Biomarkers today are used by health practitioners and researchers in a variety of settings, including clinical settings, laboratories, and community-based surveys. Epidemiological studies using biomarkers have expanded our knowledge base about aging in the community, population differences in health, and the clinical significance of many biomarkers.
Some of the most commonly used biomarkers include blood pressure as a marker of cardiovascular health and cholesterol levels and glycated hemoglobin as markers of metabolic health. However, biomarker technology is rapidly evolving, and biomarkers are being used in population surveys to assess nutritional status, genetic background, and function in many organs and systems (e.g., immune system, lung function, kidney function, liver function, reproductive function).
Fundamentally, biomarkers provide a measurement tool for observing physiological processes, including disease processes, physiological changes related to aging, and the aging process. Although biomarker research is advancing rapidly, in many ways it is still in its early stages.
Many biomarkers of disease are commonly used in clinical practice, and population-based research on biomarkers of age-related processes is leading to the discovery of additional clinically relevant bio-markers. However, consensus has yet to emerge about reliable biomarkers of the aging process that could be used to test anti-aging interventions. The great advantage of biomarkers is that they represent a window to physical processes that are difficult to observe and quantify directly, providing tools for aging researchers and health practitioners in a variety of disciplines to better understand changes in health with age.
An important area of biomarker research in aging has been the search for biomarkers of the aging process, that is, markers of “biological age” that predict age-related outcomes better than chronological age. This line of research reflects an underlying premise that the rate of aging varies in systemic ways across individuals and populations.
Normal aging appears to be quite variable, as evidenced by differences in the age at onset of disease and disability. A biomarkers of aging and biomarkers of disease would allow researchers to identify individuals aging at different rates and to directly test potential anti-aging therapies. Although some interventions, such as caloric restriction and the use of antioxidants, alter mortality patterns and extend the human life span of laboratory animal models, it is unclear whether these interventions actually delay or slow aging itself.
A trait would need to meet at least three criteria to be considered a biomarker of aging. First, it should predict physiological function across multiple domains in an age-coherent way and do so better than chronological age. Second, it should predict remaining longevity from an early age. Third, its measurement should not alter other age-sensitive traits such as life expectancy and disease state. Although biomarkers of aging and biomarkers of disease that satisfies all of these criteria has yet been identified, several promising biomarkers may prove to be useful as biomarkers of aging.
One such marker is CD4 memory T-cells. This type of T-cell is predictive of remaining life span in mice and increases with age, but it decreases in mice exposed to caloric restriction. Another potential biomarker of aging is telomere length. Each time a vertebrate cell divides, the chromosomes get shorter as their ends, or telomeres, are removed and not replaced. Although telomere length is an indicator of cell division history rather than a direct indicator of aging, it might be informative as an indicator of aging in cells where replicative potential is crucial to function.
The difficulties inherent in identifying biomarkers of aging include lack of a common definition of aging or an agreement about the role of biomarkers, extensive biological variation in aging between individuals that makes generalizations difficult, and overlapping aging and disease processes and accompanying difficulty in identifying benign versus pathogenic age-related change. Demonstrating that a given characteristic changes with age is not sufficient to call it a biomarker of aging. A biomarker must divide individuals of a given chronological age into groups hat vary in a range of other age-sensitive traits such as mortality risk and functional capacity. Despite these difficulties, the potential of biomarkers to provide insights into the fundamental causes of aging drives the search for biomarkers of the aging process.
