Humans may be able to live for between 120 and 150 years, but no longer than this "absolute limit" on human life span, a new study suggests.
For the study, published online May 25 in the journal Nature Communications, the researchers used mathematical modeling to predict that after 120 to 150 years of age, the human body would totally lose its ability to recover from stresses like illness and injury, resulting in death. If therapies were to be developed to extend the body's resilience, the researchers argue, these may enable humans to live longer, healthier lives.
Studies like this one "rely on historic and present data from populations of people," Judith Campisi, a professor at the Buck Institute for Research on Aging in Novato, California, told Live Science. "It's guessing, but based on good numbers," added Campisi, who is also a senior scientist at the Lawrence Berkeley National Laboratory. (Campisi was not involved in the new study.)
The researchers analyzed large datasets from the U.S., the U.K. and Russia, which together include anonymized medical data for more than 500,000 people. They utilized data from a simple blood test, available for almost everyone in the datasets. Individuals took the blood tests several times over the course of a few months.
The researchers looked at two numbers collected from blood tests for three different age groups: a ratio of two different types of disease-fighting white blood cells; and a measure of variability in the size of red blood cells. Just as a person might have grayer hair as they age, said Dr. Marc J. Kahn, dean of the Kirk Kerkorian School of Medicine and vice president for health affairs at the University of Nevada, Las Vegas, these two numbers go up as a person ages. Scientists call these biomarkers of aging.
From those blood tests, they then used a computer model to determine what they called dynamic organism state indicator, or DOSI, for each person — essentially a measure of "biological age" that they could use along with the time between blood tests to quantify how well a person would be able to recover from a stress, like illness or injury.
"The authors are able to use this DOSI … to measure recovery time," said Kahn, who was not involved in the current study. "The problem is at a certain point in aging, the recovery time is so great that we lose resiliency." Based on trends in the data, the researchers found that sometime between 120 and 150 years old, resiliency would entirely cease and a person would be unable to survive.
Researchers also looked at data on physical activity, measured in number of steps per day, to validate their results. They found the same pattern: Younger people tended to take more steps each day, while older people took fewer daily steps as they aged. Extrapolating from the data, the researchers found roughly the same age limit as they did from the DOSI measure.
This study isn't the first to use modeling to examine human life span. Jan Vijg, a geneticist at Albert Einstein College of Medicine, led a study detailed in 2016 in the journal Nature, that analyzed trends in life expectancy data to estimate that it would be unlikely for humans to exceed 125 years old. Other researchers have argued that there is no ultimate limit on human life span.
Even though the research suggests humans could live to 150, that number doesn't say anything about the quality of life in old age, Campisi said. In recent years, many scientists have come to refer to the number of healthy years in a person's life as their health span.
"That has huge societal implications, much more than maximum life span," Campisi said. Health in old age not only impacts a person's life, but also can have huge costs in terms of time, money, and medical resources, among others.
The researchers argue that if there were a way to increase resiliency in old age, it would not only increase human life span, but also health span, since older people would be able to recover more easily from illness and injury. To increase resiliency, Kahn could see efforts to create mechanical organs or to come up with ways to reprogram aging cells.
"Now, we're talking about the whole concept of human and mechanical constructs that are features of science fiction," Kahn said. But the study suggests "it's really going to take those types of things to extend human [life span]."
Of course, human life span is highly variable, and Campisi said that there"s always a question of whether this type of data is generalizable. The datasets used in the study, though extensive, came only from a few countries. The number the researchers came up with is also an average and applies to humans as a population; — there are still countless factors, from income to diet, that might influence how long an individual person lives. Studies like this, she said, are inexact by nature. But barring changes to the fundamental biology of humans, there is one thing that is certain, Campisi said.
"For sure, we're all going to die," she said.
The researchers of the study are from the Singapore-based biotech company Gero, Roswell Park Comprehensive Cancer Center in Buffalo, New York, and the Kurchatov Institute in Moscow.
Originally published on Live Science.