November 23, 2016
Are we fighting thousands of years of evolutionary history and the best interests of our bodies when we sit all day?
That question is at the core of a fascinating new study of the daily lives and cardiovascular health of a modern tribe of hunter-gatherers. The findings strongly suggest that we are born to be in motion, with health consequences when we are not.
Evolutionary biologists have long believed that the basic structure of human bodies and genomes were set tens of thousands of years ago, when we were hunter-gatherers. The hunter-gatherers from that time who were most adept at following game or finding tubers won the baby-making lottery and passed along their genes to us, their descendants.
But we no longer live in a hunting and gathering world. Mostly we live in offices and in front of screens, where we sit and have food brought to us, creating a fundamental mismatch between the conditions that molded our bodies and those that we inhabit.
The health consequences of this mismatch are well-established. Many scientists have pointed out that the easy availability of food creates an “obesogenic” world, in which we easily gain weight and develop related health problems.
There also has been considerable research linking sedentary lifestyles with health concerns.
But we have not really known just how much physical activity may be natural for us. The fossil record is evocative but inexact, unable to tell us precisely how our ancestors lived, while most past anthropological studies of living hunter-gatherers have been observational, meaning that researchers have estimated activity patterns.
But estimates can be wrong. So for the new study, which was published last month in the American Journal of Human Biology, researchers from Yale University, the University of Arizona, and other institutions decided to bring high-tech rigor to their ongoing examination of a group of African hunter-gatherers.
For many years, the scientists had been studying and tagging along on hunts with the Hadza, a tribe in Tanzania that lives by subsistence hunting and foraging for berries, honey, baobab fruit and tubers. As part of past research, the scientists had measured the men’s and women’s blood pressures, lipids and other markers of cardiovascular health.
They now asked some of the tribespeople if they would wear heart-rate monitors around their chests. The scientists focused on heart rates since most modern recommendations about exercise involve intensity. We are told that we should aim for at least 150 minutes per week of moderate exercise or 75 minutes of vigorous exercise.
The simplest way to determine exercise intensity is with heart rate. By most definitions, moderate exercise raises someone’s heart rate to between 55 and 69 percent of that person’s maximum heart rate, while vigorous exercise raises it to between 70 and 89 percent. Maximum heart rate can be calculated from a formula based on age.
Forty-six of the tribespeople, ranging in age from young adults to people in their 70s, agreed to participate, donning a chest strap for up to two weeks during different seasons of the year while they went about their daily lives.
The researchers then gathered the heart rate data and used it to determine how much and at what intensity the tribespeople had moved.
They moved a lot, the data proved, typically being active for more than two hours every day. The men would walk briskly in search of various game animals off and on throughout most days. The women would find, dig up, heft and prepare fruits, vegetables and other foods.
The vast majority of this activity was moderate. The tribespeople rarely ran or were otherwise vigorously active, says Brian Wood, an assistant professor of anthropology at Yale and co-author of the study.
They remained active, too, well into middle age and beyond, with those tribespeople in their 70s moving as much as or even more than the young.
Perhaps most important, the tribespeople also had enviable heart health. The scientists found that the Hadza typically showed low blood pressure and excellent cholesterol profiles across their life spans, even deep into old age.
Some of this robust, lifelong cardiovascular health is no doubt a result of diet, Dr. Wood says, but the data intimate that the Hadzas’ active lifestyle, consisting of plenty of walking, lifting and generally being up and doing, helps to protect their hearts against disease.
The underlying lesson of the study, however, is not that we should all renounce our jobs and homes “and become hunter-gatherers for the sake of our hearts,” he says.
The Hadzas’ lives remain difficult and chancy, he points out, with pronounced risks for untreated infections and illnesses, accidental deaths and no access to dental care.
The more nuanced but still potent takeaway of the new study, says David Raichlen, an anthropologist and exercise scientist at the University of Arizona who led the study, is that “human bodies likely evolved to need and respond to the kind of physiological demands” that the Hadza still undergo on most days.
Our bodies, and in particular our hearts, want to be worked, at least moderately, he says. When they are not, when our pulse rarely rises, pathology may set in.
So move, he says, and preferably often, since the need for activity seems to be built into our bones and hearts and being.