In the 19th century, Paul Bert, an innovative French physiologist, introduced a novel and somewhat morbid method to lab experimentation: joining mice together, akin to a gardener grafting branches onto a tree. The underlying concept was to explore the attributes that could be transferred between the mice through their shared blood. In the 1950s, such “parabiosis” experiments hinted that youth might be one of these transferable qualities, as older rats displayed signs of rejuvenation after being joined to younger ones for several weeks.
These experiments largely went unnoticed until a renewed interest in aging science brought them back into focus in the 2000s. Since then, the phenomenon has garnered attention from scientists and individuals, some of whom could generously be termed as opportunists. Transfusions of young human blood plasma have allowed affluent clients to gamble on the blood of financially strapped students.
There might be elements in young blood, such as taurine or humanin, that benefit the elderly. However, it appears plausible that the observed effect in older rats is more attributable to the elimination of waste than to an infusion of youthful essence. In 2020, Irina Conboy and her team at the University of California, Berkeley, discovered that substituting half of an old mouse’s blood with albumin, a blood protein, and saline solution had a rejuvenating effect comparable to young-mouse blood. It suggests that old blood might require filtering and dilution rather than supplementation.
Interestingly, blood isn’t the sole substance that, when transferred between old and young animals, impacts their aging. Gut microbes play a role as well. These microbial inhabitants of a body are integral to their host’s health, extracting nutrients from food and producing molecules the host cannot.
Some 1950s parabiosis experiments aimed to determine if the microbiome of the older mouse would be rejuvenated by young blood, which it wasn’t. However, transferring fecal matter (and its contained microbes) from a young mouse to an older one seems to improve both lifespan and healthspan in mice engineered for premature aging.
A study involving 9,000 individuals aged between 18 and 101 by researchers at the Institute for Systems Biology in Seattle unveiled three insights about the aging microbiomes in their guts. Firstly, microbiomes become increasingly diverse from person to person in middle age. Secondly, this diversification continues in healthy individuals as they age but halts in the unhealthy. Thirdly, for those aged 85 or above, a lack of such diversity is a precursor to earlier death.
There are indications of which microbes are significant. Lactobacillus plantarum both prolongs lifespan and mitigates cognitive decline in prematurely aging mice and is also enhanced by calorie-restricted diets. On the other hand, retaining a high quantity of Bacteroides bacteria into old age is a known mortality predictor. However, the primary takeaway from the institute’s research is that a microbiome conducive to old age is one that is well adapted to its unique environment. A deeper understanding of how the microbiome and its host adapt to each other may be essential before establishing a pathway to prolonged old age through the gut.