohydrate shake, or water. Participants then took heart rate variability measurements overnight using a chest strap heart rate monitor. RESULTS: There were no significant main effects or interaction effects between time and condition for any parameters of HRV. No significant differences were seen between conditions in the PNS index (F = 0.44; p = 0.6442), SNS index (F = 0.62; p = 0.5425), Mean RR (F = 0.52; p = 0.5952), SDNN (F = 0.28; p = 0.7540), RMSSD (F = 0.43; p = 0.6600), LF (F = 0.60; p = 0.7744), HF (F = 0.35; p = 0.7062), LF/HF ratio (F = 0.54; p = 0.5869), or the stress index (F = 0.83; p = 0.4868). CONCLUSIONS: Macronutrient content of the fast-interrupting shake did not affect any parameter of overnight heart rate variability. Similarly, there were no differences in heart rate variability between interrupting a fast by consuming calories and continuing to fast by drinking only water.