Ever wondered exactly what happens in your digestive system? Unless you suffer from indigestion or IBS, you might not have given it much thought. Did you know, for example, that around 70% of our immune system is located in or around our gut? Or that the gut is often referred to as the second brain, so extensive are its neuronal connections to the brain.
In the same way that thinking of food as just fuel, to think of our digestive system as just a tube running from top to bottom belies its fundamental importance to our health. It is true that we need fuel to survive and one of the functions of the digestive system is just that: to break down carbohydrates, fats and protein in food to be used by the body and brain, but it also absorbs and produces micronutrients, eliminates wastes and acts as a barrier between the outside environment and our internal environment much like our skin. As we saw in the earlier article on the microbiome, the gut also plays a fundamental role in our immunity. But just what happens to our food once it’s inside us?
Before we have even eaten a mouthful our brain responds to the sight and smell of food by revving up production of digestive juices and hormones. Saliva begins the process of carbohydrate digestion whilst teeth mash up the food ready for the journey south into the stomach.
Once inside the stomach, hydrochloric acid is secreted by cells in the lining and mixed into the food by twisting muscular movements. Almost as acidic as battery acid, gastric juice is responsible for the breakdown of protein into smaller amino acid chains as well as activating an enzyme called pepsin, which then chops the chains up further. Stomach acid also kills harmful invaders or microbes that might be in our food and activates minerals enabling them to be absorbed further down the gut. The stomach also secretes intrinsic factor - a glycoprotein which binds to vitamin B12 from food allowing it to be absorbed lower down, as well as fat digesting enzymes known as lipases.
Food stays in the stomach for 2-4 hours depending on how much protein and fat there is in each meal, after which time it is released gradually out of the bottom of the stomach and into the duodenum - often causing sound effects! Once in the duodenum, this rather acidic soupy mixture is quickly neutralised by juices squirted in by the pancreas and gallbladder. Pancreatic juice contains enzymes which start work on digesting the fats, carbohydrates and proteins, whilst bile (a greeny yellow liquid produced in the liver and concentrated by the gallbladder) emulsifies fats for absorption.
By this stage, digestion is largely complete, food is broken down into its smallest building blocks, intrinsic factor is bound to B12, harmful microbes have been destroyed and minerals are activated and all is ready for absorption.
The journey to this point is not always plain sailing. Sometimes the stomach does not release enough stomach acid, or the pancreas enough enzymes, and digestion can be compromised leading to indigestion or even bloating, pain and wind further down the gut. Feeling full for too long after meals, burping, bad breath and acid reflux are all classic signs that these digestive processes might not be working optimally. Feeling nauseous or particularly tired after rich food can also indicate poor liver function.
Digestion is just the first part of our digestive journey and as we continue to descend south into the abdominal region, it is absorption that takes centre stage. Over the course of the next 20 feet of the small intestine, minerals, fat soluble vitamins, B vitamins, fats and monosaccharides are absorbed, followed by amino acids, disaccharides, water and finally vitamin B12. These pass through the intestinal lining, the barrier between inside and out, to end up in the blood vessels ready for distribution to each cell in the body.
Finally, the bowel contents arrive at the ileocecal valve - the junction between the small and large intestine, the last part of the tube. Ironically the large intestine is much shorter than the small intestine at around 5 feet, but it is here in this short space that re-absorption of some 400-800ml of water occurs. Here the final stages of digestion take place, as resident bacteria which form part of our microbiome ferment or convert any remaining carbohydrates or proteins into smaller parts. These same bacteria also produce B vitamins and vitamin K as well as maintaining integrity of the gut lining, crowding out harmful bacteria and training the immune system to discriminate between friend and foe. As water is reabsorbed, stools begin to form, ready for excretion from the body taking toxins and indigestible matter with them.
As with digestion, the course of absorption does not always run smooth, and many of us suffer common bowel complaints such as constipation, haemorrhoids, diarrhoea, diverticular disease, IBS, food intolerances, candidiasis and dysbiosis.
Fibre also supports the gut microbiome or flora, as fibre is their fuel. Whole grains, fruit and vegetables, pulses, nuts and seeds are great sources of fibre
Drink plenty of water - aim for around 1.5 litres each day. This helps to keep stools from drying out and becoming hard to pass
So, as we come to the end of our whistle-stop tour through your digestive system, spare a thought for the wonderful job being carried out inside, and consider how what you put in has a bearing on what you get out - both in terms of your health and well-being and quite literally!
Ponder too the powerful, persuasive words of Elie Metchnickoff commenting on the primary role the gut plays in shaping our health more than a century ago whilst working as a biologist at the renowned Pasteur Institute:
“ Death begins in the colon.” A little scary perhaps, but definitely food for thought.