Running through the core of every one of us is an empty space, but it’s not of the existential variety. It’s the long tube, stretching up to seven metres from mouth to anus, that we call the gut or gastrointestinal tract. Its main jobs are to digest food, eliminate undigested gut contents, protect us against pathogens, and keep everything moving along from top, to, er, bottom…
At its most basic, food is fuel, and we get what we need out of it – whether that’s energy, nutrients or water – through the process of digestion, whereby it’s broken down in the gut into soluble units small enough to be absorbed through the gut wall and delivered to different parts of the body.
So let’s start at the top. And no, that doesn’t actually mean the mouth. Digestion begins in
the brain. Before you even raise a cheese sandwich to your lips, the very thought of it is sparking physiological changes in your gastrointestinal tract: the salivary glands start secreting saliva to begin the breakdown of anticipated carbohydrate (in the bread) and lipids (fats in the butter and cheese); the stomach produces gastric juices to kick-start digestion of prospective protein (in the cheese); and the pancreas releases insulin so it’s ready to move glucose from carbohydrate digestion into target cells to keep blood-glucose levels within the normal range.
Chewing it over
Now take a bite of that sandwich, and mechanical digestion begins – chewing, in other words, to increase the food’s surface area and make it more accessible to chemical digestion, and also to stimulate insulin and gastric juice secretion. Tasting food on your tongue prompts the production of saliva, and the salivary amylase enzymes it contains start to break down long-chain carbohydrates (such as the starch in bread) into shorter chains, while another salivary enzyme, lingual lipase, gets to work on fats.
Your tongue rolls the food into a small ball (bolus) and forces it to the back of your mouth, upon which you swallow. This is a reflex reaction which pushes the bolus down into the oesophagus – a muscular tube leading to the stomach – with the larynx rising up to meet the epiglottis and close off your windpipe. If this didn’t happen, you could choke.
It takes eight to ten seconds for the bolus to travel down the oesophagus and into your stomach, a bag-like organ which, after a meal, can stretch to over 30 times its ‘resting’ size, then snap back again when it’s empty. Contrary to popular belief, the capacity of an adult’s stomach stays more or less the same – you can’t stretch it permanently by overeating, and neither can you shrink it by dieting (the only way you can do that is by surgery). It’s also worth pointing out that there doesn’t seem to be a link between stomach size and weight, either; naturally skinny people can have stomachs bigger than those who struggle with their weight.
So, what happens when food enters that incredibly flexible stomach? The combination of a highly acidic environment (thanks to hydrochloric acid secretions) and rhythmic contractions of its muscular walls blends the food into a thick, soupy mixture called chyme, which is easier for digestive enzymes to penetrate than big lumps of food. The acid also helps in the production of the enzyme peptin, which starts the breakdown of protein. One of its other jobs is to kill off harmful micro-organisms in the food.
Little and large
A typical meal, including our cheese sandwich, passes through the stomach in two to six hours. Chyme is expelled into the small intestine, where its acidity is neutralised and the bulk of digestion takes place – enzymes finish the breakdown of carbs into glucose and fructose, of proteins into their constituent amino-acid building blocks, and of lipids into fatty acids. The small intestine is highly adapted for maximum absorption of these nutrients, as well as most vitamins and minerals, into the bloodstream; its inner walls are lined with tiny villi, finger-like projections that are in turn covered with even tinier ones called microvilli, and this increases the surface area to around that of a tennis court. It’s your own digestive Tardis.
As well as nutrients, a huge quantity around nine litres of fluid enters the gut each day, some of it from water in food, some from things you have drunk and the rest from the secretions which are made by using previously digested fluids –, some seven of which comprise digestive secretions, with the remainder from food and drink. Roughly 8.5 litres of this fluid are absorbed through the walls of the small intestine, and what’s left passes with the chyme into the large intestine. The 1.5m large intestine is far shorter than the 5m ‘small’ intestine, but much wider, hence the name.
It takes the chyme four to six hours to reach the long intestine, by which point around 96% of its nutrients will have been absorbed, leaving it comprising of mostly fibre, salts and water. The long intestine’s resident bacteria (gut flora) ferment and break down non-digestible fibre into chemicals such as nitrogen, carbon dioxide, methane and hydrogen sulphide. They also synthesise key vitamins, including thiamin (B1), riboflavin (B2), B12 and K, which are then absorbed, along with the remaining water. By the time what’s left – waste products like dead bacteria, indigestible fibre, mucus, bile pigments and other digestive secretions – reaches the rectum (which typically takes 12-24 hours), it’s solidified into faeces to be expelled through the anus. Digestion is truly a process that gives a whole new meaning to the phrase ‘getting your shit together’.
The gut is not just a digestive pathway, however; it’s also a huge nerve network, second only to that in our heads, and it’s in constant contact with the brain. It’s where 90% of the feel-good hormone and neurotransmitter serotonin is secreted. And the bacteria in our gut – which collectively weigh more than the brain – might even affect our minds. Studies have found that transplanting the gut bacteria of nervous and normal mice switches their dispositions, while giving lab animals a high-sugar diet leads to gut-flora changes that impede their ability to cope with novel situations.
What is certainly indisputable in humans is the physiological reaction we experience when we simply imagine eating – the release of hormones, enzymes and digestive secretions to break down food that at this point only exists in our imagination. This is a graphic example of how our thoughts influence our bodies and actions – which has a huge affect on how, and how much, we eat. Thinking about food all the time is not the way to lose weight and keep it off, yet it’s how most diets function, with the emphasis on ingredients, cooking, meal planning… food, food, food. No wonder most diets fail. A growing body of evidence shows that changing how we think about food, using techniques such as cognitive behaviour therapy to change our mindset, is far more effective than simply trying new recipes and counting calories. Truly food for thought.