By Maya Shetty

By now, we have all heard that exercise is good for us. But why is this true? And if it’s so good for us, why is it so hard to get up and do it? The Center for Disease Control (CDC) claims we need 150 minutes of moderate exercise and 2 days of muscle strengthening exercise a week to reduce our risk of chronic disease and other adverse health outcomes. However, nearly 80% of US adults are not meeting these guidelines, and 6 in 10 have one or more preventable chronic diseases. To understand this disconnect, we need to begin by examining the evolutionary perspective of exercise – why did humans evolve to exercise in the first place?  

The first Homo Sapiens emerged around 200,000 years ago and their lives looked very different from our lives in the modern day. While we complain about having to take the escalators instead of the stairs or our food delivery taking too long, our ancient ancestors were running from predators and hunting for prey. Humans subsisted through hunting and gathering for food, and this was the primary way of life for 95% of human history. This means the majority of our evolution was spent living as nomadic hunter-gatherers. Therefore, our body and behavior is primarily adapted for this lifestyle.

Now, you may be wondering how this translates to exercise in today’s society. Well, as hunter-gatherers, our ancestors’ main advantage was endurance. We are not the strongest or fastest animals out there, so survival was dependent on our ability to outrun our predators and prey. Evolutionarily, we are endurance athletes adapted for consistent, long bouts of physical activity. If this is the case, then why does the average American spend most of their time relatively immobile? This is because we are also adapted for inactivity and energy conservation whenever possible. Thinking again about the hunter-gatherer lifestyle, our ancestors were constantly trying to maintain their energy balance – food intake vs. energy expenditure. It made sense to exercise only when necessary for survival, and conserve energy whenever possible. In today’s society, however, this biological tendency no longer serves us, as our environment has been engineered for an extremely positive energy balance: excess food with little energy expenditure. Now we must go against our biological tendencies and make the decision to exercise, even when our body is telling us not to, in order to maintain good health. 

We can see just how much our physical activity differs from our hunter-gatherer ancestors by studying the few modern day hunter-gatherer communities. These populations are often used as models in public health due to their remarkably low rates of chronic disease and disability with age, a stark difference from modern day America. Researchers analyze the behaviors of these populations to have a better understanding of the evolutionary causes of chronic diseases – Why are they so common now vs. then? The most commonly studied population is the Hadza, a hunter-gatherer population in Tanzania. Decades of research has quantified their daily physical activity and how it changes throughout the lifetime. Most notably, the Hadza people average 15,800 steps, about 6 to 9 miles, per day. Meanwhile, Americans average less than 4,800 steps, about 2 miles, per day – about ⅓ the steps of modern hunter-gatherers. On top of this, the average American reduces the amount of steps they take per day by about half between the ages of 40 to 70. The Hadza people, on the other hand, barely change their physical activity levels with age. These behaviors have measurable effects on our physiology. By the age of 60, most Americans walk 33% slower, have less muscle mass, and their VO2max – an indicator of cardiovascular health – decreases by 25%. These functional losses are seen at a significantly lesser rate, if at all, in hunter-gatherer populations. These findings align with the theory of disuse and aging brought forth by Walter Bortz II, a Professor of Medicine at the Stanford University School of Medicine, and one of America’s most distinguished scientific experts on aging and longevity. Dr. Bortz claims the changes commonly associated with aging, such as loss of muscle mass and decreased maximum oxygen consumption (VO2max), are due to disuse with age, rather than aging itself. The differences in physical capabilities with age seen between the modern American and the Hadza people suggest our sedentary lifestyle may contribute to accelerated aging. By being sedentary, we oppose the evolutionary history encoded in our genes for periodic activity, leading to accelerated physiologic loss with age due to disuse. 

 Regular physical activity stimulates our body to allocate energy toward repair and maintenance, slowing cellular senescence and aging. It has also been seen to have dose dependent effects on the risk of several chronic conditions. 

These include:

-Cardiovascular disease and hypertension

-Type 2 diabetes

-Arthritis 

-Osteoporosis

-Stroke

-Lung disease

-Many cancers

-Alzheimer’s disease and dementia of any type

Many theories have been suggested about how exercise is able to elicit such powerful health effects. It is widely accepted that many benefits stem from the prevention of excessive weight gain, maintenance of normal blood pressure, decreased levels of unhealthy triglycerides, increased levels of healthy lipoproteins, decreased blood sugar levels, reduced systemic inflammation, and decreased stress levels. However, how these effects come about is still debated. 

Studies have also found that regular physical activity stimulates brain growth and improves cognitive function, counteracting the loss of memory and cognition seen with age. Specifically, running has been shown to stimulate the production of neurotrophic factors, or biomolecules that support the growth and survival of neurons. Brain-derived neurotrophic factor (BDNF), for instance, strengthens neuron synapses, increases the production of new nerve cells, and promotes the growth of dendrites. BDNF expression directly increases in response to exercise and is an exciting example of how exercise attenuates cognitive loss.

The World Health Organization suggests that about 150 minutes per week of moderate exercise or 75 minutes per week of vigorous exercise reduces the risk of all-cause of mortality by ~50% in otherwise sedentary individuals (1). Aside from reducing risk of mortality, regular physical activity has been shown to improve functionality, cognition, mood, and healthspan – the amount of time spent in good health- without the functional losses of aging. Despite the extensive benefits of exercise, most of us do not routinely exercise and spend the majority of the day sedentary. Our drive to not exercise, however, is as encoded into our genes as our necessity to exercise.  As difficult as it is to find the time and energy to exercise, we as humans are genetically selected for lifelong physical activity and, because of this, regular exercise is synonymous with good health. Our world has been engineered for our convenience, not our health, and for this reason we need to make the personal decision each day to walk more, sit less, and make physical activity a regular habit. Something is better than nothing, so find something that brings you joy! Identify activities that you like and can see yourself consistently doing throughout your life. When you are short on time, taking the stairs instead of the elevator or a short walk after a stressful day can still have tremendous benefits.  Whether it be running, dancing, boxing, walking, pilates, biking, etc., just remember to find joy in it and be proud of yourself for putting in the effort. Your health will thank you later.