The human resting metabolic rate and levels of physical activity have declined in the United States since 1830, according to a recent study by researchers in Harvard’s Department of Human Evolutionary Biology.
The study found a 6 percent reduction in human resting metabolic rate since 1830, which suggests an average decrease of 27 minutes of “moderate to vigorous” daily physical activity, such as walking or jogging. The researchers attributed the change to a decrease in physical activity due to technological advancement.
The study was conducted by Andrew K. Yegian, a postdoctoral fellow in HEB and Daniel E. Lieberman, a professor of Biological Sciences and HEB, along with Louisiana State University’s Steven Hymsfield. The findings were published last month in Current Biology.
Lieberman, whose lab the study was conducted in, said the idea to use temperature data to depict the change came from a study he and his lab read that attributed the decrease in human average temperature to changes in the demand of the immune system.
“I was particularly interested in the fact that they had inferred in the paper that the reason for this decline was less immune function,” he said. “It's not an unreasonable hypothesis, but there's a bunch of other studies which kind of disprove that, or call that into question.”
“So, I thought, I don't believe that,” he added.
Lieberman said he set out to understand metabolism as a result of industrialization and trends of declining physical fitness rather than decreasing immune function.
“Let's see if we can use body temperature as a thermometer to estimate how much less physical activity people have been doing,” he said.
Yegian said he believes it is crucial to quantify the decline in physical activity.
“It's really important to start putting numbers on that so that we can get a sense of ‘Okay, what were the levels back at a certain point in time?’ and ‘How many people get that kind of physical activity nowadays? And ‘What kind of increase in activity would it take to get to that point?’”
Lieberman said even minor technological changes to everyday life impact humans’ overall physical activity.
“Our modern world of elevators and escalators and shopping carts and, you know, all these devices that we've created that have made manual labor more rare,” he said.
Lieberman said this broader decline in physical activity among humans can be seen around campus.
“We know that the vast majority of Harvard students don't exercise very much,” he said. “Back in the day, Harvard students didn't have elevators that could take them up various buildings, and they had to walk around campus because there weren't shuttle buses.”
This change has significant repercussions for human health, Lieberman said.
“It should concern us all, because it affects long-term health, and heart disease, and cancer, and Alzheimer's, and diabetes, and the list goes on,” he said.
Lieberman said the decrease in physical activity requirements by universities in the United States contributed to this decline.
“We dropped [physical activity requirements] before we realized just how bad the physical inactivity epidemic was going to be,” he said. “We're now seeing the consequences of that.”