The Human Digestive System: How Food Becomes Energy

Your cells need energy and building materials, but they cannot use a bowl of dal and rice as it is. The molecules in food are far too big to pass into the blood. Digestion breaks them down into small pieces that can.

It works in two ways:

• Mechanical digestion: chewing, and the churning of the stomach, break food into smaller bits.
• Chemical digestion: enzymes and acid split large molecules into simple ones. Carbohydrates become simple sugars, proteins become amino acids, and fats become fatty acids and glycerol.

Food travels through the digestive tract, one long tube from the mouth to the anus. The liver, gallbladder and pancreas sit outside the tube and pour their juices into it.

Digestion starts as soon as you chew. Teeth grind food into smaller pieces, and saliva moistens it and begins breaking down starch with an enzyme called amylase.

When you swallow, the food enters the oesophagus. It does not simply fall: waves of muscle contraction called peristalsis squeeze it down to the stomach.

In the stomach, glands make acid and enzymes, and strong muscles mix the food with them. The acid kills much of the bacteria swallowed with food and helps the enzyme pepsin start digesting protein. The food is slowly turned into a thick liquid called chyme. A stomach usually empties into the small intestine within two to four hours of a meal.

The small intestine is the longest part of the tract and where most of the work happens. It has three parts: the duodenum, the jejunum and the ileum. Here, three helper organs join in:

• The pancreas makes a juice full of enzymes that break down carbohydrates, proteins and fats.
• The liver makes bile, which helps digest fats.
• The gallbladder stores bile between meals and releases it when you eat.

The lining of the small intestine is covered in tiny finger-like villi, which give it a huge surface for absorbing nutrients. It absorbs most of the nutrients in your food and about 90 per cent of the water you take in.

Whatever is left moves into the large intestine. It is shorter than the small intestine but more than twice as wide. It absorbs most of the remaining water and turns liquid waste into solid stool.

Trillions of bacteria live here. They break down some leftover material and make certain vitamins, including vitamin K. Waste usually spends 12 to 24 hours in the large intestine before the rectum stores it and it leaves the body through the anus.

Imagine a masala dosa for breakfast. The rice and urad dal batter supplies starch and some protein, the potato filling adds more starch, and the oil or ghee adds fat.

While you chew, amylase in your saliva is already working on the starch. In the stomach, acid and pepsin begin taking apart the dal's protein. Once the food reaches the small intestine, pancreatic enzymes finish the job, and bile from the gallbladder helps with the fat. Glucose and amino acids pass through the villi into your blood. A few hours after breakfast, that dosa is fuelling your brain and muscles through a school morning.

After absorption, the blood carries simple sugars, amino acids and glycerol to the liver, which stores and processes them before sending them on to the rest of the body. Fatty acids and some vitamins take a different route, entering the lymph system first. This is why a healthy digestive system matters: every cell in your body depends on it for fuel and raw materials.