The oldest man in the world achieved much more than a long lifespan...
When Masazo Nonaka died in 2019, he was 113 years old.
That wouldn't impress me if he'd spent his final years stuck in bed or in a cognitive fog. Instead, even at the end of his life, he was reading the newspaper every day after breakfast and watching sumo wrestling on TV. Finally, he died peacefully in his sleep at his family's inn in rural Ashoro, Japan.
As his great-grandson said in the year before his death, "He has not been receiving nursing care at a facility and has a clear brain. He's really amazing."
My longtime subscribers will recognize what I call a long health span. Nonaka didn't just live for a long time... He had a high quality of life for a long time.
Nonaka's family says the secret to his vitality was sugary treats and "onsen"... the hot springs that are part of their inn.
Their take on onsen's life-giving power could be biased. But science backs it up.
While saunas often get all the modern wellness hype, you don't need an expensive gym membership or a pricey home installation to get these benefits. The truth is, the same antiaging magic happens right in your own bathtub.
For Nonaka and millions of others in Japan and Finland, heat doesn't just feel good... It's also a tool that can repair DNA and even rebuild their cells' internal "power plants."
So this week, we're going to explore the health benefits of accessible, at-home heat therapy... I'll explain how it unlocks certain "longevity" genes in your DNA and share my recommendations on how to harness its powers.
Strength From Stress
You've heard the saying "what doesn't kill you makes you stronger." The biological equivalent of that adage is called hormesis.
A small, controlled dose of a stressor can trigger a repair response... one that leaves your cells more resilient than before.
That's what happens when you slip into a bathtub heated to, say, 105 degrees Fahrenheit. Your body's internal temperature begins to rise. Your body sees this as a threat. And it starts shifting into high gear to protect itself.
Namely, heat activates the gene with the blueprint for creating the Forkhead box O3 protein, or FOXO3 for short.
The "forkhead" part describes a special area in the protein that binds to DNA to turn certain genes on and off... specifically, genes that help us live longer by improving stress resistance and cellular maintenance.
FOXO3 is present all over the body. In addition to blood cells, it's in tissues of the muscle, brain, heart, liver, spleen, ovaries, and testes.
Some folks are born with a variant of the FOXO3 gene that stays active naturally. For the rest of us, heat stress gets our FOXO3 activity going.
Here's where your hot bath comes in...
A bit of uncomfortable heat turns the alarms on in your cells, triggering a chemical chain reaction that gets FOXO3 over to the DNA to work its magic.
First, there's the heat stress. When your body temperature rises, your cells feel it and let FOXO3 slip into the nucleus where the DNA is.
There, FOXO3 binds to the DNA and starts flipping the "on" switches for various repair programs such as...
Autophagy: The older we get, the more "junk" – like misfolded proteins, broken cell organelles, and other waste – tends to build up in our cells. And this buildup is a big driver of chronic inflammation – which savvy readers know as the driver behind many of the diseases of old age. In autophagy, FOXO3 activates the genes that clear out this debris.
Cell-cycle pause: You don't want a defective cell dividing... and passing on that damage to new cells. That could lead to cancer and other problems. FOXO3 triggers production of a special protein that stops this process – giving time to get some DNA fixes in. For example, a study published in the journal Cell Death Discovery found that FOXO3-induced cell-cycle pauses could stop cells from dying of too much iron accumulation. FOXO3 also stops insulin from telling cells to grow and divide unchecked.
Antioxidant defense: Unstable "free radical" or "reactive oxygen species" molecules are normal metabolic byproducts of normal metabolism. But if their numbers become too high, they can cause tissue damage. FOXO3 tells our body to ramp up production of antioxidants like catalase and superoxide dismutase, which neutralize those free radicals.
Stem-cell recovery: Stem cells' special power is to divide numerous times and transform into whatever cells your body needs more of. As you age, your stem cells start losing this power. FOXO3 triggers a process that tells your stem cells to "take a break," helping prolong their life.
That's just a tantalizing taste of what FOXO3 can do. But it's not the only player in the game when it comes to your body's response to heat stress.
When it comes down to repairing your damaged cells, there's another team that does the heavy lifting. And more important, you can launch this repair crew into action using nothing but your bathtub.
On Thursday, we'll cover exactly how to safely set up your "longevity soak" regimen. So stay tuned.
What We're Reading...
- Something different: China's humanoid robots take the stage.
Here's to our health, wealth, and a great retirement,
Dr. David Eifrig and the Health & Wealth Bulletin Research Team
March 24, 2026