Throughout life, the cells in our bodies continuously divide, making new cells. One theory of aging holds that the aging process at least partly results when cells stop dividing as fast as they used to. If this happens to cells of the immune system, there will be fewer immune system cells, and the body's defenses can become weak and more susceptible to disease. In bones, this slowdown in cell division can lead to decreased bone mass and ultimately osteoporosis. When cells under the skin start to disappear, wrinkles appear.

In animal studies, at least, it's been found that in some parts of the body (skin, liver, kidney, and pancreas), the cells in younger animals divide more frequently than the cells in older animals.

A possible biomarker of aging, therefore, would be a measure of whether the rate of cellular replication was slowing down. Several factors have an effect on the cell survival and cell replication. For example, the shortening of telomeres (small bits on the ends of chromosomes that get shorter each time a cell divides) shorten the survival of cells. Damage from free radicals (called oxidative damage) can cause cells to die before they would have died without this damage.

Scientists have been looking into the possibility of measuring the rate of cell replication as a biomarker of aging. To do this, they have conducted studies with rodents. In the past, it's been shown that mice put on a low calorie diet for an extended period of time live longer than mice that are given as much food as they want. For reasons that are not known, this strategy-called calorie restriction may serve to keep cells replicating at a rate that is comparable to when the animal was younger.

Scientists are using this information to test the theory about cell replication rates. This is done by comparing cell replication rates in rodents on a calorie restricted diet with rodents given as much food as they want. This is then correlated with extension of life span.

More studies are needed to test this. In addition, more definitive evidence is needed to show that a reduced rate of cell replication actually shortens life span. It may be that the body's organs are able to handle the loss of cells without completely breaking down.

In addition, reduced cell proliferation doesn't have any bearing on the health of the heart or the brain because the cells in these organs do not replicate, or at least very much. So far, this research has not led to a credible biomarker of aging.