Health and disease

Carbohydrates

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

The whyA 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

HintMeat, fish, eggs and beans are the richest sources.

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

fibre

HintWholemeal bread, vegetables and fruit skins all supply it.

The whyIt 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

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

The whyIt 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

HintSailors on long voyages avoided the illness by eating citrus fruit.

The whyIt 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

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

The whyTar 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

HintYears of heavy drinking scar it, a condition called cirrhosis.

The whyIt 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

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

The whyEnergy 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

HintNot every microbe is one — only the harmful kind earns this name.

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

The skin

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

The whyIt 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.

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

The whyIf 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

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

The whyAntibiotics 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.