To help you on the path to healthier eating, using the example of a doughnut and an apple, we explain the process of digestion and absorption

The science behind food

Digestion

The process of digestion starts once we have placed the food in our mouth and start to chew. A couple of things occur here: As we chew, our teeth start to break up the food, and this is mixed with saliva (produced by our salivary glands) by the tongue. The saliva contains a chemical called salivary amylase, which starts breaking down complex carbohydrates, and another called mucin, which helps to soften the food. The chewed and softened ball of food is called a bolus.

The bolus of food is then propelled down our oesophagus by the process of peristalsis - a wave-like contraction of the muscles of our digestive system. It then reaches the stomach, where the next stage of digestion occurs. Receptors in our stomach sense that food is about to enter, and a hormone called gastrin stimulates the release of hydrochloric acid and digestive enzymes. The stomach is then turned into a highly acid environment (acidic enough to burn your skin) and this helps to further break the food down.

After being processed in the stomach, the food is passed into the first part of the small intestine, known as the duodenum, which is where most of the digestive process takes place. The partially digested food is mixed with three other liquids here:

  1. Bile, a substance that is released by the gallbladder to help in the digestion of fat;
  2. Pancreatic juice and enzymes (released into the intestine by the pancreas);
  3. Other alkaline intestinal enzymes, such as maltase, lactase and sucrose, which help to further break down different types of sugars (this is why some people have reactions to dairy products - they lack, or have very low levels of lactase, so can't break down lactose, the main sugar in dairy products).

By this stage, most foods have been digested. Dietary fibre, however, is not digested and it helps to keep food remains and waste products moving through the rest of the small intestine and then into the large intestine (bowel) for excretion, preventing the build-up of toxic waste.