The process that your food goes through in order to provide nutrients that your body will use for all of its metabolic processes is amazingly efficient. As soon as we swallow, we often cease to give our food any more thought – it’s out of sight so out of mind, but there is a huge amount of activity that silently (except for the odd gurgle) takes place, providing our nourishment, energy, and ‘joie de vivre’.

Our digestive system is a long tube, separated into specific segments, that all have slightly different features. The wall of the tube is made of muscular layers that move the food through and (in most cases) doesn’t allow it to go back up the tube. Let’s look at what happens to the food we eat at the different stages of digestion.

1. Mouth

We think it all starts here but actually the process of digestion starts in our mind, when we think about food or smell it cooking, secretions begin to be released in our digestive tract, getting ready for breaking the food down. Our mouth, as we know is responsible for chewing which physically breaks apart our food, but did you know that saliva contains digestive enzymes to help the process of breaking down fat and carbohydrates?

Top tip – chewing your food well means that it will be easier to break down, as it travels down the alimentary canal of the digestive tract, enabling more nutrients to be absorbed. Take time to chew - at least 10 – 20 times each mouthful if you can.

2. Stomach

Food enters our stomach and special cells secrete a strong acid and an enzyme called pepsin which helps to break apart the proteins. The muscular layers within the stomach churn the contents (now called chyme) and physically move it so that it mixes with the acid. There are hormones produced in the stomach – gastrin triggers the production of the hydrochloric acid when food gets to the stomach, and ghrelin helps stimulate our feelings of hunger, prior to eating. Without adequate amounts of hydrochloric acid and enzymes we tend to feel uncomfortably full, can experience bloating or indigestion. It can also mean we lose energy because we aren’t breaking apart our food properly, so it is harder to absorb nutrients.

Top tip – Prepare your body for eating by taking a simple deep breath before you take a bite. This moment will make you present, which changes the state that your nervous system is in. We can either be in ‘fight or flight’ or ‘rest and digest’. The latter is optimal when you are about to eat.

3. Small intestine

When food leaves the stomach and enters the small intestine, it is met with highly alkaline secretions from both the pancreas and gallbladder. This helps in a couple of ways – it neutralises the acidic nature of the contents from the stomach to protect the tissue of the intestines, and these secretions are very helpful for assisting digestion. The pancreas produces digestive enzymes to break down fat, protein, and carbohydrates, and the gallbladder secretes bile to help emulsify fat. The structure of the small intestine is such that it is covered in tiny finger-like projections called microvilli. These increase the surface area, allowing for more nutrient absorption. The small intestine is where the majority of your nutrients are absorbed, so ensuring this part of your digestive system is healthy is very important for good nutrition. In between each microvilli is a crypt that produces hormones, one of which is leptin, which opposes ghrelin and helps us to realise we are feeling full. These villi are incredibly rich in blood supply, carrying nutrients into the bloodstream from the digestive tract.

Top tip - There should be a very small number of bacteria in the small intestine, thanks to the low pH of the stomach acid. If you’re experiencing a lot of gas and bloating as well as pain and diarrhoea or constipation, ask your health practitioner about Small Intestinal Bacterial Overgrowth (SIBO) and find out how to get tested for it.

4. Large intestine

As ‘yesterday’s food’ travels through the large intestine, the water content is reabsorbed back into the body. The large intestine houses many billions of probiotic organisms that ferment and digest the residual starches. These probiotics create metabolites that impact on many parts of the body, from how you think, to your immunity, your mood, and everything in between. There is so much research being conducted on the species found in our large bowel and how they influence the body.

Top tip – broad spectrum antibiotics kill good bacteria too. If you can avoid unnecessary use of antibiotics, do so, to reduce the chance of antimicrobial resistance. If antibiotics are essential, take a good quality probiotic as well as B Vitamins, and gut friendly foods like bone broths, fibre, vegetables, to restore the microbiome.

5. Exit

The final stage of the digestive process is excretion. Bowel motions are made from whatever is left behind after nutrients have been absorbed – usually starches, and fibres. The time it takes for the food you’ve eaten to be eliminated can depend on the amount of vegetables and fruit that is eaten, but also other things like stress, alcohol consumption, and infection. A bowel motion should feel satisfying and complete.

Top Tip - If you want to see how long your bowel transit time is, eat a good serving of corn at one of your meal times and then time how long it takes to see it in your bowel motions.

Summary

If the efficiency of the digestive tract could be summarised in a few words, “surface-area” comes to mind. The power of the digestive tract is partly that every part of the tube is made in a way that allows for a much greater surface area – there are many folds, there are micro-villi on top of villi. This surface area means that we get so much more opportunity for nutrient absorption than we would if it really was just a 5-metre segmented tube. The surface area of the digestive tract is thought to be between 30 – 40 m2 in size – about the size of half a badminton court1! We are fortunate enough to get to choose what we eat every day, despite it feeling like a chore to constantly come up with healthy and tasty dinner ideas. If we can make food choices that are nutrient dense to optimise the opportunity that our body has to absorb nutrients that’s a great start.

  • 0 min – 10 min at mouth and swiftly gets into stomach
  • 2.5 – 4 hours Food leaves the stomach
  • 4 – 8 hours Food leaves the small intestine2
  • 8 – 24 hours - Food leaves the large intestine


References

1. https://www.tandfonline.com/doi/abs/10.3109/00365521.2014.898326
2. http://www.vivo.colostate.edu/hbooks/pathphys/digestion/basics/transit.html

Liz McNamara is a Registered Naturopath with more than 16 years of experience in natural health. As the President of the Naturopaths & Medical Herbalists of New Zealand (NMHNZ) and the Natural Health Expert at HealthPost, Liz is passionate about health education and helping others lead healthy lives.