Food supplies the raw materials and the energy a body needs, but only once it has been broken down small enough to be absorbed.
A balanced diet provides carbohydrates, proteins, lipids, vitamins, minerals, fibre and water in appropriate amounts, each needed for a different reason:
Different people need different amounts of energy. It depends on age, sex, body size and how active a person is. A manual worker or an athlete needs far more than someone with a desk job.
Food labels give the energy content in kilojoules. Adding up the energy in a day’s food and comparing it with what the body uses shows whether the diet is balanced for that person.
Taking in more energy than the body uses leads to the excess being stored as fat, and over time to obesity, which raises the risk of type 2 diabetes and heart disease. Taking in far too little leads to starvation: the body breaks down its own fat and then muscle.
Missing a particular nutrient causes a deficiency disease. Too little vitamin C causes scurvy; too little vitamin D or calcium causes rickets; too little iron causes anaemia.
Food is broken down as it passes along the gut:
Enzymes are biological catalysts: they speed up the breakdown of large insoluble molecules into small soluble ones without being used up. Amylase breaks down starch into sugars, protease breaks down protein into amino acids, and lipase breaks down lipids into fatty acids and glycerol.
The small intestine is adapted for absorption: it is long, and its lining is folded into millions of tiny villi, which give a very large surface area and have a rich blood supply.
The gut contains large numbers of bacteria, and most are helpful rather than harmful.
They break down some substances the body cannot digest by itself, produce certain vitamins, and take up space and resources that harmful bacteria would otherwise use.
Plants do not eat. They make their own carbohydrates in their leaves by photosynthesis.
They still need mineral nutrients from the soil, absorbed through the roots along with water: nitrates for making proteins, magnesium for making chlorophyll. A plant short of nitrate grows poorly; one short of magnesium has yellow leaves.
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