As for everything else in our biochemistry, the shortening of telomeres with age is more complex than we'd like it to be: researchers "have shown that the shortening of telomeres, the protective structures at the end of chromosomes, as people age varies between individuals and depends on the telomeres' original length. Although prior population studies have indicated that telomeres might be used to predict lifespan, the new research shows that the process is in fact much more complicated than had previously been assumed. ... [researchers] investigated the shortening of telomeres in 959 individuals who had donated blood samples at 9- to 11-year intervals. ... while the shortening rate was strongly correlated with the initial length of the telomeres, it was not related to later tumour development. ... In roughly a third of the subjects, the telomeres actually lengthened over the study period. ... those with the longest telomeres at the first blood draw demonstrated the most pronounced telomere shortening over time, and vice versa. ... The results indicate that the telomere-maintenance machinery protects the shortest telomeres. However, other factors are likely to influence the rate of shortening as well. Telomere length at first blood draw could only explain 57% of the variation in the rate of shortening; 43% remains to be accounted for, and may well include lifestyle factors, oxidative stress or inflammation."