For centuries, people have searched for a fountain of youth—a mysterious substance capable of turning back the biological clock. A little more than a decade ago, scientists thought they might have found something remarkably close.
It was blood.
Experiments from some of the world's leading aging researchers suggested that exposing old mice to the blood of young mice could rejuvenate aging tissues. Older animals showed improvements in their brains, muscles and other organs.
The findings sparked an extraordinary possibility: Could something circulating in young blood actually slow—or even reverse—some aspects of aging?
Scientists have spent the years since trying to answer that question. And now the story has taken a surprising turn.
The secret may not simply be what's in young blood. It could also be what's accumulating in old blood.
Key Takeaways
Young blood isn't the whole story. Animal experiments found that young circulation can improve signs of aging, but scientists are still determining which circulating factors are responsible.
Old blood may contain part of the answer. Diluting older animals' plasma produced rejuvenating effects in several tissues, suggesting that reducing age-associated circulating factors may be important.
The science isn't ready for anti-aging treatments. Human evidence remains limited, and the FDA says young-donor plasma has not been approved for treating aging or memory loss.
When Old Mice Got a Taste of Youth
Much of the excitement began with experiments involving a technique called heterochronic parabiosis.
Researchers surgically joined the circulatory systems of a young mouse and an old mouse, allowing the animals to share blood.
The results were startling. In several studies, exposure to young circulation improved signs of aging in older animals. Researchers reported changes involving muscle regeneration, brain function and other tissues.
In 2014, Stanford researchers provided another piece of the puzzle.
They found that exposing older mice to young blood improved molecular and structural measures in the hippocampus—the brain region crucial for learning and memory. The older animals also performed better on certain memory tests.
Suddenly scientists wanted to know what was hiding inside young blood. Was there a molecule that told old cells to start behaving like young ones again?
Scientists Hunt for a “Youth Factor”
One early candidate generated enormous excitement.
Its name was GDF11.
Harvard researchers reported that levels of this protein appeared to decline with age and that restoring it in old mice produced rejuvenating effects in aging tissues. For a while, GDF11 was hailed as a potential “youth factor.” But science rarely moves in a straight line.
Later studies produced conflicting results about GDF11, including disagreements over whether its levels actually decline with age and exactly what increasing it does in older tissues.
The search therefore expanded. Instead of one magical anti-aging molecule, researchers began considering a much more complicated possibility:
Our blood may contain an entire mixture of signals capable of influencing how quickly—or slowly—our tissues age.
And that raised an obvious question. Would young blood work in people?
Young Plasma Gets its First Alzheimer's Test
Researchers at Stanford eventually tested young plasma in 18 people with mild-to-moderate Alzheimer's disease.
Participants received plasma from donors between the ages of 18 and 30. The study was primarily designed to determine whether the treatment was safe and practical, not whether it could cure Alzheimer's.
The results were intriguing—but far from the breakthrough many people had hoped for.
Researchers found that the plasma infusions were generally tolerated in this small study, and caregivers reported improvements on some measures of daily functioning.
But there were no significant improvements in global cognition, mood or brain functional connectivity.
Because the study was so small and short, the researchers said they could not determine whether young plasma was an effective treatment for Alzheimer's disease.
So young blood hadn't turned out to be the miracle cure some headlines had suggested. Then researchers made a discovery that changed the entire question.
What if we've been looking at the wrong blood?
Scientists at the University of California, Berkeley wondered whether the rejuvenating effects observed in earlier experiments actually required young blood at all.
They conducted an ingenious experiment. Instead of giving old mice young plasma, researchers removed approximately half of the animals' plasma and replaced it with saline containing albumin.
In other words, they weren't adding youthful substances. They were diluting substances already present in old blood.
And remarkably, the treatment produced rejuvenating effects in several tissues. Researchers reported improvements in muscle repair, liver health and neurogenesis—the creation of new neurons—in the hippocampus.
That suggested a dramatically different explanation for the original young-blood experiments. Perhaps aging blood gradually becomes crowded with proteins and other signals that interfere with the body's ability to repair itself.
