How does sugar affect relaxation exercises? A new study conducted by researchers at the University of Konstanz provides clear insight into the relationship between blood sugar and the autonomic nervous system, and shows that sugar intake interferes with relaxation.
The important role glucose plays in dealing with stressful situations is well-studied, whether it’s a little sugar before a class test, a piece of chocolate before an important negotiation, or a muesli bar before a marathon. When you consume sugar, your body responds more strongly to stress because more cortisol is released. Additionally, your heart rate will remain high for a long time. This means that in severe stressful situations, more energy is available. Long-term negative effects are also well known, including increased risk of high blood pressure, obesity, and cardiovascular disease.
What hasn’t been studied as much is how sugar intake affects relaxation. This is why researchers from the team of neuropsychology professor Jens Prussner at the University of Konstanz conducted a corresponding study, which is currently International Journal of Psychophysiology. Jens Prussner summarizes the findings as follows: “Relaxation exercises aren’t as effective if you’re full.”
Constance’s research team aims to understand what role the body’s energy system plays in relaxation and how individual metabolic factors, such as blood sugar levels, influence it. The autonomic nervous system, which includes the sympathetic and parasympathetic nervous systems, controls various processes within our bodies, such as heart rate and breathing. “Our heart has an internal pacemaker that determines how fast it beats. Sympathetic activity has a stimulating and activating effect in moments of stress, while parasympathetic activity acts like a vagal brake, slowing down the heartbeat,” explains Maria Mayer, lead author of the study and postdoctoral researcher in the Jens Prussner research team.
Sweet relaxation?
Ninety-four healthy adults participated in this study. Participants who had fasted before coming to the lab were randomly assigned to consume either glucose-containing drinks or water. Afterwards, half received a relaxing massage, and the other half rested without direct intervention. Cardiac activity was measured continuously. The authors later calculated heart rate variability, a measure of parasympathetic nervous system activity. They also assessed the pre-ejection period, a measure of sympathetic nervous system activity.
What effect did sugar have in this experiment? All participants said they felt mentally relaxed during the massage and rest phase. This was also reflected in measured heart activity, with relaxation techniques activating the parasympathetic nervous system, regardless of whether sugar was consumed beforehand. Previous research has already shown that massage provides deeper relaxation than just rest.
At the same time, the sympathetic nervous system was activated after sugar intake. “This means that even though the participants felt subjectively relaxed, their sympathetic nervous system activity remained undiminished, keeping their bodies in a higher state of alertness. The bottom line from our test results is that sugar impairs the body’s ability to relax,” says neuropsychologist Meyer.
Is it okay to have soft drinks or ice cream before a massage? “Enjoying sweet snacks is often associated with relaxing situations, such as eating a chocolate bar or ice cream while watching a movie, or having cake with the family on the weekend. In fact, the sympathetic nervous system is always activated after ingesting sugar, which seems to limit our ability to relax. Therefore, if you want to relax explicitly, such as through meditation or gradual muscle relaxation, you should not eat something high in sugar beforehand,” explains Jens Prussner.
The study also led researchers to another conclusion. “To make valid claims, we cannot look at one system in isolation, the sympathetic or the parasympathetic nervous system, because otherwise we would miss some effects,” says Maria Meyer. “If we had only studied the parasympathetic nervous system, we would not have observed significant effects on the sympathetic nervous system.”
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DOI: https://doi.org/10.1016/j.ijpsycho.2026.113367
