Maintenance of Vagal Tone with Time-Release Caffeine, But Vagal Withdrawal During Placebo in Caffeine-Habituated Men


Objective: This study aimed to determine the effects of a time-release caffeine (TR-CAF) supplement and caffeine abstinence on heart rate variability (HRV) in habitual users. Methods: Ten caffeine-habituated (>200 mg per day) men (25.9 ± 3.2 years; 1.81 ± 0.08 m; and 92.9 ± 9.9 kg) performed three trials, each separated by 1 week. During each trial, participants randomly consumed either 194 mg of caffeine in a multi-ingredient supplement containing TR-CAF; an instant-release caffeine supplement (CAF); or placebo (PL). HRV was assessed for 10 minutes in the supine position at baseline (BL), and every hour after supplementation for 8 hours. Time-domain [average heart rate, mean standard deviation of all normal-to-normal (NN) intervals (SDNN), and root mean square of differences between adjacent NN intervals (RMSSD)] and frequency-domain [low frequency (LF), high frequency (HF), total power (TP), LF/HF, and an index of parasympathetic nervous system (PNS = HF/TP)] measures were recorded. A two-way (trial × time) repeated-measures analysis of variance was used to assess all HRV parameters. Results: A significant interaction was noted for PNS (p = 0.039). There was a significant difference between trials 1-hour post-ingestion, but post hoc analysis noted a trend between CAF and PL (p = 0.079). However, at 6 hours, post-ingestion post hoc analyses revealed higher PNS influence during both TR-CAF (0.84 ± 0.06, p = 0.039) and CAF (0.84 ± 0.07, p = 0.005) compared to PL (0.79 ± 0.05). There was a significant main effect of time for PNS index during the PL condition only, but no subsequent differences were noted from BL in post hoc analyses. Conclusions: Both caffeine supplements maintained parasympathetic influence within the autonomic nervous system, while PL appeared to result in the potential for vagal withdrawal over an 8-hour span.

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Journal of Caffeine and Adenosine Research