Here’s a piece of history most people have forgotten. The first test that read a person’s age from DNA methylation didn’t use blood. It used spit.

A team at UCLA collected saliva from 34 pairs of identical twin men, ages 21 to 55. They found spots on the DNA whose methylation tracked age, then tested the model on 60 more people. It guessed their ages within about five years, on average. One of the scientists on that team was Steve Horvath, who went on to build the clock I wrote about earlier.

So the idea that spit can tell your age is not new. The real questions are how well it works and why it matters.

Same DNA, different cells

Every cell in your body carries the same DNA. That’s why a cheek swab can identify you as well as a blood sample can.

Methylation is different. It’s the layer of switches that makes a cell what it is, so it varies by cell type. The DNA in a blood sample comes almost entirely from white blood cells. Saliva is a mix: immune cells, plus cells shed from the lining of your mouth.

Those two kinds of cells differ at about one in five of the methylation sites researchers measured. And the mix changes from person to person.

That matters because most of the famous clocks were built on blood. Read a blood-built clock on a saliva sample and you’re using a ruler marked for something else.

What the comparisons show

One recent study measured the same people across five kinds of samples, from cheek cells and saliva to several kinds of blood. The clocks did not agree across tissues.

On most clocks, saliva and cheek samples read older than blood from the same person. On one mortality clock, cheek samples read as much as 40 years older than blood. Pace-of-aging scores ran more than 50 percent faster in saliva than in blood.

One clock held up better. It was built from skin and blood samples to work across tissues, and it gave more consistent readings between saliva and blood.

The researchers’ conclusion is the one I’d underline. Clocks work best in the tissue they were built for. If you want to measure aging from saliva, build and calibrate the clock on saliva.

That’s not a reason to give up on spit. It’s a design requirement.

Why spit matters

If blood works, why bother? Because of who shows up.

A blood draw needs a trained person, a needle, an appointment and a lab pickup. A saliva sample needs a tube and a mailbox. You can do it at your kitchen table in a few minutes, on your own schedule.

And plenty of people don’t like needles. A review of 119 studies found that 20 to 30 percent of young adults fear them. About 16 percent of adults said they skipped a flu shot because of that fear.

Every test loses people along the way. The more a test asks, the fewer people finish it. In medicine, that means missed screenings. In life insurance, it means people who start an application and never finish.

I watched that happen for years. It’s a big part of why I went looking for a better way.

A test people will take beats a perfect test they won’t.

What I’m not wrong about

I’ve been betting on saliva since my team was testing it in 2017. The science since then has made the case clearer. The signal is in the DNA, and it tracks the odds of dying sooner. The work is in calibrating it to the sample and proving it in people the model has never seen.

I believe most people will learn their biological age from a tube of saliva, not a vial of blood. I could be early. I bet I am not wrong.

This is not medical advice.

Sources

Jon Sabes is a named inventor on U.S. Patent No. 11,817,214 and managing member of LongevityFP Technologies, LLC, which holds an exclusive license to FOXO Technologies’ epigenetics technology.