How Nutrients Are Digested and Absorbed in the Body

Enzymes, hormones and the remarkably coordinated journey of food

This article continues my mini‑series focused on the body, the digestive system, and the processes that influence appetite — with a particular emphasis on how nutrients are digested and absorbed in the human body. In the previous post, I focused on the digestive system as a whole — how its different parts are involved in digestion, how long food typically spends in each section of the digestive tract, and why factors such as proper chewing play an important role in supporting digestive efficiency and overall health. If you’d like to revisit that foundational overview, you can read it here:

👉 https://www.katrinpeo.com/post/the-digestive-system-a-remarkably-intelligent-system-supporting-our-health-every-day

In this article, we take a step back and look at the bigger picture: how the digestive system works as a coordinated whole, how enzymes and hormones guide digestion, and how macronutrients and micronutrients are ultimately broken down and absorbed to support health and homeostasis.

Digestion as a foundation of health

Digestion is the first critical step in maintaining homeostasis — the stable internal environment that allows our cells, tissues and organs to function optimally. Its ultimate goal is to provide cells with the building blocks and energy they need for survival, growth and repair.

Each section of the digestive tract has a highly specialised role. When digestion proceeds smoothly, nutrients are efficiently broken down, absorbed and utilised. When something goes wrong, the consequences can ripple throughout the body.

Disturbances in digestion and absorption may lead to:

• maldigestion (insufficient breakdown of food)

• malabsorption (insufficient uptake of nutrients from food)

• motility disorders (diarrhoea, constipation, nausea, vomiting)

• heartburn and reflux

Long‑term imbalanced eating patterns are one of the key contributors to digestive dysfunction — highlighting why organism‑centred, balanced nutrition is essential for both digestive and overall health.

The journey of food through the digestive tract

Mouth

The mouth is the starting point of digestion. It performs:

• mechanical breakdown (chewing)

• moistening and lubrication with saliva

• initial carbohydrate digestion via salivary amylase

  
  

  With thorough chewing, up to 50% of starch digestion can already begin in the mouth.

Stomach

The stomach provides an acidic environment that:

• denatures proteins

• activates pepsin from its inactive precursor pepsinogen

• initiates protein digestion

• destroys many ingested microorganisms

Only a small portion (10–18%) of proteins are broken down here, but this step is crucial for efficient digestion downstream.

Small intestine

The small intestine is the central hub of digestion and absorption.

• Final digestion of carbohydrates, proteins and fats occurs here

• Most nutrients are absorbed into blood or lymph

• Digestive juices from the pancreas, liver and intestinal lining are added

  
  

  The small intestine is divided functionally into:

• duodenum – intensive digestion and early absorption

• jejunum – main site of nutrient absorption

• ileum – absorption of vitamin B12 and bile acids

Large intestine

In the large intestine:

• water and electrolytes are absorbed

• gut microbiota ferment undigested carbohydrates

• short‑chain fatty acids and bacterial metabolites are produced

Digestive enzymes – breaking food down into absorbable units

Digestion relies on a finely tuned cascade of enzymes, each specific to certain substrates and locations in the digestive tract.

Carbohydrate‑digesting enzymes

• Amylase (saliva, pancreas) – breaks starch into maltose

• Maltase, sucrase, lactase (intestinal brush border) – convert disaccharides into monosaccharides

Carbohydrates are ultimately absorbed as glucose, fructose and galactose.

Protein‑digesting enzymes

• Pepsin (stomach) – initiates protein digestion

• Trypsin, chymotrypsin, elastase (pancreas) – further cleave peptides

• Aminopeptidases (intestinal brush border) – release free amino acids

Proteins are absorbed mainly as free amino acids.

Fat‑digesting enzymes

• Pancreatic lipase (with colipase)

• Phospholipases and cholesterol esterase

Fats must first be emulsified by bile acids to allow enzyme access. The final digestion products include fatty acids, monoglycerides and glycerol.

Hormonal regulation of digestion

Digestion is not controlled by enzymes alone. It is orchestrated by a sophisticated hormonal signalling network, ensuring that digestion proceeds at the right time, in the right place, and at the right speed.

