Inside Digestion: How Your Nervous System Turns Food into Fuel

7 minute read

Digestion isn’t just something that happens in your stomach- it’s a coordinated series of neural, chemical, and hormonal events that begins before you take your first bite. The process unfolds in three main phases: the cephalic, gastric, and intestinal phases. At the centre of this orchestration is the parasympathetic nervous system (PNS)- particularly the vagus nerve- which governs much of the body’s “rest and digest” activity. Proper PNS function is essential for stomach acid production, protein digestion, vitamin B12 absorption, and the release of digestive enzymes.

Phase 1: The Cephalic Phase - Digestion Begins in the Brain

The cephalic phase of digestion starts before food enters the stomach, triggered by the sight, smell, taste, or even thought of food. These sensory stimuli activate the vagal centers in the medulla oblongata, which send signals via the vagus nerve to the stomach.

Key actions during this phase:
- Gastric glands begin secreting hydrochloric acid (HCl) and pepsinogen.
- The pancreas prepares to release digestive enzymes.
- Salivary glands release salivary amylase, starting carbohydrate breakdown.

This anticipatory activation accounts for up to 30% of total gastric secretions (Guyton & Hall, 2021). When parasympathetic tone is low - for example, during stress - these signals are blunted, leading to reduced acid and enzyme output, which can impair digestion downstream (Furness, 2012).

Phase 2: The Gastric Phase - The Stomach Takes Centre Stage

Once food reaches the stomach, the gastric phase begins. Mechanical and chemical digestion now take over as the stomach churns food and secretes digestive juices.

What happens during this phase:
- Stretch receptors in the stomach wall detect distension, stimulating further acid release via vagal reflexes.
- Gastrin, a hormone released by G-cells, amplifies HCl and pepsinogen secretion.
- Parietal cells produce both hydrochloric acid and intrinsic factor (IF) - the latter essential for vitamin B12 absorption in the ileum (O’Leary & Samman, 2010).

Adequate stomach acidity is crucial for protein breakdown, activation of pepsinogen into pepsin, and intrinsic factor stability. Without strong parasympathetic stimulation, HCl production declines, potentially leading to hypochlorhydria - a state associated with poor protein digestion, B12 deficiency, and increased risk of intestinal dysbiosis (Martinsen et al., 2005).

Phase 3: The Intestinal Phase — Enzymatic Fine-Tuning and Absorption

As chyme moves into the next part of the gut, the duodenum, the intestinal phase begins. This stage fine-tunes digestion and maximizes nutrient absorption.

Key events:
- The duodenum releases secretin and cholecystokinin (CCK) in response to acidic and fatty chyme.
- Secretin stimulates the pancreas to release bicarbonate, neutralizing stomach acid.
- CCK signals the pancreas to secrete digestive enzymes and the gallbladder to release bile for fat emulsification (Johnson, 2013).
- Intrinsic factor–B12 complexes are absorbed in the terminal ileum.

Parasympathetic activation maintains gut motility, enzyme secretion, and bile flow. Low vagal tone can impair these processes, contributing to bloating, malabsorption, or constipation (Breit et al., 2018).

Why the Parasympathetic Nervous System Matters

The parasympathetic nervous system, primarily through the vagus nerve, coordinates much of the digestive sequence. When the body is in a “rest and digest” state, gastric acid and enzyme secretion are enhanced, intestinal motility increases, and nutrient absorption improves. Conversely, chronic stress activates the sympathetic system, which suppresses these processes and can lead to digestive disorders (Bonaz et al., 2018).

Summary of Digestive Phases

Cephalic Phase: Triggered by the thought, smell, or sight of food; initiates digestive secretions via the vagus nerve.
Gastric Phase: Begins when food enters the stomach; involves acid and enzyme release for protein digestion and intrinsic factor production.
Intestinal Phase: Manages enzymatic breakdown, bile release, and nutrient absorption; coordinated by parasympathetic control.

Supporting Parasympathetic Activity for Better Digestion

- Eat mindfully and chew thoroughly.
- Practice deep breathing before meals.
- Manage stress through relaxation or meditation.
- Ensure adequate intake of zinc and B-vitamins to support gastric secretions.

References

• Bonaz, B., Bazin, T., & Pellissier, S. (2018). The vagus nerve at the interface of the microbiota-gut-brain axis. Frontiers in Neuroscience, 12, 49.

• Breit, S., Kupferberg, A., Rogler, G., & Hasler, G. (2018). Vagus nerve as modulator of the brain–gut axis in psychiatric and inflammatory disorders. Frontiers in Psychiatry, 9, 44.

• Furness, J. B. (2012). The enteric nervous system and neurogastroenterology. Nature Reviews Gastroenterology & Hepatology, 9(5), 286–294.

• Guyton, A. C., & Hall, J. E. (2021). Textbook of Medical Physiology (14th ed.). Elsevier.

• Johnson, L. R. (2013). Physiology of the Gastrointestinal Tract (5th ed.). Academic Press.

• Martinsen, T. C., Bergh, K., & Waldum, H. L. (2005). Gastric juice: a barrier against infectious diseases. Basic & Clinical Pharmacology & Toxicology, 96(2), 94–102.

• O’Leary, F., & Samman, S. (2010). Vitamin B12 in health and disease. Nutrients, 2(3), 299–316.