Diluting those substances might allow tissues' existing regenerative machinery to work better again. In other words: Maybe young blood isn't powerful simply because it contains something magical. Maybe it's powerful partly because it doesn't contain as much of what's accumulating in old blood.
Could Our Blood Become More “Pro-Aging” as We Get Older?
Blood is far more than a delivery system for oxygen.
It carries hormones, proteins, inflammatory molecules, metabolites and countless chemical messages throughout your body.
Those signals help your organs communicate with one another. As we grow older, that circulating environment changes. Some potentially harmful proteins and inflammatory signals become more abundant, while other protective or regenerative signals may diminish.
Researchers increasingly suspect that this changing environment may help explain why aging occurs throughout the body at the same time.
Your muscles age.
Your brain ages.
Your immune system ages.
Your blood vessels age.
Rather than each organ aging completely independently, some scientists believe the substances circulating between them may help coordinate the process. That possibility has opened an entirely new field of anti-aging research.
Don't Rush Out for a Young-Blood Transfusion
Unfortunately, scientific excitement surrounding young blood also created a market for clinics offering young-donor plasma infusions costing thousands of dollars.
The U.S. Food and Drug Administration first warned consumers about these treatments in 2019. Then, in December 2024, the FDA issued another warning.
The agency said it was still aware of businesses marketing plasma from young donors for aging, memory loss and conditions including Alzheimer's and Parkinson's disease.
The FDA emphasized that young plasma has not been approved for these purposes and that it is not aware of evidence demonstrating clinical benefit for them. Plasma transfusions can also cause adverse reactions and other medical complications.
So, despite the fascinating science, the fountain of youth probably isn't flowing through the veins of a 20-year-old donor waiting to be tapped. But the increasingly detailed map scientists are creating of the substances circulating through our blood may eventually reveal something even more valuable: why our cells receive the message to grow old—and whether some of those messages can safely be changed.
But you don't have to wait...
What To Do Instead
Many of the same biological systems scientists are studying—chronic inflammation, metabolic health, mitochondrial function and the body's ability to repair itself—are influenced by everyday habits.
Regular exercise, particularly a combination of aerobic activity and strength training, a nutrient-rich diet emphasizing fruits, vegetables and other plant food and supplements, adequate sleep, maintaining a healthy weight, and controlling blood pressure and blood sugar are all associated with healthier aging.
These are proven ways to protect your brain, heart, muscles and other tissues as you get older, and you can start putting them to work for you today.
Summary
Early experiments suggested that young blood could rejuvenate aging tissues, prompting scientists to search for youthful factors capable of turning back aspects of biological aging. But newer research has complicated the story. Studies suggest that removing or diluting substances that accumulate in older blood may also produce rejuvenating effects, while researchers are investigating extracellular vesicles and other circulating signals that could influence aging. Human evidence remains preliminary, and the FDA warns against commercial young-plasma treatments for anti-aging purposes. Still, the research is revealing something potentially profound: the changing composition of our blood may play an important role in how our bodies age.
Frequently Asked Questions
Can young blood reverse aging?
Animal studies have found that exposure to young blood or young circulating factors can improve certain age-related changes. However, there is currently no convincing evidence that transfusions of young blood reverse aging in humans.
What happens when old mice receive young blood?
Studies have reported improvements in measures involving the brain, muscles and tissue regeneration. These findings led researchers to investigate which substances circulating in blood might influence aging.
What did scientists discover about old blood?
Researchers found that removing and replacing part of the plasma of older mice produced rejuvenating effects without adding young plasma. This suggests that reducing certain factors that accumulate with age may be important.
What are extracellular vesicles?
Extracellular vesicles are tiny packages released by cells that carry proteins, RNA and other molecular signals. Researchers are investigating whether vesicles circulating in young blood contribute to some of its effects in animals.
Are young-plasma transfusions an approved anti-aging treatment?
No. The FDA says young-donor plasma is not approved for treating normal aging or memory loss and has warned consumers about businesses offering young-plasma infusions for unproven purposes.
