Age is a one-way street, but only if you think in years. Organisms have another underlying age that still baffles scientists. If you’ve ever met an octogenarian who was in much better shape than you, or a twenty-something who you could have 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 it is the case that 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 researcher in medicine at Harvard University, says Vise versa 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 a person’s age in years. The better and more efficiently cells function, the lower their biological age. Or, if these cells are declining, then their biological age would be higher.

Scientists can measure biological age based on biomarkers made as a person’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 one carbon atom bonded to three hydrogen atoms. Although these groups are not normally reactive, their presence can significantly affect cellular function. As methylation occurs, cell functions may deteriorate, indicating an increase in biological age. A 2013 study delineates the relationship between aging and methylation by creating a predictor that can estimate DNA methylation age for many tissue and cell types. Steven Horvath, the study’s author, developed an epigenetic clock used in other studies called the Horvath DNAmAge clock.

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

How does biological age change?

An important 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, such as 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, professor of medicine at Harvard Medical School. However, this damage is not necessarily permanent. “When the organism recovers from that state, the damage decreases and the 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 in the body can increase or decrease biological age. They surgically joined a young mouse and an old mouse in a procedure called heterochronic parabiosis. As a result, their shared circulatory systems pumped each other’s blood through their bodies. The younger of this duo had a significant increase in biological age when exposed to the older mouse’s blood. The exciting part came after the mice were separated again and recovered, and the biological age of the younger one was 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 is still out on exactly how that damage works when it’s psychological. Unlike physiological damage, psychological damage 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 analogue, making it difficult to quantify.

How can I age backwards?

A 2019 study found that a pharmaceutical trio of recombinant human growth hormone and two diabetes drugs (dehydroepiandrosterone and metformin) beat biological clocks right in the face: After one year of treatment, the nine participants’ biological age dropped two years from their baseline, and remained at six months after the end of treatment.

But in terms of daily practice, biological age is best managed with regular exercise and a healthy diet, a 2021 study found. In the clinical trial, 18 healthy men between the ages of 50 and 72 received an eight-week treatment that modified 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 mean biological age of the experimental group at the end of the intervention was 1.96 years younger than at the start, whereas that of the control group was 1.27 years older (although the change within the latter group was not statistically significant). In addition, 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. However, lower triglycerides are also associated with a healthier diet and lower risk of stroke and heart disease, which are always worth pursuing regardless of biological age.

The answer may be unsatisfactory, but this study shows that solid habits such as adequate sleep, a nutritious diet and exercise all keep biological age low – as well as the proper time to recover from illness or surgery.

Poganik says these first glimpses of answers encourage him to understand 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.”