We prefer to offer upbeat reasons to exercise, but occasionally we resort to provoking fear. Today's alarm: the metabolic syndrome, a common yet pernicious cluster of health conditions that elevates risk of cardiac disease. People with three or more of these conditions are diagnosed as having the syndrome, according to the American Heart Association:

* Obesity, especially around the abdomen.

* Blood fat disorders -- mainly high triglycerides and low HDL cholesterol -- which foster plaque buildup in arteries.

* Elevated blood pressure (130/85 mmHg or higher).

* Insulin resistance or glucose intolerance (which is to say, high blood sugar).

* Two conditions with names nobody needs to know (prothrombotic or preinflammatory states, identified by a variety of things in your blood, including C-reactive protein and some other stuff only your doctor can tell you about).

Those with the syndrome are at increased risk of coronary heart disease, stroke, peripheral vascular disease and type 2 diabetes. They are also far more likely than others to die earlier than they'd prefer.

An estimated 47 million Americans -- roughly 25 percent of adults -- have metabolic syndrome. It is most common in persons over age 55 but can strike others, including obese, sedentary children.

The syndrome "is basically a heart attack waiting to happen," said Michael J. LaMonte, director of epidemiology at the Cooper Institute in Dallas. LaMonte led a study of 9,007 men and 1,491 women aged 20 to 80 that showed that people with higher levels of fitness were far less likely than unfit individuals to develop metabolic syndrome. (The participants were all free of the syndrome when they entered the study, though some may have had one or two of the qualifying conditions at the start.) The study appeared in the July 26 issue of the journal Circulation.

LaMonte said the syndrome is especially troubling because it lacks distinctive symptoms, so medical testing for the underlying conditions is the only way to learn if you are afflicted.

In the study, LaMonte and his colleagues categorized participants as having low, medium or high fitness, based on a treadmill walking test. The researchers then reexamined the participants periodically over an average of 5.7 years.

The correlation between fitness and syndrome was obvious: Men with high fitness were 56 percent less likely to develop metabolic syndrome than low-fitness men; men with middle fitness were 26 percent less likely. Numbers were comparable for women.

"That's a strong statement for lifestyle change" among non-exercisers, LaMonte said. To achieve middle fitness, people should work up to walking briskly 30 to 45 minutes five or more days per week. (That burns about 200 calories per session.) Or one could exercise more vigorously (jogging, for example) for 20 to 30 minutes three to five days a week.

To achieve high fitness, LaMonte prescribes upping the duration of each exercise session by 50 to 100 percent. Greater intensity can raise the bar, too.

So, the march of science continues to provide proof that exercise will boost the odds of staying healthy and, if the gods are with you, deferring death. You knew that. But when it's hard to tie on the shoes, grab the gym bag or turn off that new plasma TV, it's good to remember.

No matter your level of fitness -- low, medium or high, zero or hero -- jump online with us today, from 11 to noon, to rap about all fitness topics at www.washingtonpost.com.

-- John Briley