The Effects of a High-Carbohydrate Versus a High-Fat Shake on Heart Rate Variability When Used to Interrupt a 38-Hour Fast

Heart rate variability (HRV) indicates the heart’s capacity to respond to stimuli from the autonomic nervous system. While previous research has explored the cardiovascular effects of fasting and food intake, the impact of the post-fast meal on cardiovascular health remains understudied.PURPOSE: This study aims to investigate how the macronutrient composition of a post-fast meal affects the autonomic nervous system, as measured by HRV.METHODS: Twenty-seven individuals (12 females and 15 males) participated in three separate 38-hour water-only fasts, each separated by a rest period of 6 to 10 days. Two of the three fasts were interrupted after 24 hours with either a high-carbohydrate, low-fat shake (HC/LF) or a low-carbohydrate, high-fat shake (LC/HF). Each shake represented 25% of the participants’ daily calorie needs. The third fast, serving as a control, was not interrupted. Each fasting period began at 6:00 pm and was preceded by a standardized meal tailored to individual energy needs, calculated based on height (in cm), weight (in kg), and age (in years). To measure HRV, participants wore HRV monitoring equipment throughout the night while fasting. RR intervals were collected using the Polar H10 chest strap (Polar Inc., Finland) and recorded via the Polar Beats mobile application (Polar Inc., Finland).RESULTS: Out of 200 applicants, 29 qualified and were randomly assigned to conditions, with 27 completing all sessions and 2 dropping out. The average caloric intake for standardized meals and shakes was 628.7 ± 102.6 kcal. No main effect was observed between conditions for any HRV parameter, including the PNS index (F=0.44, p=0.64), SNS index (F=0.6160, p=0.5425), mean RR (F=0.5221, p=0.5952), SDNN (F=0.2833, p=0.7540), RMSSD (F=0.4299, p=0.6600), LF (F=0.2564, p=0.7744), HF (F=0.3499, p=0.7062), LF/HF ratio (F=0.5362, p=0.5869), and Stress Index (F=0.8284, p=0.4868). Additionally, there were no significant differences over time or interaction effects between time and condition for these parameters.CONCLUSION: The macronutrient composition of a post-fast meal does not significantly impact HRV parameters. Further research is needed to explore other potential factors influencing HRV.