Some people's hearts stay strong well into their 60s, but their kidneys begin to fail. Others may have the kidneys of a 30-year-old but fall victim to constant infection.

Now, scientists may be one step closer to understanding why the aging process varies so drastically between people.

Even within a single person, aging unfolds at different rates in different tissues, sometimes striking the liver before the heart or kidney, for example. People fall into distinct categories depending on which of their biological systems ages fastest, and someday, doctors could use this information to recommend specific lifestyle changes and design personalized medical treatments, according to a new study, published Jan. 13 in the journal Nature Medicine .

What's your "ageotype"?

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.

"Now, it's going to be a lot more than just four categories," said senior author Michael Snyder, a professor and the chair of genetics at the Stanford University School of Medicine in California. For instance, one participant in the study appeared to be a cardiovascular ager, meaning their cardiac muscle accumulates wear-and-tear at a greater rate than other parts of their body. "If we [surveyed] 1,000 people, I'm sure we'll find other cardio agers and that category will become better defined." And with more research, even more patterns of aging may emerge, Snyder added.

Related: 8 Tips for Healthy Aging

In the past, scientists have hunted for markers of aging in enormous datasets for large populations, Snyder, told Live Science. Researchers pinpointed markers of aging by comparing data from young people to that of older people, but for individuals, that kind of data captures only a specific moment in time. It cannot reveal how a given person might change as they age, Snyder said.

In a clinical setting, that means population-based markers might not be the best measure to determine how a patient is aging, or what combination of medical treatments might suit them best, he added.

"Population-based decisions are crude at best," Synder said. They won't necessarily hold up for you, per se."

By tracking specific people through time, Snyder and his co-authors hoped to learn how aging markers differ between individuals. Their study participants ranged in age from 29 to 75 and provided at least five biological samples over the course of two years. Even within that relatively short time frame, several patterns of aging emerged.

For example, immunological agers accumulated more markers of inflammation through time, while metabolic agers accrued more sugar in their blood , indicating that their bodies were metabolizing glucose less efficiently. Similar to scores on a personality test, each individual's aging "profile" included a combination of traits, mixed and matched from different ageotypes.

Personalized medicine

Snyder and his co-authors plan to follow the study participants to see how their aging profiles morph over time. They also aim to develop a simple ageotype test that could be used in the doctor's office to quickly assess a patient's health status, and potentially point them toward the best possible treatment options.

"There are drugs and various kinds of dietary interventions and lifestyle interventions through which it may be possible to modulate some of these aging processes," Dr. James Kirkland, a gerontologist and head of the Kogod Center on Aging at the Mayo Clinic in Rochester, Minnesota, told NBC News .

"But in order to apply those correctly, we have to know which people to apply which drugs or which dietary interventions in order to get the most bang for the buck," said Kirkland, who was not involved in the new study.

Related: 7 Ways the Mind and Body Change With Age

While existing drugs, diets and exercise regimes can target some signs of aging, other markers aren't fully understood yet.

For example, over the course of Snyder's study, a marker of poor kidney function decreased in 12 individuals, eight of whom took statins. The marker, a waste product called creatinine , accumulates in the blood as muscle tissue naturally breaks down, but the kidneys typically filter the substance and expel it through the urine. Creatinine levels fell in the eight individuals on statins, suggesting that the medication improved their kidney function, though it's unclear why levels also dipped in four additional people, the authors noted.

The team also found that concentrations of several microbes seem to change with age, but we don't yet know how that may affect health. Certain microbes may proliferate in response to age-related changes in the body, while others help drive them, Snyder said. The authors also spotted differences in how diabetic and pre-diabetic people aged as compared to insulin-sensitive people, but it's unclear whether these markers indicate meaningful differences in health status. Many studies suggest that insulin plays a central role in aging throughout the animal kingdom, but more research is needed to clarify its exact influence over human aging.

For now, ageotypes present as many questions as they do answers about human aging. Until scientists understand what various aging markers really mean, clinicians will continue to rely on standard vital sign assessments to track patients' health over time. In the near future, perhaps ageotypes could serve to motivate people to take better care of areas of their body that appear to be aging faster than others, Snyder said. For instance, if someone fits the profile of a cardiovascular ager, they might focus on improving their cardiovascular health and undergoing relevant medical tests to check on their progress.

"As we collect a lot more information, we are going to be better able to follow how people are aging, [as well as] what interventions they did that actually reduced their aging," Snyder said.

Originally published on Live Science .