How Old Are You Inside? Blood Test May Tell

June 18, 2009 -- You know how many birthday candles are on your cake, but a new blood test may be able to tell whether you're aging faster, or slower, than you think.

That blood test is described in a new study, published in Aging Cell. It measures a protein called p16 that rises with age, and rises particularly fast if you smoke and don't exercise.

Researcher Norman Sharpless, MD, an associate professor of medicine and genetics at the University of North Carolina School of Medicine in Chapel Hill, N.C., says other scientists identified the p16 gene years ago as a gene that helps prevent cancer. Later research showed that the p16 gene becomes more active as mammals age.

Sharpless and colleagues have created a blood test that measures p16 levels in certain immune system cells called T-cells.

Sharpless says it's the first p16 test for cells that are easy to check. Other cells that contain p16 are in organs that are harder to get to, like the pancreas, he says. "You can't really sample those things," Sharpless tells WebMD. "The advance here is that this is a cell that's easily attainable."

In their new study, 170 healthy North Carolina adults took the p16 blood test and answered questions about their lifestyle. Sharpless and colleagues looked for patterns to gauge participants' "molecular age," based on their p16 blood levels.

Molecular Age Results

The results showed that p16 levels were most strongly linked to chronological age -- the older a person was, the higher their p16 level was -- long before that person would be considered old.

"Even at very young ages, when p16 is not very highly expressed, you can still see a difference with a decade. So you can tell a difference between 20 and 30 year olds, and 30 and 40 year olds," Sharpless says. "Aging occurs well before you're aged."

Smoking was strongly linked to higher levels of p16.

"If you smoke, your p16 is higher at any age, and if you smoke a lot, your p16 is a lot higher," Sharpless says. "I think it's very likely, based on other data, that carcinogens make p16 go up."

Exercise was linked to lower levels of p16. It's not yet clear if exercise lowers p16 levels, or if people who exercise have low p16 levels for some other reason. Perhaps those people have diet habits that keep their p16 level low, Sharpless says.

"I really wouldn't want to make the mistake that has been made recurrently in epidemiologic association studies that try and place some causal relationship between the two, other than just note that there's a strong relationship that's reproducible," he says.

BMI ( body mass index) wasn't related to p16 after the researchers took age into account. That finding surprised Sharpless.

"I would have thought that people who were thinner would have lower p16 for their age. That wasn't true," Sharpless says. "One could infer from that that body mass is not a great predictor of fitness ... it just turned out to be a bad marker of molecular age for us."