A study by researchers from the University of Konstanz, Germany, revealed interesting insights into the connection between blood glucose and the autonomic nervous system, suggesting that the intake of sugar counteracts relaxation, reports News Medical.
The important role of glucose in coping with stressful situations has been well researched – when we consume sugar, the body reacts more strongly to stress by releasing more cortisol, and additionally, our heart rate remains elevated for longer. This means that more energy is available in acute stressful situations.
The negative long-term consequences are also well known: increased risk of high blood pressure, obesity and cardiovascular disease.
But what has been less well researched is how sugar intake affects relaxation – which is why researchers led by Jens Pruessner, Professor of Neuropsychology at the University of Konstanz, conducted a corresponding study, which was published in the International Journal of Psychophysiology.
In essence, said Pruessner, “If your stomach is full, relaxation exercises will not be as effective.” The Konstanz team aimed to understand what role the body's energy systems play in relaxation and what effect individual metabolic factors have – for example, blood glucose levels. The autonomic nervous system, which includes the sympathetic and parasympathetic nervous systems, controls various processes in our organism, such as heart rate and breathing.
“Our heart has an internal pacemaker that determines how fast it beats. While sympathetic activity has a stimulating and activating effect in moments of stress, parasympathetic activity works like a vagal brake, slowing down the heartbeat,” said Maria Meier, first author of the study and postdoctoral researcher the research team.
Sweet relaxation?
The study involved 94 healthy adults, all of whom fasted before coming to the laboratory, and were randomly allocated to consume either a drink containing glucose or water. One half them received a relaxing massage, while the other half rested without a direct intervention.
Cardiac activity was measured continuously. The authors later calculated heart rate variability, a measure of the parasympathetic nervous system’s activity. They also assessed the pre-ejection period, which is a measure of the activity of the sympathetic nervous system.
The effect of sugar
All participants said they had found the massage or the resting phase mentally relaxing. This was also reflected in the measured cardiac activity: the relaxation techniques activated the parasympathetic nervous system, regardless of whether sugar had been consumed beforehand or not.
That massages provide deeper relaxation compared with simply resting had already been shown in previous studies.
At the same time, the sympathetic nervous system was activated after sugar intake, meaning that although the participants subjectively felt relaxed, their sympathetic nervous system did not slow down, but kept the body in a higher state of arousal.
“Enjoying a sweet snack is often associated with relaxing situations …in fact, the constant sympathetic activation after sugar intake seems to limit the ability to relax. So, if you want to explicitly relax, e.g, through meditation or progressive muscle relaxation, you should not eat something high in sugar beforehand,” said Pruessner.
The study also leads to another conclusion for the researchers: “To make valid statements, we can’t just look at one system in isolation – that is, either the sympathetic or the parasympathetic system – because otherwise we would overlook some effects,” said Meier. “If we had only investigated the parasympathetic nervous system, we would not have observed the important effect on the sympathetic nervous system.”
Study details
The effect of glucose on cardiac reactivity to a standardised massage in healthy adults.
Maria Meier, Eva Unternaehrer, Jens Pruessner et al.
Published in International Journal of Psychophysiology in June 2026
Abstract
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 standardised 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.
News Medical article – Sugar intake may reduce effectiveness of relaxation exercises (Open access)
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