Nutrients, Digestion, and Gut-Brain Axis: Key Concepts

Essential nutrients

Nutrients that must be provided from the diet as the body cannot produce them.

Non-essential nutrients

Nutrients that can be synthesized by the body from other substances.

Macronutrients

Nutrients that provide energy and are needed in large amounts.

Carbohydrates

Types of carbohydrates include glucose, fructose, sucrose, and galactose.

Essential amino acids

Amino acids that must be obtained from the diet, such as tryptophan, threonine, methionine, and lysine.

Lipids

Fats that include fatty acids and cholesterol.

Fat-soluble vitamins

Vitamins that include A, D, E, and K.

Water-soluble vitamins

Vitamins that dissolve in water and are not stored in the body.

Folate (B9)

A B-complex vitamin found in grains, meat, and leafy greens.

Macro minerals

Minerals needed in larger amounts, such as calcium (Ca), phosphorus (P), potassium (K), sodium (Na), and magnesium (Mg).

Trace minerals

Minerals needed in smaller amounts.

Phytochemicals

Substances that are not classified as essential nutrients but have health benefits, such as antioxidants and anti-inflammatory properties.

Probiotics

Live bacteria cultures that support gut health.

Prebiotics

Substances that create an environment for good bacteria and serve as food for them.

Fatty acids

Important for the evolution of brain size and function.

Free radicals

Chemicals with unpaired electrons that can cause oxidative stress.

Electron transport chain (ETC)

The process in the mitochondria that produces free radicals.

Free radical damage

Can lead to DNA damage, protein damage, and lipid peroxidation, resulting in neurodegenerative diseases.

Antioxidants

Substances that neutralize free radicals and protect the body from oxidative stress.

Myenteric plexus

Controls motility, including peristalsis and segmentation in the gastrointestinal tract.

Submucosal plexus neurotransmitters

Includes ACH, VIP, serotonin (5-HT), NPY, and somatostatin.

Vagus nerve

Carries signals from the gut to the brain and vice versa.

Short-chain fatty acids (SCFAs)

Produced by bacterial fermentation of dietary fiber and have various health benefits.

SCFAs

Produced by bacterial fermentation of dietary fiber and have various health benefits.

Gastrin

Stimulates HCI secretion, pepsinogen release, and gastric motility.

Secretin

Stimulates bicarbonate secretion from the pancreas to neutralize stomach acid.

Cholecystokinin (CCK)

Triggers pancreatic enzyme secretion and gallbladder contraction for bile release.

Carbohydrate Digestion

Carbohydrates are broken down by pancreatic amylase and brush border enzymes into monosaccharides.

Pancreatic Lipase

Breaks down triglycerides into monoglycerides and free fatty acids.

Jejunum

The main site of nutrient absorption in the small intestine.

Insulinotropic Peptide (GIP)

Stimulates insulin secretion and decreases gastric acid secretion.

Motilin

Stimulates the migrating motor complex, promoting peristalsis during fasting.

Somatostatin

Inhibits gastrin, secretin, insulin, and glucagon secretion.

Histamine

Stimulates HCI secretion via H2 receptors on parietal cells.

Parasympathetic Input

Enhances motility and secretion in the digestive system.

Protein Digestion in Stomach

Proteins are broken down into polypeptides by pepsin, which is activated from pepsinogen.

Bile Salts

Emulsify fats, aiding in their digestion and absorption.

Final Products of Lipid Digestion

Monoglycerides and free fatty acids.

Blood-Brain Barrier (BBB)

Regulates the passage of molecules between the blood and the brain, protecting the brain from harmful substances.

Gut Microbes Influence on Brain Function

Produce signaling molecules that can affect gut-brain communication and influence brain function.

Neurotransmitter-like Molecules from Gut Bacteria

Regulate gut motility and can signal to the vagus nerve, influencing brain signaling.

Antioxidants on Neurons

Protect neurons from damage caused by free radicals.

Enteric Neurons

Act as local relay stations for signals between the gut and the brain.

Enteric Nervous System (ENS)

Controls gastrointestinal function independently of the central nervous system.

Dietary Fiber for Gut Health

Fermented by gut bacteria to produce SCFAs, which support gut barrier integrity and overall health.

