Doctors routinely urge their patients to quit smoking and exercise regularly. But what if there were a blood test that could show smokers and couch potatoes the damage their lifestyle was actually wreak­ing on their chromosomes?

Two groups of prominent researchers have started companies to provide just such a test, which would measure the length of one’s telomeres. Telomeres are caps on the ends of chromosomes, protecting them much as plastic tips on the ends of shoelaces keep the laces from fraying. Whenever chromosomes—the store­houses of our genes—are replicated in preparation for cell division, their telomeres shorten. That shrinking has led many scientists to view telomere length as a marker of biological aging, a “molecular” clock ticking off the cell’s life span, as well as an indicator of overall health. Studies comparing the telomere length of white blood cells among groups of volunteers show distinct correlations between telomere length and lifestyle. Those who exercise regularly have longer telomeres than those who do not. Folks who perceive themselves as the most stressed have shorter telomeres than those who see themselves as the least. Certain diseases, too, correlate with shorter telomeres, including cardiovascular, obesity and Alzheimer’s.

“Knowing whether our telomeres are a normal length or not for a given chronological age will give us an indication of our health status and of our physiological ‘age’ even before diseases appear,” says María A. Blasco, who heads the Telomeres and Telomerase Group at the Spanish National Cancer Research Center and who co-founded the company Life Length in September. Telomere research pioneer Calvin B. Harley, who co-founded Telome Health last spring with Nobel laureate Elizabeth H. Blackburn, considers telomere length “probably the best single measure of our integrated genetics, previous lifestyle and environmental exposures.” Beginning as early as this spring, the companies will offer telomere-measurement tests to research centers and companies studying the role of telomeres in aging and disease; the general public may have access by the fall through doctors and laboratories, perhaps even directly.

Although enthusiasm for the research services runs high, some telomere experts question the tests’ current value for individuals. “We haven’t defined what we consider to be a norm and what we consider to be abnormal, either long or short,” says Nilesh J. Samani, head of cardiovascular services at the University of Leicester in England. But telo­mere length is not a diagnosis or a prognosis, Harley says. The data, he insists, are sufficient to help people make “personal lifestyle decisions,” regarding, say, diet, exercise and stress.