Being a fully paid-up member of the worried well, I wasn't counting my chickens. Arriving at Lloyds pharmacy to have tests done on my blood pressure and cholesterol, aka the "silent killers", my feelings were mixed: four parts curiosity, one part trepidation. I might be a non-smoker under 40, a regular exerciser and unnaturally devoted to broccoli, but all my grandparents died following heart attacks or strokes and, besides, I have a dark chocolate habit that would give Augustus Gloop pause for thought. I furrowed my brow and stepped inside.

It was all so civilised. I was ushered into a little anteroom that was curtained off (with velvet). After taking a brief history, including questions about heart disease in my immediate family, the pharmacist, Nikki, took some blood with a pipette from a pinprick in my finger; this was then deposited into a special slide and inserted into a machine. While it whizzed away making calculations, I had my blood pressure taken. All the results were put into the computer and no more than 10 minutes later I had a print-out of my results. They showed my blood pressure to be 115/77, "as normal as you can get", and my cholesterol, apparently, to be OK (total cholesterol 5.27 millimoles per litre; good cholesterol 1.45mmol/l; and the ratio of total to good, 3.6). It also calculated my CRA (cardiac risk assessment) - my chance of developing heart disease - to be negligible. I was moderately delighted.

But things got more interesting when, back home, I keyed these figures into Flora Love Your Heart's heart age calculator. This little online gizmo, for which you need your BP and cholesterol scores, was developed by Flora's parent company, Unilever, and Boston University's Statistics and Consulting Unit, the department involved in establishing the risk factors for heart disease in the internationally renowned cardiovascular research project known as the Framingham Heart Study.

The user-friendly calculator gives an indication of how old your heart is compared to your real age, as certain activities, such as smoking, can age a heart rapidly. It asks you your systolic blood pressure, total and good cholesterol scores, and a series of questions about you, your lifestyle and family history.

It turns out that I'm young at heart: 31, in fact, three years lower than my actual age. I felt smug about that, until I read on to the bit about my cholesterol, which said it was "higher than normal". That wiped the smile off my face. A link on the page referred me to current guidelines, which are that total cholesterol should be no more than 5mmol/l, bad cholesterol no more than 3mmol/l and good should be above 1mmol/l. My total cholesterol was 5.27: should I be concerned?

Chest, Heart and Stroke Scotland (CHSS) confirms that Scottish GPs have agreed that they will treat patients according to these guideline figures (there are separate guideline figures for patients with particular risk factors, such as having had a heart attack or having diabetes). But they are only that: guidelines. As a GP friend confirms, one number in isolation is not terribly meaningful. All an individual's risk factors, such as age, body mass index and family history, must be taken into account. In my case, when these things are looked at as a piece, my heart and I are lucky in having little to worry about, as the CRA score indicates.

Partly, that is to do with age. As Andy Murdock, Lloyds' pharmacy director, says: "People move into a higher coronary risk category when they reach their forties." Lloyds recently carried out a survey demonstrating this.

Using evidence from more than 3000 pharmacy heart health checks on adults under 60, it found that the average male non-smoker has a chronological age of 45 and a heart age of 49. For smokers, the picture was much worse: the average female smoker has a heart 14 years older than her actual age, while with male smokers, the difference is 15 years. Only female non-smokers had a heart age that matched their chronological age.

"Millions of people are at risk of developing heart disease within the next 10 years," says Murdock. "The good news is that there is a lot that can be done to reduce risk."

Coronary heart disease, when a fatty substance (atheroma) builds up in the lining of the blood vessels, is caused by a combination of factors. One of the biggest is smoking, though public awareness of this is not as great as it is about the link between smoking and lung cancer. Smoking makes the blood vessels less elastic, encourages the build-up of atheroma and makes the blood stickier, increasing the chances of a clot. What most people do know is that obesity and a diet high in saturated fat also put you at greater risk. David Clark, chief executive of CHSS, regards bad diet as the most worrying trend for the future. Being overweight puts strain on the heart, raises blood pressure and leads to abnormal blood fats, while high-fat diets increase the level of cholesterol in the blood.

There are two types of cholesterol - "good" (HDL, or high-density lipoprotein), and "bad" (LDL or low-density lipoprotein). It's LDL that causes the fatty deposits. Replacing fat from animals in meat and dairy products with oils and fish can help reduce bad cholesterol, as can eating more fruit and vegetables, while achieving a healthy weight and taking regular exercise help lower cholesterol and blood pressure.

People with high blood pressure are often unaware they have it. It is a risk factor for heart disease because, over time, high BP makes blood vessels narrower and less flexible, putting the heart under extra strain and making it easier for clots to form. Blood pressure should be checked by a health professional every five years in adulthood and a healthy lifestyle, including a low-salt diet, should help control it.

Another hard-to-quantify factor is stress. "Stress certainly pushes up blood pressure and it makes it more likely that you'll smoke, eat and drink more, and exercise less," says Clark. A moderate alcohol intake is not a risk factor, but binge-drinking is. Diabetes puts you at particular risk, due to damage to blood vessels, while a family history of cardiovascular disease may indicate a genetic tendency to high cholesterol or blood pressure. Finally, in Scotland, coming from a deprived area makes it more likely that you will develop heart disease.

The good news is that not only can heart disease risk be greatly reduced by living a healthy lifestyle, but even if you already have a build-up of atheroma in your arteries, quitting smoking, cutting down on saturated fats and doing regular exercise can help reduce the chance of a heart attack. It's never too late. Not getting any younger? Your heart will, if you make a concerted effort.

If anyone doubts that, they need only look at the figures. In the 18 months after the smoking ban was introduced, heart attacks in Scotland dropped by 17%. "We have had major success in reducing deaths from heart disease and we are beginning to see a reduction in the incidence of heart disease, too," says Clark. "We have lots of people in cardiac heart groups across Scotland and what they say to me time and again is, I've changed, I've stopped smoking and reduced the fat in my diet,' and they're not having that second heart attack which used to carry them off."

So what if you are of healthy weight? That's no reason to be complacent. "It's amazing how many slim, fit people have heart attacks," says Clark. After all, slim people might still have undiagnosed high blood pressure (it often runs in families) or eat high-fat diets. That underlines the importance of "knowing your numbers" (blood pressure and cholesterol) - even if you're just one of the worried well.