Several companies are commercializing tests for telomere length such as this one here. I understand the basic mechanism for why telomeres shorten during DNA replication, but how good is the evidence that telomere length is a reliable indicator of healthspan/lifespan?
I disagree with Ctina. So many factors go into determining loss rate, activity of telomerase, gene conversion or unequal exchange at chromosome ends, etc to ever say that length is a function of age. There is far more variation between individuals and between cells within an individual than aging ever shows. Here is one study:
Das B, Saini D, Seshadri M. Telomere length in human adults and high level natural background radiation. PLoS One. 2009 Dec 23;4(12):e8440.
however I've seen similar studies published elsewhere. Telomere length as diagnostic for age is very poor at best. Here's an article that gives a formula, but acknowledges the R2 is 0.04, which is useless: http://preview.ncbi.nlm.nih.gov/pubmed/18378481
It would be reliable if we don't take into account the environment, and if we are comparing two genetically identical persons.
First, because there are many other factors that can cause genetic mutation and consequently shorten lifespan. Second, giving an extreme example and not taking into account the environment and modern medicine, we cannot expect a hemophilic to have the same lifespan as a non-hemophilic.
More, not all the individuals have the exactly same rate of cell division thus, two persons with the same telomere length can have slightly distinct ages, and consequently die at different ages.
I would say, it is not 100% reliable since there are many other factors to take into account, but it could give a rough idea of lifespan
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A number of studies have compared replicative capacity of cells with lifespan. Since Replicative capacity of cells is linked to telomere-length, these studies may provide indirect evidence for the association between telomere length and lifespan. One interesting study in particular compared animal life spans and in vitro replicative capacity of skin fibroblasts in groupings of small, middle, large, and very large breeds of dogs of specific ages (Li et al, 1996). It was found that the life spans were inversely correlated to the frame sizes of the breeds. It was shown that all the small breeds studied have a longer life span than that of the large breeds. The replicative capacity of fibroblasts from the large dogs (Great Dane and Irish Wolfhound) was significantly decreased compared with that of the small dogs. The reasoning behind these observations may again be due to varying degrees of cell turnover between the species. Large dogs consist of more cells than small dogs and as a result more cell turnover was initially required in their development compared to small dogs. This increase in cell turnover would subsequently lead to a decrease in replicative potential (due to telomere shortening) and an increase in the rate of senescent cell formation.
Sperms and eggs have full-length telomeres but they may carry some problematic genes or mutations. Longer telomeres cause more health but it doesn't mean all the possible genetic related problems are also gone.