The Digestive System – a Remarkably Intelligent System Supporting Our Health Every Day

Mini-series: The Body, the Digestive System and Processes Influencing Appetite

I am currently attending a month-long training programme titled “The Body, the Digestive System and Processes Influencing Appetite”, which has provided me with deeper and more nuanced insights into how the human digestive system works. Because I find this knowledge both fascinating and highly practical, I wanted to share parts of it with you in a way that is easy to understand and applicable to everyday life.

As the topic is extensive, I will be sharing it through a series of blog posts. This first article focuses on the physiology of the human digestive system — what happens in the different parts of the digestive tract, how long digestion takes at each stage, and why supporting digestion through our daily habits is so important.

An important takeaway is that every part of the digestive system can be either supported or overburdened by our eating behaviour, food choices and eating rhythm.

The course is taught by Laine Parts, one of my favourite lecturers from my nutrition counselling studies at the Public Health Academy. She is a lecturer at the Tallinn Health Care College, a senior analytical chemist at the Estonian Environmental Research Centre, and a certified nutrition counsellor and nutrition therapist. With a strong background in chemistry and material sciences, she also teaches biochemistry, toxicology and nutrition-related subjects to future healthcare professionals. Her multidisciplinary, science-based background gives the training a very solid foundation.

The deeper one delves into the functioning of the digestive system, the more respect and awe one develops for the human body. What our body does for us every single day — mostly unnoticed — is truly remarkable.

What is digestion?

Digestion is the process by which large food molecules are broken down by digestive enzymes into smaller building blocks (monomers) that can be absorbed through the intestinal lining into the bloodstream or lymphatic system.

The ultimate goal of digestion is nutrient absorption, allowing the body to use nutrients:

• to produce energy

• as building material for cells

• to support metabolic processes

The more balanced, diverse and individualised our diet is, the more efficient digestion becomes. Equally important is how we eat — in a calm environment, without rushing or excessive stress.

The digestive tract – 7 to 9 metres of coordinated work

The total length of the human digestive tract is approximately 7–9 metres, consisting of:

• the mouth

• pharynx

• oesophagus

• stomach

• small intestine (duodenum, jejunum, ileum)

• large intestine

Each section has a specific and irreplaceable role, and digestion functions optimally only when the entire system works in harmony.

Digestion begins in the mouth

The mouth is the most important mechanical stage of digestion. Food is broken down by chewing and mixed with saliva.The better food is chewed, the easier it is for digestive enzymes to access it later in the digestion process.

About 1.2 litres of saliva are produced daily. Saliva:

• helps reduce the entry of harmful bacteria with food

• initiates carbohydrate digestion

  
  

The Oesophagus – an Often Overlooked Part of the Digestive Tract

The oesophagus is a muscular tube that connects the mouth to the stomach. Its role is to transport swallowed food to the stomach through coordinated muscular contractions. Food moves through the oesophagus regardless of body position, but its function is strongly influenced by eating speed, stress levels, and body posture after meals.

Food typically remains in the oesophagus for a few seconds up to around 10 seconds.

At the lower end of the oesophagus is a sphincter muscle that prevents stomach acid from flowing back into the oesophagus. Eating too quickly, swallowing large bites, overeating, or lying down immediately after meals can increase the risk of heartburn, acid reflux and digestive discomfort.

👉 To support oesophageal function:

• eat slowly and chew food thoroughly (20-30 times)

• avoid very large bites

• remain upright for at least 20–30 minutes after eating

The stomach – a chemical and mechanical processing unit

An adult stomach typically holds 1.5–3 litres, yet when empty it is small and folded. The stomach:

• mixes and grinds food through muscular contractions

• creates an acidic environment

• initiates protein digestion

Food remains in the stomach for approximately 2–6 hours, depending on its composition. Protein and fat rich food remains in the stomach longer, carbohydrate and liquid based food shorter time.

The liver and pancreas – hidden heroes of digestion

The liver, the body’s largest gland, acts as a biochemical laboratory:

• producing bile acids for fat digestion

• processing and storing nutrients from food

• supporting detoxification

👉Learn more about the many functions of liver here and how can you support it with your food and lifestyle choices: https://www.katrinpeo.com/post/your-liver-why-and-how-to-take-care-of-it.

The pancreas produces:

• digestive enzymes (amylases, lipases, proteases)

• hormones insulin and glucagon, which regulate blood sugar and metabolism

  (these hormones work in opposition to maintain metabolic balance (homeostasis).

  
  

The small and large intestine – absorption and the microbiome

Most nutrient digestion and absorption occur in the small intestine, where food moves through over 3–6 hours. Its surface area reaches up to 300 m², making it highly efficient in absorbing nutrients.

Food reaches the large intestine approximately 6–8 hours after eating and may remain there for 2–3 days. Here:

• water and minerals are absorbed

• billions of microorganisms process remaining material

• waste is formed

Why does this matter?

Overall, the complete digestive process — from the first bite to waste elimination — can take 24 to 72 hours. Digestion is therefore a highly energy-demanding process.

Supporting digestion and digestive organs is essential for maintaining:

• energy levels

• immune function

• hormonal balance

• mood

• body weight

For this reason, allowing the body rest from eating, especially at night, is crucial. During longer eating breaks, the body can redirect energy toward:

• cleansing processes

• recovery

• cellular regeneration

👉 You can read more about intermittent fasting, how to practise it and its health benefits here: https://et.katrinpeo.com/post/vahelduv-paastumine-kasu-tervisele-ja-kuidas-seda-teha

Coming up next in this mini-series:

• how nutrients are digested and absorbed

• the role of gut microbiota in health

• mechanisms behind appetite regulation

• the connection between nutrition and biological rhythms

If you would like to improve your eating habits and support your health, but are not sure where to start, feel free to reach out to me for personalised nutrition counselling at info@katrinpeo.com.

During online or in-person session(s), I provide feedback on your current nutrition and lifestyle habits, based on information you share with me before our meeting. Together, we identify your goals and focus areas, and you decide what you would like to work on and improve.

I support you with evidence-based information, practical guidance and realistic steps, so you can move towards your health goals in a way that fits your daily life.