Blood cholesterol is a risk factor for coronary artery disease and heart attack, so reducing your risk of high cholesterol is a worthy goal.

A smarter way of looking at cholesterol risk is by component.

However, the next time you brag that your cholesterol is nice and low -- or lament that your number is in the mid-200s -- know this:

"Your total cholesterol is a pretty meaningless number," says Maureen Mays, M.D., a preventive cardiologist and lipid specialist at Oregon Health & Science University in Portland.

"Not only does the general public not know this, some doctors don't either."

Here's why "the number" is so misleading.

Total cholesterol is calculated by adding LDL (bad cholesterol), HDL (good cholesterol), and one-fifth of your triglyceride total.

"We have been using this formula of adding a bad thing to a good thing and factoring in one-fifth of a bad thing, and it's not useful," Dr. Mays says.

That's one reason 50 percent of people who have a heart attack have normal cholesterol readings. Health.com: Slash your heart disease risk by 83 percent

Effects of diet and exercise

A smarter way of looking at cholesterol risk is by component.

LDL, or bad cholesterol, is very responsive to good nutrition and exercise. The target number is less than 100 mg/dL. It's not uncommon for LDL to swing up by 40 percent in response to a sedentary lifestyle and a diet high in saturated and other unhealthy fats, according to Dr. Mays.

It can also drop by up to 40 percent in response to a heart-healthy diet and regular exercise. Health.com: The new cholesterol math

When should you have a cholesterol test?

One in 500 people has an inherited risk of extremely high LDL and should be put on statins to control their risk of heart disease. Health.com: Statins cut cholesterol and heart disease risk

Being overweight can also raise your triglycerides, for which the goal is 150 mg/dL or under.

High triglycerides put you at risk for type 2 diabetes, which is a coronary heart disease risk equivalent; this means that if you have diabetes, you have the same risk of dying from cardiovascular problems as someone who already has coronary heart disease.

While increasing age and stress will slightly change your cholesterol panel, "stopping smoking is the best way to raise your good cholesterol," says Dr. Mays.

It is critical for women nearing menopause to maintain a healthy diet and exercise plan to counteract the effects of estrogen loss. Health.com: 10 simple food choices that can help your heart

Because estrogen suppresses LDL levels, women who reach menopause may notice a surge of bad cholesterol, says Denise Janosik, M.D., a cardiologist and professor of internal medicine at Saint Louis University School of Medicine.

Effect of drugs and alcohol

Certain medications, including the steroid prednisone and HIV drugs, can affect your cholesterol panel negatively, so much so that people who are on protease inhibitors for HIV need to be concerned about developing heart disease, according to Dr. Mays.

One to two drinks a day is fine for keeping your cholesterol in check. More than that may raise triglycerides because of the high sugar and calorie content of alcoholic drinks. Alcohol also raises HDL slightly, but this increase in good cholesterol isn't as great as that caused by a healthy lifestyle.

Hypothyroidism, too, can result in skewed cholesterol numbers.

"If you are fatigued and have sudden weight gain, it is good to have a thyroid screening," says Dr. Mays. "If your thyroid isn't working properly, your lipid panel will make no sense."

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Copyright Health Magazine 2009