Health in the Headlines: Life span vs. Health span
by Lorien E. Menhennett
Good news, if you’re a roundworm: researchers at Stanford reported last month that blocking the expression of a particular protein can extend a worm’s life span up to 30 percent.
That is good news . . . right?
It depends on whom you ask. Clearly, the Stanford researchers, as well as other researchers who have studied caloric restriction (another method purported to promote long life) think so. But there is another camp of scientists dedicated to improving not so much life span, but health span.
So what does “health span” mean? According to the Macmillan Dictionary’s online “Buzzword” feature, “health span” is “the period in a person’s life during which they are generally healthy and free from serious or chronic illness.” So the focus is on living better, as opposed to living forever.
Sounds simple enough, doesn’t it? But this concept is really a very complex one which scientists are only beginning to grapple with, understand, and research. Ironing out some of the complexities, and getting more scientists, policy makers, and regular people to think this way, though, is key to making sure that “aging gracefully” is not just a catch-phrase.
One major problem in the world of aging research is a disconnect between the viewpoints of clinicians and “basic” science researchers on this very topic. In an article published in the Journal of Gerontology in 2009, authors Drs. James Kirkland and Charlotte Peterson write that “Geriatricians and others providing health care for the elderly have long recognized that disability, frailty, and age-related disease onset are the critical end points that need to be addressed in older populations.” Hence, many clinicians are on the “health span” wagon. However, Kirkland and Peterson also claim that “Most investigators in the basic science of aging use survival curves and maximum life span as key end points for studies of effects of interventions, rather than health span or function.” Hence many basic science researchers are on the “life span” wagon. And when researchers and clinicians are on two different wagons, little if any progress will be made in terms of going from the scientific “bench” to the bedside with new treatments.
What needs to happen, writes Marc Tatar, a biologist at Brown University, is first clarification of the health span concept. While Tatar explores a more scientific definition of health span than the one I previous listed, his comments relate to our more simple definition as well. For example, how do you define “healthy”? What is the threshold between healthy and unhealthy, in terms of time and quality of life? And even if you use baseline performance as a judge, that baseline performance declines over time — how do you factor that in? These issues need to be resolved before any progress can be made, Tatar says.
Once some of that ambiguity is resolved, the next step is to develop a better animal model for studying health span, Tatar says. That will allow researchers to take results and translate them to human models. For example, researchers should look at how best to study osteoporosis — clearly a factor in human health span — in mice, flies, or worms (three of the best animals for studying aging issues).
Kirkland and Peterson agree that better animal modeling is needed, although they focus on the concept of frailty: “Frailty usually describes a condition in which a critical number of impairments occur in parallel, becoming evident after a threshold is reached, and if a stress such as an infection or injury is applied.” They say that indicators of “frailty syndrome” include weakness, fatigue, weight loss, impaired balance, decreased physical activity, slowed motor performance, social withdrawal, mild cognitive dysfunction, and increased vulnerability to physiological stress.
According to Kirkland and Peterson, screening for frailty in humans is being developed and validated, and could be adapted for use in animals. Testing animals for frailty — after giving them an anti-aging compound, for example — could help show whether the added longevity compromised health span, and therefore whether the compound was potentially appropriate for trials in humans.
Living longer, obviously, can be a good thing. But only if that extended life is a healthy one. That’s what the study of health span is about. And while the idea is catching on, it has some catching up to do.