David Raichlen

This is the second of our op-ed features, contributed by David Raichlen, Associate Professor in the School of Anthropology at the University of Arizona.

What hunter-gatherers can teach us about exercise.

Exercise is beneficial to health. This may be the most obvious statement you will ever read on The Evolution & Medicine Review. But knowing this information does not seem to influence behavior. Less that 5% of adults in the United States regularly meet recommendations for daily physical activity – approximately 150 minutes per week (Troiano et al., 2008). However, simply repeatedly announcing the benefits of regular exercise is not proving a successful strategy for changing our behaviour. So, what would be more effective and can evolutionary medicine help?

Recent theoretical work by Daniel Lieberman (Harvard University), has provided much needed nuance to the discussion, allowing us to consider more carefully how an evolutionary perspective may help us alter behavior. In addition, new relevant data are available from work on exercise patterns in modern day hunter-gatherers. With collaborators Brian Wood (Yale University) and Herman Pontzer (Hunter College), I have examined physical activity patterns in people living lifestyles similar, in many ways, to those of our ancestors, and we can use these datasets to help model our physiological evolution. Figuring out how much and what kind of exercise we need requires understanding both our unique evolutionary history, and how patterns of activity in our ancestors may have affected the evolution of human physiology. There is much more evolutionary physiologists need to investigate and debate, in particular how best to apply evolutionary theory to practical prescriptions for the public, but there is also much we already know.

To begin, we need to better understand how and why our evolutionary history may lead us to derive physiological benefits from exercise. Daniel Lieberman has written extensively on this topic in his recent book, “The Story of the Human Body”, and in a recent review article, “Is exercise really medicine? An evolutionary perspective”. In these works, Lieberman introduces the perspective that exercise represents a kind of paradox. Nearly two million years ago, humans shifted to a novel hunter-gathering pattern of foraging that required intense aerobic effort to find food. Our physiology has become adapted to respond to this physical activity-induced stress in ways that increase capacity: increased bone mass, and increased vascular elasticity, for example. However, our bodies are also adapted to reduce energy expenditure when possible, through rest or through reductions in physiological capacity. This makes good evolutionary sense since energy expended must be replenished through foraging, and energy not spent on activity can be used to improve reproductive success. However, the longer we remain in sedentary and inactive states, the greater the reductions in those stressors, and consequently we experience reductions in physiological capacity that are often associated in today’s world with morbidity – sarcopenia and cardiovascular disease, for example – and mortality.

Thus, Lieberman argues that we are not adapted to exercise for health benefits, per se, but that our physiology is adapted to respond to stress in a dose-dependent way. Paradoxically, reductions in capacity often associated with negative health outcomes today are actually adaptive in an evolutionary sense – all that saved energy, remember, could have been spent by our ancestors on improving their reproductive success. But in modern industrialized societies we gravitate towards the couch when we don’t need to be active, leaving many of us susceptible to chronic diseases associated with physiological capacity reductions including cardiovascular disease, osteoporosis, and perhaps even neurodegenerative diseases (although the jury is still out on this connection).

So, while we are not really adapted to exercise for health, our physiological health is improved by the increased capacity that occurs in response to physical activity. If this is the case, then how much exercise do we need to tax our systems in ways that will improve our health and well-being? The answer has remained elusive to public health researchers, with the best response being that some is better than none, and, up to a point, more aerobic exercise is better than less (O’Donovan et al., 2017). However, this is unsatisfying in many ways, and to provide more detailed prescriptions from an evolutionary perspective, we need to have a better understanding of the types of physiological stresses our bodies have been responding to throughout our evolutionary history. We can do this by beginning to look more carefully at physical activity levels in modern groups of hunter-gatherers that are still living in ways that bear some resemblance to the lifestyles of our ancestors.

Now come the major caveats. Living hunter-gatherers are not close models of hunter-gatherers in the past. There are differences in lifestyle, food sources, and access to medicine and technology that clearly affect activity and health today. In addition, it is simplistic to assume that patterns of exercise in hunter-gatherers are optimal for humans today. As Lieberman points out, we did not evolve to be healthy, we evolved to be reproductively successful. Thus, patterns of activity in hunter-gatherers may maximize reproductive success at the expense of optimal health. Finally, we did not stop evolving when many human populations left the hunting and gathering lifestyle for agriculture, and later for industrialized societies. Simply engaging in activity levels seen in hunter-gatherers may not be optimal to maintain health, despite the fact that they may be optimal for finding food. With these caveats in mind, we can delve into recent work that sheds light on activity levels in hunter-gatherers and consider how we might use these data to model activity in the evolutionary past and apply these studies to life today.

For many years, researchers have used observational data to estimate activity levels in modern day hunter-gatherers. However, these factorial-type models are notoriously inaccurate. Recent advances in wearable technologies have allowed physiologists to gain a more detailed perspective on activity levels in hunter-gatherers. Along with Herman Pontzer (Hunter College) and Brian Wood (Yale University), I have had the good fortune to work with the Hadza – hunter-gatherers living in Northern Tanzania. The Hadza inhabit a highly seasonal woodland-savannah habitat, composed of rocky, uneven terrain, and dominated by Acacia, Commiphora, and Baobab trees. The individuals we work with capture nearly all of their food from wild resources, including hunting large and small game, and gathering honey, tubers, berries, baobab fruit, and other plant foods (Marlowe, 2010). This lifestyle requires considerable movement during the day to hunt and gather foods, to collect water, to gather firewood, and to make social visits to neighboring camps (Marlowe, 2010; Pontzer et al., 2012; Raichlen et al., 2014).

