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Dracula may have been on to something. It wasn’t just blood, but the blood of youth that was the secret to staying alive for centuries.

The rejuvenating effect of young blood has been demonstrated in strange, draculoid experiments first done 150 years ago. Two genetically compatible animals, one young and one old, are sewn together. With their circulatory systems connected, the old animal gains access not only to the younger animal’s blood but also to the detoxifying and metabolizing function of its organs. The state is known as heterochronic parabiosis.

In a paper published in 2014, researchers at Harvard University’s Department of Stem Cell and Regenerative Biology described the results of connecting two-month-old mice to 15-month-old mice. (The average life span of the animals is 27 months.)

After attachment, the number of neural stem cells and the number of new olfactory cells in the smell center of the old animals’ brains increased. Those animals were also able to detect odors at lower concentration — evidence that the changes had a practical effect.

Interestingly, the younger animals were largely unchanged by the procedure unless they were attached to very old (21-month-old) animals. In that case, neurogenesis — the growth of new neurons — in the young animals’ brains declined, suggesting there was a toxic effect from the old blood.

Neurogenesis, however, wasn’t the only thing happening in the old animals’ brains. Heterochronic parabiosis also caused new blood vessels to sprout, restoring a more youthful blood flow to the brain. (The researchers gave the mice MRI scans to show that.)

The Harvard researchers showed that many of the effects they saw were attributable to a growth factor called GDF11, which is in higher concentration in young blood than in old blood. Solitary old mice given daily shots of GDF11 experienced growth of brain cells and improvement in circulation (although not to the degree of the surgically attached animals).

That wasn’t all that GDF11 did. It also enhanced the ability of damaged muscle to repair itself, improved exercise capacity in old mice and reversed the stiffening of the heart that comes with age. The research team wrote, with notable understatement, that its findings “should encourage further investigation of its therapeutic potential for a variety of age-related diseases.”

So why aren’t people lining up to get infusions of young blood?

Actually, they are. Sort of. Since September, a company named Ambrosia has been enrolling people age 35 and older to receive two liters of plasma from young people. To date, 61 people have signed up and paid $8,000 for the service. Most are in their 60s; the oldest is 92. The average age of the plasma donors is 19.

The project is a scientific study, although it has no control group or placebo arm. About 100 blood biomarkers are measured before the infusions (which take place over two days) and then one month later. There’s no effort to determine whether the plasma recipients show improvement in abilities that tend to decline with age, such as cognition, balance and grip strength.

Jesse Karmazin, the 32-year-old physician who started the company after graduating from Stanford University’s medical school in 2012, said early results show a decline in some biomarkers of inflammation — a healthful trend. No results have been published.

The project is entirely funded by the patients.

“I think a double-blind, placebo-controlled trial would be great, but there’s nobody who wants to put up the millions of dollars to do one,” said Karmazin, who lives in Washington (although the infusions take place in Monterey, Calif.).

Other researchers say they think young blood won’t ever slow aging. That’s because in parabiosis experiments, much of the benefit to the old animal comes from its access to the young animal’s organs — a situation that’s never going to happen with human beings.

Irina and Michael Conboy, a wife-and-husband research team at the University of California at Berkeley, performed experiments in which half the blood of an old mouse was replaced with blood from a young mouse, and vice versa. The animals, however, weren’t attached to each other and didn’t share circulatory systems.

They found that young blood did, indeed, help old muscle repair itself. However, it didn’t improve endurance in old animals, and old blood markedly reduced young animals’ endurance. Furthermore, old blood suppressed the growth of brain cells in young animals more than young blood enhanced it in old animals.

Altogether, this suggests that the real problem lies with the toxicity of old blood, not the rejuvenating properties of young blood.

“This tells us that old, damaged tissues produce something that is bad for them, and secrete it into the bloodstream, every day, all the time,” Irina Conboy said. “Young blood is not a medicine, and it will never be a medicine until the inhibitory substances of old blood are neutralized.”

The Conboys say they believe they’ve identified two physiological pathways — one that becomes more active with age, the other less active — that are responsible for producing the toxicity of old blood. They are now doing experiments in which they turn the first pathway’s pace down and the second one’s up. Early results suggest the strategy is helping restore old animals to a healthier and more youthful state, they said.

Will there by human applications?

“Certainly,” Michael Conboy said.