Glucose

A type of sugar.

Fructose

Sugar found in fruits and juice.

Sucrose

A type of sugar.

Galactose

Monosaccharide from lactose.

Proteins

Composed of essential and non-essential amino acids.

Cholesterol

A type of lipid.

Micronutrients

Include water, vitamins, and minerals.

Vitamin A

Found in carrots and sweet potatoes.

Vitamin D

Obtained from sunlight and fatty fish.

Vitamin E

Found in nuts and seeds.

Vitamin K

Found in leafy greens.

Vitamin C

Found in citrus and peppers.

Choline

A water-soluble nutrient.

Minerals

Include macro and trace minerals.

Polyphenols

Antioxidants that have anti-inflammatory properties.

Inulin

A type of prebiotic.

Oxidative Stress

Can cause DNA damage, protein damage, and lipid peroxidation.

Malnutrition in-utero

Increased risk of schizophrenia.

Schizophrenia

A mental disorder characterized by abnormal dopamine and GABA signaling.

DNA hypomethylation

A condition leading to dysregulated gene expression.

Dutch Hunger Winter

A historical event that resulted in alerted methylation.

Altered RA receptor activity

Leads to disrupted cortical and hippocampal development.

Iron (Fe)

A cofactor for neurotransmitter synthesis that supports myelination.

Hypomyelination

Impaired dopamine and serotonin metabolism.

Epigenetics and developmental programming

Nutrients supply substrates and cofactors for chemical reactions.

DNA Methylation

The addition of a methyl group to DNA.

Histone (de)acetylation

The addition or removal of acetyl groups from histones.

Acetyl CoA

Increases transcription by opening chromatin.

DNA Demethylation

The process of removing methyl groups from DNA.

Metabolic State

Shifts the balance of epigenetic marks based on nutrient availability.

Superoxide Dismutase (SOD)

An enzyme that catalyzes the reaction 2O2− + 2H+ → H2O2 + O2.

Catalase

An enzyme that catalyzes the reaction 2H2O2 → 2H2O + O2.

Glutathione Peroxidase (GPx)

An enzyme that catalyzes the reaction H2O2 + 2GSH → 2H2O + GSSG.

Uric acid

A non-enzyme that looks for reactive oxygen species (ROS) and reactive nitrogen species (RNS).

SOD

Converts superoxide to hydrogen peroxide.

Catalase and GPx

Convert hydrogen peroxide to harmless water + oxygen.

Non-enzymatic antioxidants

Mop up residual free radicals.

Myenteric Plexus (Auerbach's)

Controls motility: peristalsis, segmentation, sphincter relaxation.

Excitatory neurotransmitters

ACh, Substance P, 5-HT stimulate contraction & secretion.

Inhibitory neurotransmitters

Cause muscle relaxation, vasodilation.

Parasympathetic Input (Vagus)

Increases peristalsis, secretion, and blood flow (rest-and-digest).

Gut-brain communication

Involves physical routes and cell types for signaling between gut and brain.

Neural Route

The vagus nerve carries signals from gut → brain (sensory) and brain → gut (motor).

Circulatory / HPA axis

Stress activates the HPA axis, releasing cortisol that affects gut and immune cells.

Barrier systems

Regulate passage of molecules; compromised barriers allow gut metabolites to access the brain.

Gut microbes

Active messengers that digest nutrients and release signaling molecules.

Butyrate

Acts as an HDAC inhibitor, influencing brain gene expression.

Serotonin (5-HT)

~90% of serotonin in the body is made in gut enterochromaffin cells, regulating gut motility.

Dopamine

Some gut bacteria produce dopamine precursors; peripheral dopamine modulates gut-brain signaling.

Digestion process in stomach

Mechanical mixing (peristalsis) + acidic environment (HCl).

Chief cells

Secrete pepsinogen, activated to pepsin by HCl.

Lipids digestion

Gastric lipase begins triglyceride breakdown.

Chyme

Partially digested food + gastric juice.

Duodenum

Entry to small intestine.

CCK (Cholecystokinin)

Hormone released by I cells that triggers pancreatic enzyme secretion and gallbladder contraction for bile release.