Last month, a paper published in Nature grabbed headlines by claiming that human lifespan is capped at 115 years. As disappointing as the news may be to anyone wanting to live forever, I’m okay with this shelf life, so long as I can be an energetic, pain-free supercentenarian. Still, despite tangible progress in medicine that is helping us live longer than ever, there’s no magic bullet in sight that could stave off aging.
Scientists, from the Netherlands, based their claim that human lifespan is finite on global demographic data that showed modern medicine failed to extend life for those who reach 100 years of age, despite helping more people reach their mid-nineties. Their analysis sparked a debate because it did not include future progress in regenerative medicine that could potentially counteract age-related tissue damage.
Ultimately, our bodies age because the cells slowly succumb to molecular damage that accumulates over time. The cells are continuously assaulted, either by radiation or byproducts of their own metabolism. Thankfully, any damage is quickly fixed by superb cellular repair machinery. But this warranty on cell function expires once we are past the age of reproducing. That’s when we become invisible to natural selection and start to decay.
Ever since scientists discovered in worms a single gene that acts as a switch to extend lifespan, there’s been hope that one day we may be able to slow down aging, or even stall it. But it’s now clear that aging is much more complex than that, underpinned by many genes that influence each other.
At one point, a low calorie diet looked like it could hold the key to longevity because mice that ate less lived longer. But before you start restricting your calorie intake, know that dieting does not have the same effect in primates (i.e. humans). Lab mice are inbred animals and lack genetic diversity – the basis for all wonderful differences among human beings – which often complicates drawing conclusions about health from findings in mice.
Stem cells to the rescue?
Advances in regenerative medicine may make it possible to generate replacement tissue and organs for our aging bodies. If we could safely use stem cells to make transplant tissue and avoid immune rejection, or spur tissue repair from within, then we might get to be 115 and still kicking.
But stem cells also change over time and, amazingly, they don’t necessarily disappear altogether. Scientists can isolate roughly 10,000 stem cells from donated eyes regardless of whether they come from eight- or 80-year olds, according to University of Toronto professor Derek van der Kooy. Instead, just like some people, stem cells become selfish in their old age: they clamp down on making other cell types, while continuing to self-renew, effectively inching towards a cancer-like state without rebuilding new tissue.
The trick then will be to find a source of stem cells that is both young and tolerated by the immune system. This could be cord blood, which is already used to treat a variety of blood and metabolic disorders.
While it is generally accepted that younger stem cells are better at tissue regeneration than older ones, I was surprised to learn that there’s actually little hard evidence to support this.
I spoke with Dr. Ian Rogers, who studies stem cells at the Lunenfeld-Tanenbaum Research Institute in Toronto and is a co-founder of Insception Lifebank, a cord blood bank. Rogers’ research into wound healing suggests that cord blood stem cells drive tissue repair better than adult stem cells from the bone marrow. But he says that the data are hard to interpret and adds, “Is it because of the source of stem cells or because of their age? I think it is because of age, but it’s going to be very difficult for us to get infant bone marrow to do the proper control so I don’t know if I’ll ever be able to really answer that question.”
Rogers also warned against a “big disconnect” between what happens in a lab dish and in the body and that we draw too many conclusions from studies that take stem cells out of their natural context.
Inside the body, stem cells exist in a complex environment, surrounded by other cells and a cocktail of molecules that instruct them how to behave.
In fact, experiments in mice showed that the cells’ environment alone holds rejuvenating properties. When old animals received young animals’ plasma – what remains in the blood after all the cells have been removed from it – they became rejuvenated. Not only did their raggedy old fur become all shiny again, but their organs looked younger too, including the brain.
Although the young blood did not extend the lifespan in mice, the effect it had on the brain spurred a clinical trial testing if blood taken from 30-year-olds can stave off the loss of brain cells in older patients with Alzheimer’s. But remember, humans are not mice and it’s too early to hope that this relatively simple procedure will defy aging.
For the time being, just stick with exercising and eating well.
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