Woman eating spaghetti, good digestion

Digestion: what you need to know

From feeling butterflies to stomach ache, our digestive system has a big impact on our wider wellbeing. Understanding how it works is the first step to staying well

Written by Madeleine Bailey on March 1, 2019 Reviewed by Amanda Hamilton on March 14, 2019

From the boom in friendly bacteria supplements to ‘free from’ supermarket aisles, we’re increasingly interested in our digestive wellness. And no wonder – 40% of us have at least one digestive issue, such as indigestion, at any one time.1 Yet, how many of us really how what’s going on in the full nine metres of our digestive tract?2 It’s time to find out.

What is the digestive system?

Although we eat food, our digestive system doesn’t absorb food – it absorbs nutrients. So our food has to broken down into amino acids from proteins, fatty acids from various fats, and simple sugars from carbohydrates, as well as vitamins, minerals and other essential nutrients.

Your digestion is a complex network of organs that manages this process, breaking down food down into different compounds, and passing out what we don’t need as waste. The whole process takes on average one to three days.3

The part of the digestive tract known as the gut also contains trillions of various microorganisms, or beneficial bacteria, many of which help support our immune system.4 Plus, it has over 100,000 nerve cells and is closely linked to our emotions, which is why we may get butterflies or diarrhoea when we’re feeling nervous or stressed.5

How does the digestive system work?

Let’s take a close look at your digestion, from start to finish – quite literally.

The mouth

Your teeth break food down into small pieces, while the sight and smell of a delicious meal wakes up your salivary glands. Saliva lubricates and further breaks down food with the help of an enzyme called amylase, which starts to turn carbohydrates into sugars.6

What can go wrong? Rushing your meal or talking while you eat mean you can swallow air, leading to burping, bloating or flatulence.7

Keep it healthy. Friendly bacteria aren’t just beneficial for your gut – studies show they can be good for our dental health  too.

Did you know? The average person produces one litre of saliva every day.8

The oesophagus

Once swallowed, food passes down your oesophagus, or gullet, to your stomach via automatic waves or contractions, known as peristalsis.9

What can go wrong? If stomach acid travels back up into your oesophagus, you can get heartburn: a burning sensation in your chest. Stress, obesity, smoking, and some foods and medicines can make heartburn more likely.10

Keep it healthy. Stopping smoking can help prevent heartburn and stomach ulcers, and reduce your risk of developing Crohn’s disease and gallstones, too.11

Did you know? Peristalsis is so powerful that food would reach your stomach even if you were eating upside down.12

The stomach

Pressure from food travelling down the oesophagus sends a signal to the valve at the top of your stomach to open. In the stomach, gastric juices containing stomach acid and various enzymes break down proteins, creating a liquid called chyme.13 Chyme is then moved out of your stomach by peristalsis into your small intestine.

What can go wrong? If the protective layer lining the inside of your stomach gets damaged, contact with stomach acid can result in an ulcer. This damage can be caused by the H.pylori bacteria or overuse of anti-inflammatory drugs such as ibuprofen.14

Keep it healthy. Avoid rushing your meals and maybe practice some yoga – research shows certain yoga postures can help support your gastrointestinal health, while exercise in general helps increase the friendly bacteria in your gut.

Did you know? Your stomach acid is strong enough to dissolve metal!15

The small intestine

The small intestine, also known as the duodenum or small bowel, is where most nutrients are absorbed into your bloodstream through villi – tiny, finger-like structures that line the walls of your small intestine.16

What can go wrong? A lack of some enzymes can make it difficult to digest certain foods, causing food intolerances. Symptoms include stomach ache, bloating, flatulence and diarrhoea.17

Keep it healthy. Help rebalance the bacteria living in your gut by eating a high-fibre diet – aim for around 30g a day – and consider more fermented foods, such as sauerkraut, kefir or kombucha. If you don’t currently eat a lot of fibre, introduce it slowly by gradually increasing the amount every few days.

Did you know? The small intestine is an impressive six metres long.18

The liver, gallbladder and pancreas

Your liver makes bile, which is then stored it in your gallbladder and released into your small intestine to break down fats. Meanwhile, your pancreas produces enzymes that are used by the digestive system to help break down carbohydrates, protein and fats until they are small enough to be absorbed into your bloodstream.19

What can go wrong? A fatty diet with too many junk foods, or drinking too much alcohol can cause fat to build up in your liver, which significantly raises your risk of developing liver disease.20,21

Keep it healthy. Look after your liver by reducing your drinking and upping your intake of liver-supporting foods, such as broccoli.

Did you know? Your liver has more than 500 functions and is your body’s largest solid organ, weighing between 1.3kg and 1.8kg.22

The large intestine

Finally, food reaches your large intestine, also called the colon or large bowel, where water and remaining nutrients are absorbed. This makes waste products – yes, we’re talking about poo – solid and easier to pass. When enough waste builds up in your rectum, the lowest part of your large intestine, you feel that the urge to go to the loo.23

What can go wrong? Not eating enough fibre can slow your digestion down, leading to constipation.24

Keep it healthy. Drink plenty of fluids, as this can reduce the risk of constipation.25 Keep your stress levels in check, too – stress hormones can contribute to digestive problems, such as diarrhoea.26

Did you know? Stools stay in the large intestine for an average of 33 hours in men and 47 hours in women.27

When to see your GP

See your doctor if any of the symptoms above continue for longer than is normal for you. If you also experience loss of appetite, black or bloody stools, or unexplained weight loss, make an appointment straight away.28 Advice is for information only and should not replace medical care. Please check with your GP before trying any remedies.


1. NHS. Common digestive problems – and how to treat them 2. Salford Royal NHS Foundation Trust. The upper gastrointestinal tract 3. Stephanie Watson. Healthline. How long does it take to digest food? 4. Science Daily. Gut microbes closely linked to proper immune function, other health issues 5. Adam Hadhazy. Scientific American. Think twice: how the gut’s “second brain” influences mood and well-being 6. BBC Bitesize. What is digestion? 7. Mayo Clinic. Belching, intestinal gas and bloating: tips for reducing them 8. Mayo Clinic. How your digestive system works – slide 2

9. As above – slide 3.

10. NHS. Heartburn and acid reflux 11. Love Your Gut. Lifestyle tips 12. Joseph Castro. Live Science. 11 surprising facts about the digestive system 13. Kim Ann Zimmermann. Live Science. Digestive system: facts, functions and diseases 14. NHS. Stomach ulcer – overview 15. Li PK, et al. In vitro effects of simulated gastric juice on swallowed metal objects: implications for practical management

16. As Source 13

17. Christian Nordqvist. Medical News Today. What is a food intolerance?

18. As Source 13
19. As Source 8 – slide 5

20. NHS. Non-alcoholic fatty liver disease 21. NHS. Alcohol-related liver disease 22. British Liver Trust. Liver health

23. As Source 8 – slide 7

24. NHS Inform. Causes of constipation 25. NHS Inform. Treating constipation

26. As Source 11

27. Mayo Clinic. Digestion: How long does it take? 28. Love Your Gut. When to see a doctor
Digestive Health