"To fight aging, we first need an objective way of measuring it. Pinpointing a set of biomarkers that keeps time throughout the body has been a four-year challenge," Steve Horvath, a professor of human genetics at the David Geffen School of Medicine at UCLA and a professor of biostatistics at the UCLA Fielding School of Public Health, said in a press release. "My goal in inventing this age-predictive tool is to help scientists improve their understanding of what speeds up and slows down the human aging process."
The team looked at the process of so-called "methylation," which is a natural chemical process that occurs when a methyl compound is added to one of the four building blocks of our DNA and causes chemical changes. They used 121 sets of previously collected data, which included almost 8,000 samples from 51 different types of tissue and cells. Researchers discovered there were 353 biological markers linked to methylation found in the body that change as a person gets older.
They used the data to create a chart that mapped DNA methylation levels from pre-birth through 101 years of age.
Researchers then tested the tissue for its "biological age" and compared it to its chronological, or actual, age. For the most part, the two numbers matched. Even people who had progeria, a genetic condition that causes premature aging, had cells that matched their chronological age.
"It's surprising that one could develop a predictive tool that reliably keeps time across the human anatomy," he said. "My approach really compared apples and oranges, or in this case, very different parts of the body -- including brain, heart, lungs, liver, kidney and cartilage."
The team also discovered that cells don't age at the same pace throughout our lives.
"The clock's ticking rate isn't constant," he explained. "It ticks much faster when we're born and growing from children into teenagers, then slows to a constant rate when we reach 20."
There were some cases where the biological age did not match the chronological one. One of the starkest differences were found in a woman's breast tissue, which may provide more clues into breast cancer.
For the most part, healthy breast tissue was two to three years older than the woman's chronological age. But, if the woman had breast cancer, the tumor tissue was an average of 36 years older than her healthy tissue. The tissue surrounding the tumor was about 12 years older than the rest of her unaffected tissues.
The researchers believe this could explain why so many cancers are linked to aging.
"These data could prove valuable in furthering our knowledge of the biological changes that are linked to the ageing process," said Veryan Codd, a researcher at Leicester University who was not involved in the study, told the Guardian. "It will be important to determine whether the accelerated ageing, as described here, is associated with other age-related diseases and if it is a causal factor in, or a consequence of, disease development.
However, Dr. Darryl Shibata, professor of pathology at the University of Southern California's Keck School of Medicine, pointed out to Forbes that while this study shows that biological aging and chronological age go hand in hand, it doesn't explain why our bodies break down as we get older.
"The general idea that you can read a genome and it reflects the aging process is probably correct," Shibata, who was not involved in the research, said. "But the weakness is that this study doesn't provide a mechanism, and without a mechanism it's just a correlation."
Horvath called for more research to look at why the process of methylation and the molecules it affects make it possible to determine a person's biological age. He also noted that pluripotent stem cells -- which are adult stem cells that are turned into to embryonic stem cells, which in turn can be programmed into any other cells -- have the biological age of a newborn. This could mean there might be a way to stop aging, but more studies need to be completed.
"In principle, the discovery proves that scientists can rewind the body's biological clock and restore it to zero," he said.
The study was published in Genome Biology on Oct. 21.