Key gastrointestinal hormones

Gastrin

Produced mainly in the stomach, gastrin:

• stimulates hydrochloric acid secretion

• increases pepsin release

• promotes gastric motility

Its release is triggered by protein intake and stomach stretching.

Cholecystokinin (CCK‑8)

Secreted by the duodenum and small intestine in response to fats and amino acids, CCK:

• stimulates gallbladder contraction and bile release

• activates pancreatic enzyme secretion

• slows gastric emptying

Secretin

Released when acidic chyme enters the duodenum, secretin:

• stimulates bicarbonate secretion from the pancreas

• neutralises gastric acid

• inhibits gastric acid production

Glucagon

Produced in the pancreas, glucagon:

• raises blood glucose

• stimulates fat breakdown during fasting

Adequate fasting intervals between meals are necessary for glucagon‑driven fat metabolism to occur.

Other regulatory hormones and peptides

• Somatostatin – inhibits acid, gastrin and insulin secretion

• Histamine – stimulates gastric acid production

• Ghrelin – regulates hunger and energy balance

• Leptin – signals long-term energy availability and satiety, helps regulate appetite over time, and interacts with digestive and metabolic hormones

• Motilin – stimulates intestinal motility between meals

• GLP‑1 and GIP – enhance post‑meal insulin secretion and slow gastric emptying

Together, these hormones ensure efficient digestion, nutrient absorption and metabolic balance.

Digestion and absorption of macronutrients

Carbohydrates

Carbohydrate digestion begins in the mouth and is completed in the small intestine. Absorption occurs as monosaccharides, which enter the bloodstream and influence blood glucose and insulin regulation.

Non‑digestible carbohydrates (dietary fibre) reach the colon, where they support gut microbiota and are fermented into beneficial short‑chain fatty acids.

Proteins

Protein digestion starts in the stomach and continues in the small intestine. Most amino acids are absorbed before reaching the colon. Efficient protein digestion depends on adequate stomach acid, pancreatic enzymes and healthy intestinal lining.

Fats

Fat digestion is the most complex process, requiring:

• bile acids for emulsification

• pancreatic enzymes for hydrolysis

• micelle and chylomicron formation for absorption and transport

Disruptions in bile production or pancreatic function can severely impair fat absorption and lead to deficiencies in fat‑soluble vitamins.

Micronutrient digestion and absorption

Micronutrients — vitamins and minerals — rely on many of the same digestive processes as macronutrients. Their absorption is influenced by:

• stomach acidity

• bile acids

• intestinal health

• gut microbiota

• interactions with other nutrients and medications

Because this topic is extensive, I have covered it in detail in a separate article and downloadable one‑page guides.

📘 You can explore these resources here: 👉 Resources page (downloadable vitamin & mineral guides)

When digestion doesn’t work optimally

Digestive‑related nutrient deficiencies may arise due to:

• low stomach acid (impaired protein digestion, reduced mineral absorption)

• bile or pancreatic enzyme insufficiency (fat malabsorption, bloating, digestive discomfort)

• intestinal inflammation or resection (reduced absorptive surface, increased nutrient losses)

• gut microbiota disruption (e.g. antibiotics, altered fermentation, reduced short-chain fatty acid production)

• long-term medication use (e.g. stomach acid suppressants (proton-pump inhibitors, metformin, NSAIDs)

Supporting digestion through balanced nutrition, adequate meal timing, and digestive health awareness is therefore foundational to long‑term wellbeing.

Final thoughts

Digestion is not a passive process — it is a highly intelligent, responsive and adaptive system. Enzymes, hormones, organs and microbes work together seamlessly to transform food into life‑sustaining molecules.

Understanding how this system works empowers us to make nutrition choices that truly support our bodies — not just on paper, but at the cellular level.

If you’d like to continue exploring nutrient absorption in more detail, I warmly recommend revisiting the previous article and downloading the vitamin and mineral guides from the Resources page.

If digestive symptoms, appetite changes, or ongoing nutrient concerns feel familiar, personalised nutrition support can make a meaningful difference.

Digestion, absorption and appetite regulation are highly individual processes, influenced by lifestyle, health history and physiology. If you’d like guidance tailored specifically to your body and needs, you’re welcome to reach out to me for personalised nutrition counselling at info@katrinpeo.com.