When you think of the digestive process, you might picture the (pretty creepy) plastic human body in the Healthy Harold van with their stomach lighting up. Or perhaps you see a diagram of a mishmash of worm-like intestines, like the image below.
Although we eat (and get rid of waste) every day, not everyone has a clear idea of what's actually happening in the digestive system, if at all. In reality, the process of digesting food and absorbing its nutrients is much longer and more complex than we believe.
Here's what happens as soon as you put food in your mouth.
The digestive process:
- Stage one: begins in the mouth via chewing, saliva and enzymes;
- Stage two: food travels to the stomach via the esophagus;
- Stage three: enters the stomach, where acids begin to break down food;
- Stage four: food enters the small intestine, where it is further broken down and many nutrients are absorbed;
- Stage five: passes through to large intestine, where other nutrients are absorbed, with the remaining solids finally making their way to the colon;
- Stage six: solid waste is stored in the rectum and egested.
How long does it take to digest food?
"Digestion starts to happen immediately," accredited practising dietitian Chloe McLeod told HuffPost Australia.
"Some carbohydrates will be absorbed in the mouth as the food is chewed and broken down by enzymes. Enzymes in the stomach further break the food down, before most of the absorption taking place in the small intestine."
It normally takes 6-8 hours for food to pass through your stomach and small intestine, and to enter the large intestine, where it becomes fully digested.
"From there onward, it takes 40 hours for the waste to actually be excreted. It is a bit of a process, so what you're eating for breakfast today isn't going to be fully digested until the end of the day. What you eat for dinner is being digested overnight while you're asleep."
The exact time it takes for food to be digested depends on which nutrients the food contains and the quantity of the meal.
"Plant-based foods such as fruit and vegetables will usually move through more quickly than high-protein or fatty foods," McLeod said.
"Fat actually slows down digestion, so if the meal is higher in fat (whether it's healthy or unhealthy fats) it will take longer to digest. Really high-fibre meals take a bit longer to digest, as well, as the fibre is bulking everything out and slowing the transit time down."
How long does it take to absorb nutrients from food?
As we digest food, our body transports and utilises various vitamins, minerals, protein, carbohydrates and fats at different points along the digestive tract. The absorption process begins around 3-6 hours after eating.
"Nutrients get absorbed as the food is broken down, with the majority of nutrients being absorbed in the small intestine, where they're then transported into the blood stream," McLeod explained.
"In particular, sugars will be digested quickly and absorbed through the stomach wall and the wall of the small intestine, to then be utilised by the body.
"Various enzymes break down carbs, fat and protein. For example, bile acid breaks down fat, pancreatic juices help break down carbohydrates. Some nutrients are also absorbed via the large intestine."
Some of the vitamins and minerals are absorbed by what's called 'active transport' across intestinal membranes.
"So they will attach to another compound and be transported across a passage. Whereas other nutrients will passively diffuse through the walls," McLeod said.
"For example, as fat is digested, it is broken down into small compounds in the small intestine before it is absorbed through the intestinal wall. The fatty acids then bind to a protein called albumin and are transported to the liver for energy or turned into longer chain fatty acids.
"Some fats form into triglycerides and are then moved into the blood stream. Any unused fat ends up being stored in adipose tissue."
For carbs, they are transported into the blood stream, and then to the muscles and stored as glycogen to be used for energy.
"Excess consumption also ends up with conversion to fat and stored in the adipose tissue."
Pretty cool, eh?