Built to Endure

Human heart study explores risk of sedentary lives

Human heart

Major physical changes occurred in the human heart as people shifted from hunting and foraging to farming and modern life. As a result, human hearts are now less “apelike” and better suited to endurance types of activity. But that also means those who lead sedentary lives are at greater risk for heart disease.

Those are the main conclusions from a unique study led by Aaron Baggish, Harvard Medical School associate professor of medicine and director of the Massachusetts General Hospital Cardiovascular Performance Program. Baggish and his collaborators examined how ape hearts differ from those of humans, why those differences exist and what that means for human health.

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The researchers measured and compared heart function in apes and four groups of humans, ranging from sedentary to elite runners and including indigenous subsistence farmers. Their research is presented in the Sept. 2019 issue of PNAS.

Baggish’s collaborators on this paper include Daniel Lieberman, Department of Human Evolutionary Biology, Harvard University, and Robert E. Shave, School of Health and Exercise Sciences, University of British Columbia

Chimpanzees are humans’ closest known relatives, based on genetics and evolutionary studies. There are, of course, some stark differences between the species. For example, in terms of exertion, chimpanzees mainly engage in short bursts of activity, such as climbing and fighting, which puts intense pressure on the heart but only for a limited time.

In contrast, it’s believed that up until the industrial revolution, humans were active for longer periods of time in order to hunt and farm. Survival of pre-industrial humans, it is thought, depended on moderate-intensity endurance activity (e.g., hunting and gathering and then farming). 

It is also well established that some physical features of the heart change in response to certain physical challenges. Walking and running, for example, require more blood to be pumped to deliver fuel to active muscles.

In contrast, brief but intense exertion from activities such as climbing or fighting creates pressure in heart, which over time can make the heart chambers develop stiffer and thicker walls.

“The heart remodels in response to two main forces: pressure and volume,” said Baggish. As a result, “humans have longer, thinner and more flexible-walled hearts, while chimps have smaller hearts with thicker walls.”

What Baggish and his collaborators wanted to know was, could those differences have evolved in response to humans’ new activity levels? And if so, what implications does that have on human health today?

Using a group of more than 160 study participants, the researchers carried out detailed heart function studies, including measuring blood pressure and using ultrasound to examine the heart’s structure and function during many different activities.

The subjects were fairly evenly divided into elite runners, American football players, indigenous Mexican subsistence farmers and people who engage in little physical activity. They made similar measurements in about 40 semi-wild chimpanzees and five gorillas.

“The goal was to compare heart structure and function in each “type”—whether the subject was very active to barely active,” said Baggish. 

In addition, the investigators sought to determine whether adaptation to either pressure or volume comes at the expense of the ability to handle the alternative form of stress. This was done by giving the pressure adapted subjects (football linemen) and volume adapted subjects (long-distance runners) a “volume challenge,” by a large intravenous saline infusion and a “pressure challenge,” by a sustained, forceful handgrip, and simultaneously measuring heart function.

The goal was to see whether there is a tradeoff between having a heart adapted for endurance versus one that performs better for short bursts of intense activity. Or could the heart adapt to both? 

Baggish and his collaborators found that human hearts appear to have evolved to be better at handling endurance activity. The researchers also confirmed that people who train specifically for endurance sports have hearts with longer, larger and more elastic left ventricles, the part of the heart that pumps the blood out to the body.

Those features, and others, make the heart better able to cope with pumping higher volumes of blood over a sustained time. In contrast, sedentary people, even at a relatively young age, have hearts that appear more apelike and are better suited to cope with short bursts of high activity.

These findings help answer the question about the heart’s evolution. “The human heart has evolved over hundreds of thousands of years as our activity levels gradually became more sustained,” said Baggish.

“We now understand that the human heart, coupled with changes in the musculoskeletal and thermoregulatory system, evolved to facilitate extended endurance activity rather than spurts of intense exertion.”

This study has important implications for understanding heart health today. For example, people who live a sedentary lifestyle appear to develop more apelike hearts and are more prone to hypertension.

This study is unique for several reasons, said Baggish. “Not only were we able to study heart function in three types of primates, but we also had the opportunity to work with people who are among the last groups of truly subsistence-based farmers, the Tarahumara in Mexico’s Copper Canyons.”

Adapted from a Mass General news release