Biological Frailty Index in centenarians

This study measured the subclinical frailty of centenarians by looking at the accumulation of their biological abnormalities. For this aim, a biological Frailty Index (FI) was computed in centenarians living in Northern Italy. The median value of the biological FI was 0.33 (interquartile range, IQR 0.28–0.41). The biological FI did not significantly differ between women (0.34, IQR 0.31–0.39) and men (0.32, IQR 0.26–0.43). The biological FI seems to have a narrower distribution compared to clinical FI we previously computed in the same cohort. In conclusion, our study suggests that centenarians benefit from exceptional biological reserves that might be underestimated by clinical appearances.


Introduction
The amount of people reaching old age has been growing exponentially in the last decades, and centenarians represent the fastest-growing group (World Population Prospects 2019: Highlights; https:// popul ation. un. org/ wpp/). Centenarians are persons with an extraordinary adaptive capacity, probably thanks to unusual functional reserves. Centenarians may live with debilitating disease, but still present an advantage in terms of incident disability and death [1,2].
They constitute a very heterogeneous population as result of lifestyle habits, environmental factors, and histories that have differently affected their biological and clinical profile over the life course [3,4]. Thus, centenarians may be subjects with not only good but also very poor health status, as demonstrated by the different degrees of frailty we previously reported [5].
Aging occurs at molecular and cellular levels [6]. Interestingly, centenarians seem to express molecular signatures suggestive of a slower process compared to other persons [7,8].
Recently, it has been explained that Frailty Index (FI) exclusively based on biological parameters may define the biological age of the individual, potentially capturing variations in the health status before the manifestation of clinical deficits [6,9].
The aim of this study was to measure the subclinical frailty of centenarians by looking at the accumulation of their biological abnormalities. Since available measures of biological age are not optimized to disentangle the heterogeneity that characterizes centenarians [10], in this study, we have computed a biological FI by the means of blood tests in a cohort of well-characterized centenarians living in Northern Italy.

Study design
The participants belonged to a large cohort enrolled during a study conducted between 2007 and 2014 and funded by the Italian Ministry of University and Scientific Research. The cohort was composed by 125 centenarians. Forty-six registry offices in Northern Italy were contacted to collect dates of birth of living people close to 100 years at the enrolment. Sixty-five out of 125 centenarians with all available variables needed for the computation of the biological FI were included. All these persons had a clinical FI already described [5]. Briefly, a trained multidisciplinary team went to each centenarian's house or nursing home to administer a standard structured questionnaire and collect blood samples [11].
The biological FI was computed considering a total of 42 variables including routine blood tests [6], telomere length [12] and Apolipoprotein E genotype [9]. The 20th and 80th percentiles of each variable were considered as cut-points. The values under the 20th percentile and over the 80th percentile were considered abnormal. These biomarkers and their cut-points are presented in Table 1.
Each biomarker was categorized to assume the value of 0 if its value fell within the range of normality or 1 if abnormal. The biological FI was then calculated as the ratio between the number of biomarkers presenting abnormal values and the number of considered biomarkers (n = 42).

Results
Overall, a total of 65 centenarians (46 women and 19 men) were included in this study. The mean age of the sample was 101.3 (standard deviation, SD 2.0) years. The age was similar between women and men (101.2, SD 2.1 and 101.6, SD 2.0, respectively). As expected, the prevalence of women was higher than men (71% and 29%, respectively).

Discussion
To our knowledge, this is the first study measuring a biological FI in a cohort of well-characterized centenarians. Interestingly, it seems to have a more narrow distribution compared to the clinical FI we previously computed [5].
In fact, in the same cohort, we reported a higher clinical FI (median 0.50, IQR 0.40-0.58), and a wider spectrum of values (ranging between 0.13 and 0.73) [5].
In a cohort of persons aged 80 years and older, it has been reported that the clinically fittest persons (FI values between 0 and 0.02) had a mean biological FI of 0.33, indicating that this latter is able to detect the subclinical accumulation of deficits and anticipate the clinical phenotype [9]. Similarly, community-dwelling men aged 40-79 showed a higher biological FI (based on routine blood tests) compared to the clinical one and a significant association with mortality and adverse health outcomes [6]. Nevertheless, our findings suggest that centenarians benefit from exceptional biological reserves that might be underestimated by clinical appearances. Indeed, in our cohort of centenarians, we got the counterintuitive finding of a biological FI lower than the clinical FI we previously reported.
This result may suggest that, at very advanced age, the biology of the system might be "better" than what clinically manifested. The hypothesis might be explained by the lower relevance that clinical constructs (e.g., definition of the diagnoses) may have with increasing age, especially if compared to the biological substratum feeding them [13]. After all, it is possible that several clinical deficits could be overestimated in centenarians. For example, some tools (e.g., Mini-Mental State Examination) are not validated for extremely old persons [14] and do not often consider peculiar characteristics (e.g., fatigue) potentially affecting their results.
We found a weak association between age and biological FI in centenarians probably because of the narrow range of chronological age and the similar biological FI observed in men and women. This last result is apparently in contrast with the so-called "sex-frailty paradox", describing women as frailer than men but, at the same time, presenting longer life expectancy [15].
It is possible that, at an extremely advanced age (as in centenarians), the paradox may lose value because of the ceiling effect determined by the exceptional age and the favourable biology that allows it.
The main limitation of our study resides in the relatively low number of participants, which might have affected the statistical power of our analyses. We cannot also exclude that our sample does not represent the population of centenarians, and that third factors not considered in the study may differently explain our findings. For all these reasons, this study has to be considered an exploratory analysis that needs to be confirmed in a larger population.
In conclusion, our study suggests that centenarians benefit from exceptional biological reserves that might be underestimated by clinical appearances. Further studies are needed to disentangle the relationship between chronological age, biological age, and clinical complexity in older persons, especially at a very advanced age.

Conflict of interest
The authors declare that they have no conflict of interest.

Ethical approval
The protocol received approval from the Ethical Committee of the Fondazione IRCCS Ca' Granda Ospedale Maggiore Poli- Human and animal participants All procedures involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards.

Informed consent
The informed consent was obtained for all individual participants included in the study.
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