RUNNING flat out, climbing stairs, digging the garden, sawing wood, sex: all make us breathless because we are burning up energy faster than the heart and lungs can supply the muscles with oxygen. Working with too little oxygen, our muscles produce chemical wastes that cause cramps and need oxygen to neutralise them. So we go on breathing fast for some time after the exertion has ceased, to repay the 'oxygen debt'.

People who are physically fit have strong hearts and lungs and can pump large amounts of oxygen-rich blood to the muscles; and their muscles have efficient enzymes that can extract most of the oxygen from the blood. A trained athlete can go on running or playing football or tennis for a long time without building up an oxygen debt and getting out of breath.

The wimp's breathlessness on slight exertion is due, then, essentially to his or her being physically unfit. And people in their twenties who never take exercise may become embarrassingly puffed if they have to run for a bus. This unhealthy breathlessness will be made much worse by being overweight; someone who is two stones too heavy has to do as much work going upstairs as someone of average weight carrying 28lb of potatoes up the same staircase.

A recent onset of breathlessness in middle or old age is generally the result of something other than a simple lack of fitness. There are three main possibilities.

The blood may be too thin and unable to carry a full load of oxygen. Middle-aged women may become anaemic because they lose a lot of blood in heavy periods at the menopause. Old people of both sexes lose blood through internal bleeding from stomach ulcers (often brought on by taking pain- relieving drugs for arthritis) or from inflammation or a cancer of the bowel. Other causes of anaemia include a poor diet.

The lungs may be failing to deliver enough oxygen into the blood. This may be due to asthma or some reaction to dust (such as farmer's lung). The lungs may have become overstretched (emphysema), usually from years of smoking-induced bronchitis. Lung infections still occur, despite antibiotics, and breathlessness may be due to pneumonia or to acute bronchitis.

The heart may be failing in its pumping action. Earlier this century the most common cause of heart failure was damage to the heart valves from the long-term effects of rheumatic fever in childhood or from syphilis. Valve damage is less common today and is usually treatable by replacement of the defective valve with a man-made one. The main cause of heart disease in middle life is narrowing of the coronary arteries which supply it with blood; and the causes of coronary artery disease are the familiar public health bogies - smoking, a high-fat diet, lack of exercise and a raised blood pressure.

With so many possible explanations, breathlessness can be a diagnostic puzzle and may take time to sort out. Self-diagnosis is not a good idea; certainly it is unwise to guess that the cause might be anaemia and buy iron tablets. Once the cause is found, many heart and lung diseases can be cured; but even those that cannot can often be relieved by drug treatment. And no one is too old or too frail to benefit from exercise.