It can be taboo to ask a person his or her age, so people have long thought up roundabout ways to figure it out: you can ask someone what year she graduated, Google his name, ask for her work history ... and now you can get some spit.
Scientists have discovered that the way the DNA in your spit changes over time reveals your age to within five years.
They did this with 0.1 ounces of saliva, about half a teaspoon.
The researchers are currently looking into whether crime scene investigators could someday use this finding as a forensic tool to narrow a suspect's age range using trace amounts of saliva left on a coffee cup or from a tooth bite.
Dr. Eric Vilain and a team of geneticists at the University of California, Los Angeles, discovered spit's relation to age by looking at a process called methylation.
As we get older, our DNA gets methylated, meaning that certain genes are told to turn on or off based on our lifestyle and environmental factors, such as how we eat or the toxins to which we're exposed.
When this happens, Vilain told MSNBC,
the sequence of the genes themselves is not modified, but their expression is. What we found is that the degree of methylation at a small number of places in the human genome is linked to our age. The correlation is high enough that we can predict what the age of a person is by just having access to a sample of their saliva.
An accidental discovery
Vilain and his colleagues initially explored methylation -- in 34 pairs of identical male twins, aged 21 to 55 -- to study a person's sexual orientation. Methylation and sexual orientation turned out to have no correlation, but age and methylation did -- at 88 points on DNA.
Intrigued, the team then did the test in non-twins -- 31 men and 29 women ages 18 to 70 -- and found a similar correlation.
They used two of the three genes with the strongest link between age and methylation to create a model that could predict a person's age. When they tested it with the data they had already collected from the twins and the second group, they found they could predict a person's age to within five years.
"Methylation's relationship with age is so strong that we can identify how old someone is by examining just two of the three billion building blocks that make up our genome."
first author Sven Bocklandt, a former UCLA geneticist now at Bioline, said in a press release.
The researchers, who published their results in PLoS ONE on June 22nd, are now looking at the minority of the population in which methylation does not correlate with their actual age. This discrepancy could lead to scientists one day calculating a person's "bio-age" -- or their biological age -- as opposed to their chronological age.
Doctors could use bio-age instead of chronological age to screen patients for age-related diseases. For instance, instead of requiring all 50-year-olds to undergo a colonoscopy, they would instead recommend it to patients whose bio-age was 50.
In the press release, Vilain said:
Doctors could predict your medical risk for a particular disease and customize treatment based on your DNA's true biological age, as opposed to how old you are. By eliminating costly and unnecessary tests, we could target those patients who really need them.
The UCLA team is now exploring whether people whose bio-age is younger than their chronological age live longer and have healthier lives and whether people whose bio-age is older than their actual age experience a higher rate of disease and early death.
Author’s note: The original version of this post stated that 0.1 ounces of saliva was equivalent to the amount of saliva left on a coffee cup or tooth bite. That is incorrect; it is actually half a teaspoon. It has been corrected. I regret the error.