Previous research suggested that young blood could rejuvenate old mice, but a new study suggests that old blood is far worse for health than young blood is good
In 2005 we discovered that joining the circulation of old and young mice appeared to have a rejuvenative effect on the older mice. While the older linked mice did indeed appear to reap the benefits, displaying improved neurogenesis for example, the researchers noted that joining a circulatory system doesn't necessarily imply the blood alone is behind the rejuvenation. Joining circulatory systems links up many different organs like younger lungs, heart and liver, which may be behind the difference rather than the blood itself.
"What we showed in 2005 was evidence that aging is reversible and is not set in stone. Under no circumstances were we saying that infusions of young blood into elderly is medicine"
Exploring the issue further
A team from UC Berkeley set out to explore the phenomenon in more detail by precisely controlling the flow and exchange of blood to limit some other confounding factors. They developed a system in which old and young mice are connected and disconnected to measure and control exactly the influence on their health. The researchers repeated 2005's experiments in this manner, until each mouse had been infused with about 50% of blood from the other. Again, effects were seen within 24 hours.
Old blood is carrying something detrimental to health
Old mice did see some benefits from the linkage, but they were mild and in some cases non-existent. The young mice however went into noticeable decline very quickly; muscle strength and repair evaporating and displaying a two-fold decline in brain cell development. This suggests that young blood may carry some beneficial factors, but that old blood contains an assortment of factors that contribute more strongly to organismal wide decline. Old mice showed little improvement in neural stem cell activity after the controlled infusion, but young mice saw a very stark drop.
"Under no circumstances did young blood improve brain neurogenesis in our experiments. Old blood appears to have inhibitors of brain cell health and growth, which we need to identify and remove if we want to improve memory"
A mixed picture
The new study doesn't necessarily discount the validity of young blood entirely, but it does hint that it's more important to remove whatever is in old blood that's inhibiting healthy function. It may be that aging triggers changes in mammalian biology that limit healthy function, perhaps by tuning down tissue replenishment to protect against cancer. At this point we don't know why this is the case, but it fits within the theory that there are certain harmful systemic signals that kick in as we get older. If we can work out what the inhibitory signals are, and which initiate more youthful function, then we may be able to rejuvenate the old (to a point anyway).
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