Reaching New Heights
The first researchers to examine height loss as we age are from the Department of Economics in USC Dornsife. Their study has been published in the American Economic Journal: Applied Economics.By Suzanne Wu
April 2, 2013
Even if you didn’t eat your veggies or drink your milk as a child, your height is still in your hands, according to new findings by economists from USC, Harvard University and Peking University.
Using unique data from a new longitudinal survey of 17,708 adults beginning at age 45, the researchers showed for the first time that lifestyle choices people make in adulthood — and not just the hand they’re dealt as children — influence how tall they stand as they age.
“Had we only examined the correlations between measured height and health, we would have missed this important insight,” said John Strauss, professor of economics in USC Dornsife, an investigator on a study published in the April issue of the American Economic Journal: Applied Economics. “The evidence shows that it is not only early-life events that are associated with how we age, but health decisions in later life as well.”
While prior work has looked for the connection between height and health — both in childhood and adulthood — the researchers are the first to examine height loss as we age. They showed that regardless of one’s maximum height, the loss of height over time is also an important indicator for other health issues as people age.
For example, the research revealed an especially strong relationship between height loss and cognitive health. Those who had lost more height were also much more likely to perform poorly on standard tests of cognitive health, such as short-term memory, ability to perform basic arithmetic and awareness of the date.
Among the socioeconomic factors that correlate to height loss, urban dwellers had much less height loss than those in rural areas, the researchers found, in a country where there has been significant migration to urban areas in the last few decades. In addition, having completed primary school, rather than being illiterate, is associated with 0.9 cm less height shrinkage in men — a large difference when considering that overall average height loss for men is 3.3 cm. Completing high school meant an additional 1 cm less in shrinkage.
For women, having completed primary school was the difference in 0.6 cm of shrinkage, compared to average overall height decrease of 3.8 cm.
“Height has been recognized as an acceptable proxy for childhood health conditions, but there are complications there,” said Geert Ridder, professor and interim chair of economics in USC Dornsife, a co-investigator on the study. “Some of adult health might be determined by childhood circumstances, but people shrink differentially, and that shrinkage is also a measure of adult health conditions.”
All humans go through physical changes with age, including an increase in body fat and decrease in bone mass. But a decrease in height can be further exacerbated by certain kinds of arthritis, inflammation of spine joints or osteoporosis, which other studies have shown are associated with diet, exercise and smoking, among other lifestyle choices.
The researchers used new data from the China Health and Retirement Longitudinal Study, a sampling project led by Strauss, Yaohui Zhao of the China Center for Economic Research at Peking University and Gonghuan Yang of the Chinese Academy of Medical Sciences and Peking Union Medical College, that covers 150 counties randomly chosen throughout China.
The baseline for the survey, which was collected from June 2011 to March 2012, includes both subjective self-reported responses to survey questions, as well as objective physical measurements such as blood tests. These physical measurements and personal interviews will be followed up with the same 17,708 people every two years — capturing, for the first time, critical data about human aging in the most populous and most rapidly aging country in the world.
For example, recent changes in social security policy and health insurance in China provide a valuable opportunity for researchers to study how health care actually affects health and aging in a large population, with insights for other developing health care systems worldwide, as well as an opportunity to identify possible under-diagnosis of various chronic conditions.
The researchers will also be able to examine the role specific historical events in China may have had on long-term health, including whether there are health and aging differences among those who were “sent down” during the Cultural Revolution.
The research was supported by the National Institute on Aging, the National Natural Science Foundation of China, the Fogarty International Center of the National Institutes of Health and the World Bank.
To estimate full adult height for older study participants, the researchers examined relationships between current height and the length of limbs, which do not shrink with age, from younger survey participants who have not yet started shrinking.
Wei Huang, a doctoral student in economics at Harvard who started this work as part of his master’s thesis at Peking University, and Zhao and Xiaoyan Lei of Peking University, were co-authors of the study.