Average Telomere Length is a Terrible Measure of Aging
Here I'll point out one of numerous studies providing evidence to illustrate that telomere length isn't all that useful as a biomarker of aging. Telomeres cap chromosomal DNA, a length of repeated sequences that shortens every time a cell divides. This forms a part of the limiting mechanism that stops ordinary somatic cells from dividing indefinitely. Stem cells and cancer cells maintain their ability to divide by periodically extending their telomeres via various mechanisms. At present telomere length is usually measured from a blood sample, taking the average of lengths in immune cells. This will reflect some combination of cell division rates, cell replacement rates, and the immune status of the individual.
Statistically, considered over large populations, average telomere length in immune cells trends downward with aging, indicating that cell populations are dividing more frequently, or not receiving as great an influx of new cells with long telomeres as they were in youth, or both. The latter is a part of the well-known decline of stem cell activity with age. Unfortunately telomere length is nonetheless a terrible measure of biological age on a practical, individual basis, as the correlation just isn't that good, and measures can vary greatly over time for reasons that have little to do with aging, such as ill health due to infectious disease. This all ties in with the idea that the age-related statistical trend towards diminishing telomere length in cell populations is a reflection of the effects of damage on other processes, as well as the influence of numerous other environmental circumstances, and not a cause of aging in and of itself.
Advances in technology allow scientists to measure intricate details about the human body that greatly enhance understanding of health, disease and aging. Yet, when it comes to predicting death, more rudimentary measures - like a person's age or a person's ability to climb stairs or walk a short distance - are much more powerful predictors of survival than certain biomarkers. Using data from the United States, Costa Rica and Taiwan, researchers compared a broad set of predictors of death - like age, smoking habits and mobility - with the length of telomeres, DNA sequences that generally shrink with age.
Decades ago, researchers discovered that telomeres - which are protective caps on the ends of our chromosomes - act as a 'molecular clock' in human cells. Every time cells divide, telomeres shorten until they become critically short and signal the cell to stop dividing. Telomere length is typically measured in white blood cells (leukocytes), and shorter leukocyte telomeres have been associated with disease, aging and death. For these reasons, there has been great interest in the ability of this biomarker to predict mortality.
After evaluating data, the research team found that using telomere length to predict a human's death was only marginally better than a "coin toss." Chronological age was, by far, the single best predictor of death in all three countries. "Scientific evidence on telomere length has been sensationalized and, in some cases, exaggerated by the media and by companies that have capitalized on the research to market products that may promise more than they can deliver. This is what fueled our research. We wanted to determine whether telomere length could predict mortality better than other well-established predictors of survival, most of which are less invasive and much less costly to measure."
The researchers note some potential limitations of the findings. People who are critically ill might exhibit changes in the distribution of different types of leukocytes that makes their telomeres appear longer. In this study, telomere length is measured in leukocytes, which is common across most research. But some types of leukocytes tend to have longer telomeres than others. "Telomere length tends to be longer in the type of leukocyte that becomes more dominant when a person is ill. Therefore, a sick person might appear to have 'longer' telomere length, but that is deceptive. In fact, these critically ill individuals may be much more likely to die in the short-term despite the appearance of 'longer' telomeres."
It also is plausible that telomere length is a better predictor of long-term mortality, compared to short-term survival, since it reflects the gradual process of cellular aging. "Alternatively, telomere length might be a predictor of mortality only for certain groups of patients, such as those with cancer. An interesting possibility is that telomere length might not be a good predictor of mortality, but it could be a good predictor of healthy aging. Increasing evidence demonstrates that shorter telomeres are associated with cardiovascular disease, but additional research is needed to clarify the association between telomere length and other diseases of aging such as cancer."