Health and disease

Carbohydrates

提示Bread, pasta, rice and potatoes are mostly this, and it is the body's main energy source.

为什么A balanced diet supplies carbohydrate, protein, fat, vitamins, minerals, fibre and water in suitable proportions. Carbohydrates are the body's main source of energy; starch is digested into glucose before it can be absorbed.

Protein

提示Meat, fish, eggs and beans are the richest sources.

为什么It is digested into amino acids, which the body then reassembles into its own molecules for muscle, skin, enzymes and antibodies.

fibre

提示Wholemeal bread, vegetables and fruit skins all supply it.

为什么It is mostly plant cell wall. It adds bulk that the muscles of the gut can grip, which prevents constipation and helps keep the gut healthy.

Yeast

提示A single-celled fungus that gives off a gas as it feeds on sugar.

为什么It respires anaerobically on sugar, producing carbon dioxide that inflates the dough and ethanol that bakes off. The same process makes beer and wine.

Vitamin C

提示Sailors on long voyages avoided the illness by eating citrus fruit.

为什么It is needed to make collagen, the material that holds tissues together, so without it gums bleed and wounds fail to heal. Fresh fruit and vegetables supply it.

Nicotine

提示It is the drug that keeps a smoker reaching for the next cigarette.

为什么Tar causes the damage to the airways and the cancers, and carbon monoxide takes the place of oxygen in red blood cells. Only one of the three is what makes stopping so hard.

The liver

提示Years of heavy drinking scar it, a condition called cirrhosis.

为什么It works slowly, at roughly one unit an hour, and nothing speeds it up. Alcohol also depresses the nervous system, which is why reaction times lengthen.

The extra is stored as fat

提示The surplus does not vanish — your body puts it away for later.

为什么Energy in has to balance energy out. A steady surplus raises the risk of type 2 diabetes and heart disease, while a long shortfall causes weight loss and tiredness.

A pathogen

提示Not every microbe is one — only the harmful kind earns this name.

为什么Pathogens include bacteria, viruses, fungi and protists. They spread through air, water, food, direct contact and vectors such as mosquitoes.

The skin

提示An unbroken outer layer no germ can cross — until you cut it.

为什么It blocks entry and seals itself by clotting when cut. Mucus and cilia trap and sweep out pathogens in the airways, and stomach acid kills many of those that are swallowed.

A harmless form — it triggers antibodies and memory cells.

提示Enough of the invader for your body to learn it, but not enough to hurt you.

为什么If the real pathogen arrives later, the memory cells recognise it at once, and the response is fast enough to stop the illness developing.

A cold is caused by a virus; antibiotics kill bacteria

提示These medicines target just one group of microbes, and that group is not what gives you a runny nose.

为什么Antibiotics attack structures that only one group of microbes has, such as the cell wall. Taking them for a viral illness does nothing and helps resistant strains spread.