Age is a one-way street, but only if you think in terms of years. Organisms have another underlying age that still baffles researchers. If you’ve ever met an octogenarian who was in much better shape than you, or a twenty-something whom you could’ve sworn was at least 40, you’ve seen how age in years is just a number.

Scientists use the term biological age to describe how well our bodies, and our cells, are performing. Most often, the lower our biological age is, the healthier we are. And, unlike chronological age, which we can’t change, there are many things we can do on a daily basis that can lower our biological age.

Jesse Poganik, a research fellow in medicine at Harvard University, tells Inverse what we can do to keep our biological age as low as possible. “We hypothesize that biological age is actually much more dynamic than chronological age.”

What is biological age?

Biological age reflects how old our cells are, while chronological age is simply someone’s age in years. The better and more efficiently cells function, the lower their biological age. Or, if those cells are on the decline, then their biological age would be higher.

Scientists can measure biological age based on biomarkers made as someone’s DNA changes over time. One biomarker comes from DNA methylation, an epigenetic modification in which methyl groups are added to a cell’s DNA. Methyl groups are stable molecules composed of a carbon atom bonded to three hydrogen atoms. While these groups are usually unreactive, their presence can significantly influence cellular function. As methylation occurs, cell functions may worsen, indicating an increase in biological age. A 2013 study depicts the relationship between aging and methylation by creating a predictor that can estimate the DNA methylation age of many tissue and cell types. Steven Horvath, the study’s author, developed an epigenetic clock employed in other studies called the Horvath DNAmAge clock.

These clocks synthesize information about DNA methylation at particular sites in the genome, which researchers can track. “Using various algorithms and different combinations of these sites, you can estimate biological age quite accurately,” Poganik says. He explains that data from these biomarkers are “mathematically transformed” to correspond to chronological years, but this measurement is “artificial in some sense,” he says. “We don’t really understand what one year of biological age means. it may or may not have the same meaning as a difference of five years in chronological age.”

How does biological age change?

A key difference between biological and chronological age is that only the latter is fixed. Not only can biological age differ from chronological age, but it can roll forward or backward. Physical stressors, like giving birth or undergoing major surgery, can add years to biological age because the stress radiates down to the cellular level.

“Molecular damage underlies the nature of aging,” says Vadim Gladyshev, a professor of medicine at Harvard Medical School. This damage, however, isn’t necessarily permanent. “Once the organism recovers from that condition, the damage then decreases, and biological age also decreases.”

For example, Gladyshev and Poganik co-authored a recent study published in April in the journal Cell Metabolism demonstrated that changes to the body can increase or decrease biological age. They surgically connected one young mouse and one old mouse in a procedure called heterochronic parabiosis. As a result, their joint circulatory systems pumped each other’s blood through their bodies. The younger of this duo had a marked increase in biological age as he was exposed to the older mouse’s blood. The intriguing part came after the mice were separated again and recovered, and the younger’s biological age reset to its pre-surgery baseline.

Research like this confirms that physiological damage can age a body and also that recovery can reset that age. But the jury’s still out on exactly how that damage works when it’s psychological. Unlike physiological harm, psychological harm leaves no clear biomarkers. Researchers like Gladyshev can check biomarkers like DNA methylation against changes like healing wounds, but mental stress doesn’t have a clear analog, making it difficult to quantify.

How can I age backward?

One 2019 study found that a pharmaceutical trio of recombinant human growth hormone and two diabetes medications (dehydroepiandrosterone and metformin) punched biological clocks right in the face: After one year of treatment, the nine participants’ biological age decreased by two years from their baseline, and remained for six months after stopping treatment.

But in terms of daily practices, biological age can be best managed with regular exercise and a healthy diet, as a 2021 study found. In the clinical trial, 18 healthy men between 50 and 72 received an eight-week treatment modifying diet, sleep, exercise, relaxation, and intake of probiotics and plant nutrients. Another 20 healthy men in the group went about their days as usual.

The experimental group’s average biological age at the end of the intervention was 1.96 years younger than at the start, whereas the control group’s was 1.27 years older (though the change within the latter group was not statistically significant). Moreover, there were significant changes in two blood biomarkers researchers used. Serum 5-methyltetrahydrofolate, a dietary folate that supports cell metabolism, increased while triglycerides, fat in the blood, decreased. Lower triglycerides, however, are also associated with a healthier diet and lower risk of stroke and heart disease, which is always worth pursuing regardless of biological age.

The answer may be unsatisfying, but this study shows that solid habits like sufficient sleep, a nutrient-rich diet, and exercise all keep biological age low — as well as the proper time to recover after illness or surgery.

Poganik says that these first gleams of answers encourage him in understanding the many things we still don’t know. “Establishing the fact that you have this flexibility and fluctuations in biological age is a strong impetus to understand what are the factors that drive these, and we don’t have a very good understanding yet.”

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