The research team behind the study sorted 43 people into aging categories, or "ageotypes," based on biological samples collected over the course of two years. The samples included blood, inflammatory substances, microbes, genetic material, proteins and by-products of metabolic processes. By tracking how the samples changed over time, the team identified about 600 so-called markers of aging — values that predict the functional capacity of a tissue and essentially estimate its "biological age."
So far, the team has identified four distinct ageotypes: Immune, kidney, liver and metabolic. Some people fit squarely in one category, but others may meet the criteria for all four, depending on how their biological systems hold up with age.
Expanding the study will surely reveal more than four categories. One of the study participants was clearly a cardiovascular ager, whose cardiac muscle was accumulating damage at a greater rate than other parts of their body.
This reminds me of Atul Gawande's great description of how complex systems wear down and crash:
...complex systems—power plants, say—have to survive and function despite having thousands of critical components. Engineers therefore design these machines with multiple layers of redundancy: with backup systems, and backup systems for the backup systems. The backups may not be as efficient as the first-line components, but they allow the machine to keep going even as damage accumulates...within the parameters established by our genes, that’s exactly how human beings appear to work. We have an extra kidney, an extra lung, an extra gonad, extra teeth. The DNA in our cells is frequently damaged under routine conditions, but our cells have a number of DNA repair systems. If a key gene is permanently damaged, there are usually extra copies of the gene nearby. And, if the entire cell dies, other cells can fill in.
Nonetheless, as the defects in a complex system increase, the time comes when just one more defect is enough to impair the whole, resulting in the condition known as frailty. It happens to power plants, cars, and large organizations. And it happens to us: eventually, one too many joints are damaged, one too many arteries calcify. There are no more backups. We wear down until we can’t wear down anymore.