A growing body of research suggests that age is more than just a number.
Regardless of how many birthdays you've celebrated, your overall health may depend on the resilience and vitality of your cells.
It's a concept known as biological, or epigenetic, aging.
"Chronological age is what the calendar tells us," Elissa Epel, PhD, a professor in the department of psychiatry & behavioral sciences at the University of California, San Francisco Weill Institute for Neurosciences, told ABC News.
Biological age looks beyond the calendar to provide clues for how well someone's health stands the test of time.
Researchers commonly analyze a process called DNA methylation, which involves chemical alterations of gene expression, without any changes to the actual genetic sequence itself. As this pattern shifts, it is reflected in the age of the cells.
Genetic factors intertwine with environmental factors, such as pollutants and toxins, and lifestyle habits, like diet and exercise, to alter DNA methylation and influence cell function.
The idea is that everything can have an impact at the cellular level, which could potentially have implications for predicting disease risk and longevity.
To test for biological age, scientists use advanced tools known as epigenetic clocks.
Developed a little over a decade ago by UCLA researcher Steve Horvath, PhD, epigenetic clocks use blood, skin, or saliva samples to analyze specific patterns in the DNA, called methylation marks, and then compare them against chronological age, as well as to a database containing information from other individuals.
Evaluating various body systems, Horvath found that even healthy tissue next to a breast cancer tumor, for example, was about 12 years older than the rest of the body.
"We can't change our genes, but we can change how much they are activated or silenced," said Epel, also the director of the Aging, Metabolism, and Emotions Center in San Francisco, adding that, "the patterns of methylation, like doors being open or closed, change dramatically with age."
The top anti-aging, nutritionist-recommended foods to add to your diet nowUntil recently, epigenetic clocks have primarily been used in the lab, but a few are now available to consumers. Some have price tags upwards of $500.
Consumer tests can offer insights into lifestyle changes that might help slow biological aging, though experts are still evaluating their accuracy and utility.
Epel and her team recently found that women who followed a diet with healthy nutrients such as folate and magnesium, showed signs of younger biological age, while women who consumed diets higher in added sugar were found to have older biological ages.
These findings were independent, in that more sugar meant more accelerated biological age, even if a woman's overall diet was healthier, but sticking to a healthier overall diet was associated with slower cellular aging, even in those who consumed more added sugar.
Another team of researchers similarly looked at diet, with an 8-week twin study, where one twin followed a vegan diet while the other continued eating meat.
The "vegan" twins had lower age markers in different body systems compared to their meat-eating identical twin, and even more compelling, they had a decrease in their biological ages among multiple epigenetic clocks.
"Our health is not necessarily set in stone, it's changing all the time, and so within the realm of our own individual spaces, we have some power, in terms of, the health behaviors that we choose to engage in, and those can have an impact on our epigenetic health," Dorothy Chiu, PhD, a postdoctoral research fellow at the Osher Center for Integrative Health at UCSF, told ABC News.
Understanding biological aging can be empowering. But it's just as important to remember that it is not the end all, be all, Epel cautions.
"We don't suggest individuals go out and get tested and think they've determined how long they are going to live," she said. "It's one source of information, and it's not definitive in any way, especially since it changes."
Influencers over 50 are changing the conversation on agingThe main takeaway is that day-to-day habits can potentially influence health right down to the cellular level. Any steps, even small steps, towards staying healthy may have benefits.
While we can test biological age for some insights into how well someone is aging, for now, the best way to stay healthy is to keep up to date with screenings and follow the evidence-based recommendations from your healthcare provider.
Jade March, MD, a board-certified family medicine physician and current integrative medicine fellow at UCLA, is a member of the ABC News Medical Unit.