The effect of glucose on cardiac reactivity to a standardized massage in healthy adults

Cardiac reactivity reflects autonomic nervous system regulation, yet its determinants and modulators remain incompletely understood. Although glucose influences cardiac activity at rest, its effects on task-dependent reactivity -particularly during relaxation interventions- are unclear. We examined whether glucose modulates cardiac reactivity during massage and rest, two treatments known to increase parasympathetic activity, and whether such effects relate to sustained attention. Ninety-four adults (age mean = 22.84, SD = 2.53; 71.28% female) were randomly assigned to consume glucose, or water before receiving a standardized massage or rest treatment, followed by a sustained attention task. Blood glucose, blood pressure, subjective arousal and pleasure, and electro- and impedance cardiography were assessed. Parasympathetic activity was indexed using the root mean square of successive differences (RMSSD) and high-frequency heart rate variability (HF-HRV), and sympathetic activity using pre-ejection period (PEP). Both massage and rest increased parasympathetic activity and reduced arousal. Glucose increased sympathetic activity and prevented sympathetic withdrawal during the massage, but we found no evidence for a modulation of parasympathetic reactivity. Sympathetic activity increased during sustained attention, and higher blood glucose was associated with better performance. Overall, glucose modulated sympathetic but not parasympathetic reactivity, potentially impairing the effectiveness of relaxation treatments.