If you are a fortysomething woman carrying too much weight, still reliant on oral contraception and fond of a wild Friday night in a wine bar, beware. Or if you are a man whose mid-life bulge is the packet of cigarettes in your jacket. And even if you work out regularly, are years away from your pension and pride yourself on never bothering the doctor - beware. Every five minutes someone in Britain suffers a stroke.

Strokes occur when blood flow to the brain is interrupted, usually due to a clot, narrowed or abnormal blood vessels or a haemorrhage. The consequences can include slurred speech, impaired mobility or paralysis, diminished sight and concentration. It is also the third commonest cause of death in Britain. Yet most of us assume it is an old person's disease, affecting mainly men.

"In fact, it's a significant risk in younger people," says Peter Rothwell, professor of clinical neurology and head of the stroke prevention unit at Oxford University. "There's a long-standing misapprehension that younger men are more prone to heart attacks [than strokes], and that strokes affect the elderly, but actually the risks of a stroke affect a broad age range. A quarter of victims are under 65, and men and women are equally prone." The Stroke Association says 1,000 people under the age of 30 have a stroke in Britain each year.

In the face of such statistics, what can we can do to try to prevent a stroke? Simple lifestyle adjustments can greatly decrease the risk of clots and haemorrhages - but because younger, outwardly fit people tend to consider themselves protected they often ignore the basic safeguards.

There are some factors that cannot be helped. People of Asian, African and Afro-Caribbean extraction are, for instance, statistically more liable to strokes; family genes can contribute, as can age. And, more rarely, they can be caused by an accident. "Whiplash or an over-enthusiastic chiropractor can damage the arteries in the neck," says Rothwell. "There have even been cases of victims who have had their hair washed too roughly in a hairdresser's basin."

An accident was apparently responsible in the case of actor Samantha Morton, who has just revealed in an Observer interview that she had a stroke in early 2006 that left her "close to death" after part of the ceiling fell on her at home. Afterwards, she says, she had to re-learn to walk.

But the most lethal stroke trigger is high blood pressure, a condition that causes the arteries to fur up. This is often self-inflicted and is deemed to be responsible for half of all strokes, yet many people remain unaware that they have it. Smoking, immoderate drinking, obesity and lack of exercise can all contribute and a fundamental step for everyone, says Rothwell, is to get to the doctor at least once a year for a blood pressure check. If it is high then medication and a reformed lifestyle can help reduce it; if it is healthy then keep it so by forsaking cigarettes which double the risk of a stroke by clogging blood vessels. It doesn't matter how old you are or how long you have been smoking - giving up will halve the danger.

Next cut out the weekend binge sessions. A couple of units of alcohol a day may be a relatively harmless pleasure, although a day or two a week of abstinence is a healthy idea; more than six units in six hours causes blood pressure to soar.

A diet of high fat and high salt also furs up arteries and inflates blood pressure and cholesterol, so cut down on red meat, cut out salt-laden processed meals and substitute high fibre foods such as wholegrain cereals and wholewheat bread and pasta which help control blood fat levels. Although cholesterol is not thought to be a strong contributor to strokes, lowering cholesterol levels significantly reduces the risk of one, according to Charles Warlow, professor of medical neurology at the University of Edinburgh. "It's a scientific oddity," he says, which suggests that statins, a class of drug often prescribed to help control cholesterol levels, would help guard against a stroke.

Cutting fat consumption also fights obesity - another risk factor in strokes. The next step is exercise. Thirty minutes of exercise five times a week is enough to lower blood pressure. It can be done in three 10-minute sessions a day, if needs be, so long as it is vigorous enough to warm the body and make you slightly breathless.

Most of the safeguards against stroke risk are common sense, but there are some misconceptions. A daily dose of aspirin is frequently recommended to reduce blood clotting, which is the most common cause of strokes. This, however, could be counter-productive. "Aspirin does reduce the risk of a recurring ischemic or clot-induced stroke if you have already suffered one," says Warlow. "However, it can cause bleeding in the gut or brain and so increases the risk of a haemorrhagic or haemorrhage-induced stroke which cancels out any benefit"

Hormones can thicken blood and cause clotting which means hormone replacement therapy should be considered only if other symptoms require it. Oral contraceptives should also be used with care. "They increase the risk of a stroke by two or three times, but because it's mainly younger women who use them the existing risk is already very low," says Warlow. "It would be more of a problem if an overweight woman of over 40 was on the pill."

It is also important to avoid cocaine, ecstasy and amphetamines which can inflame the arteries in the brain and increase blood pressure. Even a single snort could cause a stroke in a susceptible novice.

Strokes will always stalk the elderly, the diabetic and those with a genetic predisposition, but many of the 150,000 who experience one each year in Britain could have spared themselves by making a few changes in their daily lives. "Research has shown it is an exquisitely preventable condition," says Rothwell. "The tragedy is people are not very aware of the risks".

· For more help and advice contact the Stroke Association or the stroke helpline 0845 3033 100.