From kai to cells: the journey of a meal

A mouthful of sandwich is far too big to get into your blood. Your digestive system is a long tube, about 5 to 9 metres long in an adult, that breaks food into pieces so small they can pass through its walls. This happens in two ways. Mechanical digestion is physical: chewing and churning break food into smaller bits. Chemical digestion uses substances like acid and enzymes to break the food particles themselves apart.

The route

Enzymes: tiny helpers

Enzymes are proteins that speed up chemical reactions. Each one fits a particular kind of food molecule, a bit like a key fits a lock. Heat can change an enzyme's shape so it no longer works. Green kiwifruit contains an enzyme called actinidin that breaks down protein, which is why fresh kiwifruit stops jelly setting but canned kiwifruit does not.

Built for the job

The inside of your small intestine is covered in millions of tiny finger-like bumps called villi. They give it a huge surface for absorbing nutrients into the blood. Your large intestine is home to trillions of helpful bacteria that break down fibre, make some vitamins and help keep you healthy. That is one reason fibre from fruit, vegetables and whole grains matters. Your body uses the nutrients from digestion as fuel and as building blocks for growing and repairing.