Autonomic nervous system (ANS)

The autonomic nervous system (ANS) is a subdivision of the peripheral nervous system (PNS) that regulates the body’s vital functions, such as blood pressure, heart rate, breathing, and digestion. It is further subdivided into the sympathetic and parasympathetic nervous systems (Gibbons, 2019).

In nutritional psychology, the ANS is the key link between what we eat and how we feel emotionally. The sympathetic branch triggers the “fight-or-flight” response, diverting blood flow away from digestion during stress, while the parasympathetic branch supports “rest-and-digest” functions, allowing nutrient absorption and gut-brain communication to occur efficiently. Chronic stress keeps the sympathetic system dominant, impairing digestion, altering gut microbiota, and disrupting the production of neurotransmitters like serotonin, much of which is synthesized in the gut.

Because the autonomic nervous system ANS directly influences the gut-brain axis, its balance affects mental health outcomes as much as physical ones. An overactive sympathetic response, common in anxiety and chronic stress, can reduce appetite, slow digestion, and worsen nutrient absorption, creating a cycle that further destabilizes mood. Nutritional psychologists often focus on parasympathetic-supporting strategies, such as mindful eating and vagus nerve stimulation through diet and breathing, to restore ANS balance and support both digestive and emotional wellbeing.