In a recent study, we tracked physical activity using heart rate monitors attached to chest straps. This technique allowed us to determine how much time the Hadza spent in activities that stress their cardiovascular system. We know from experimental and epidemiological work that most health benefits accrue from exercise at moderate-to-vigorous intensity levels (moderate-to-vigorous physical activity or MVPA) accumulated in bouts of 10 minutes or more. We defined these by heart rate as a percentage of age-adjusted maximum heart rate (MVPA = 55-89% of max HR). We found that the Hadza spent about 75 minutes per day in MVPA, mostly in the moderate range of 55-69% of max HR. By contrast, humans in the US spend about 10 minutes per day, on average, in MVPA. US Department of Health and Human Services recommends adults average 150 minutes of MVPA per week. The Hadza meet US guidelines in just two days! What’s more, these patterns hold across both age and gender. What is also noteworthy, however, is that the Hadza spend a great deal of time resting as well. It seems that when they rest, they rest, and when they move, they are moving at moderate-to-vigorous intensities.

What does this mean for our evolutionary perspective? It does not suggest that we evolved to get 75 minutes/day of MVPA, nor does it mean that we can generate a hunter-gatherer workout in the same way that mimicking hunter-gatherer diets (i.e. the PaleoDiet) is a misguided application of evolutionary thinking to health. However, I believe that studies like ours can help us understand exercise in a health context in two important ways.

First, data from these studies can provide a new context to scientific explorations of exercise. Some people might be motivated to engage in exercise once they develop a deeper understanding of the ultimate evolutionary mechanisms underlying why exercise is so beneficial to human health. Second, these types of studies can point to novel directions for experimental research. This dataset provides a new way to view the types of activity-induced stresses experienced by hunter-gatherers, and therefore, model the types of stresses that may have increased physiological capacity during our evolutionary history. What we can take from our view of hunter-gatherers is that they are highly active, except when they are not. That is, when they are active, they seem to be engaged in activity at moderate to vigorous intensities, rather than lower intensity activity. This differs greatly from what we see in studies of adults living in industrialized societies, where much of our physical activity is registered at low intensities.

Thus, we might consider interventions that increase the intensity of everyday activities into the moderate-to-vigorous range. This is a fruitful area of experimental and epidemiological research, and one that may have a stronger impact on public health than repeated recommendations to achieve 150 minutes/week of aerobic exercise. For example, researchers have already begun to show that short bouts of MVPA can be beneficial to health (Glazer et al., 2013). By designing interventions that encourage people to walk faster during everyday activities- from their car to the store, for example – we might be able to have a positive impact and encourage people to stress their physiology in ways that are central to our evolutionary biology. In this way, evolutionary models of physical activity can play a key role in directing areas of research in exercise physiology and perhaps providing novel recommendations to improve public health. Future work should continue to expand our definitions of exercise and include evolutionary perspectives that may drive experimental interventions.

References:

Glazer, N. L., Lyass, A., Esliger, D. W., Blease, S. J., Freedson, P. S., Massaro, J. M., … & Vasan, R. S. (2013). Sustained and shorter bouts of physical activity are related to cardiovascular health. Medicine and science in sports and exercise, 45(1), 109-115.

Lieberman, D. (2013). The story of the human body: evolution, health, and disease. Vintage.

Lieberman, D. E. (2015). Is exercise really medicine? An evolutionary perspective. Current sports medicine reports, 14(4), 313-319.

Marlowe, F. (2010). The Hadza: hunter-gatherers of Tanzania (Vol. 3). Univ of California Press.

O’Donovan, G., Lee, I. M., Hamer, M., & Stamatakis, E. Association of “Weekend Warrior” and Other Leisure Time Physical Activity Patterns With Risks for All-Cause, Cardiovascular Disease, and Cancer Mortality. JAMA Internal Medicine.

Pontzer, H., Raichlen, D. A., Wood, B. M., Mabulla, A. Z., Racette, S. B., & Marlowe, F. W. (2012). Hunter-gatherer energetics and human obesity. PLoS One, 7(7), e40503.

Raichlen, D. A., Wood, B. M., Gordon, A. D., Mabulla, A. Z., Marlowe, F. W., & Pontzer, H. (2014). Evidence of Lévy walk foraging patterns in human hunter–gatherers. Proceedings of the National Academy of Sciences, 111(2), 728-733.

Raichlen, D. A., Pontzer, H., Harris, J. A., Mabulla, A. Z., Marlowe, F. W., Josh Snodgrass, J., … & Wood, B. M. (2017). Physical activity patterns and biomarkers of cardiovascular disease risk in hunter‐gatherers. American Journal of Human Biology.

Troiano, R. P., Berrigan, D., Dodd, K. W., Mâsse, L. C., Tilert, T., & McDowell, M. (2008). Physical activity in the United States measured by accelerometer. Medicine and science in sports and exercise, 40(1), 181.