Exercise

By Herman Pontzer

THE Spine Challenger is a brutal race. It claws its way along the toughest 174 kilometres of the Pennines, the geological backbone of England, in the dead of winter. It must be completed in 60 hours. Finishers rack up some 5400 metres of ascent, equivalent to climbing Mont Blanc twice.

Participants in 2017 – the fast ones, anyway – would have glimpsed Dom Layfield, an irrepressibly upbeat man in his 40s, pulling away and disappearing into the low clouds and sleet. They let him go, perhaps thinking that this first-timer had underestimated the race’s difficulty and would burn out. They were wrong. After 28 hours of non-stop running and scrambling, he finished first, an hour ahead of his nearest rival, setting a course record.

If exercise is medicine – as we are often told – surely the Spine Challenger is a massive overdose. To complete it takes more than 20 times the 10,000 steps that many of us aspire to each day. Yet hundreds of these ultramarathons have sprung up around the world, and the most prestigious have to turn eager contestants away. At the same time, lifts and escalators are jammed with people who would never consider climbing the stairs. In fact, the average person in the US takes fewer than 5000 steps a day and in the UK it isn’t much more.

As a species, we have a love-hate relationship with exercise. Many people fail to get enough, some seem to get too much. So, what is the correct dose? Or, put another way for the Fitbit generation: how many daily steps should we take to make the most of this marvellous medicine?

“Mostly we avoid exercise. Our lazy inner ape calls the shots far too often”

Dom and I met as PhD students in 2001, dissecting cadavers at Harvard Medical School. Chatting as we worked, we discovered a shared love of the mountains. A friendship was born amid the grease and formalin. In the years since, we have spent many happy days climbing, skiing and running together. The one constant has been Dom pulling ahead, wearing me out. So I have a sense of how the other racers in the Spine Challenger must have felt. As a scientist working at the intersection of human evolution, energetics and health, I also find myself wondering what our species’ immense capacity for physical exertion tells us about the way our bodies are built.

We evolved from lazy stock. All animals rest when they can, saving precious calories for survival and reproduction, but by any measure, our great ape relatives are impressively sedentary, resting and sleeping for 18 hours a day. However, when our ancestors began hunting and gathering, around 2.5 million years ago, it put an evolutionary premium on physical exertion.

These activities are incredibly demanding, requiring hours of effort each day to find food. Individuals that were more active found more food and had more offspring – and these, in turn, inherited their desire to move. Over generations, the human brain evolved to reward hard work, releasing endorphins and endocannabinoids – the body’s homemade, feel-good drugs – in response to endurance exercise. The “runner’s high” was born, taking up residence in our brains alongside our ancient, simian desire to rest. These two competing drives were balanced by a lifestyle that demanded hard work, but rewarded strategic laziness.

These sirens continue to call from opposite shores inside our evolved minds, luring us towards idleness or action. But recently, and in the blink of an evolutionary eye, our environment has changed. In the well-stocked human zoos many of us now inhabit, we have largely engineered away hunger, fear and the other demons that got our hunter-gatherer ancestors moving. We have made it easy to overindulge, leading to heart disease, obesity, diabetes and other plagues of civilisation. In our Palaeolithic past, we could know what our bodies needed by listening to what they wanted. In the modern world, relying on our neural reward systems to deliver the proper dose of exercise feels a bit like trusting my 4-year-old daughter to serve herself healthy portions of broccoli and ice cream.

The perils of sloth

Our strange modern environment has also exposed our seemingly paradoxical relationship with exercise. Some of us, like Dom and other ultramarathon competitors, seek it out in large doses, feeding the evolved craving for physical activity. Yet, mostly we avoid it. Our lazy inner ape calls the shots far too often.

The health benefits of exercise and the perils of sloth have long been known. Even Socrates, not remembered as an athlete, bemoaned the lack of fitness among his students. Today, many people would like to be more active to improve their health. But how much more? To get a better sense of the amount of exercise we should be aiming for, we need to understand exactly what it does to our bodies. It has taken a surprisingly long time to figure that out, but recent work is illuminating.

escalator
Why is it that many people wouldn’t dream of climbing the stairs, while some love to run ultramarathons?

First, the obvious benefits: exercise keeps our muscles and hearts strong, our blood vessels pliant and improves aerobic fitness. When we get our heart rate up, the stresses imposed by the blood rushing through our arteries promotes the production of nitric oxide, which helps repair blood vessels and keep them elastic. Maintaining strength and aerobic fitness is particularly important as we age. Older adults who can cover at least 365 metres in a standard 6-minute walk test have half the risk of dying in the subsequent decade as their peers who can’t make 290 metres.

Exercise does more than strengthen our hearts and muscles, though. It also has helpful suppressive effects all over the body. It reduces chronic inflammation, moderates levels of the reproductive hormones testosterone, oestrogen and progesterone, and blunts our physiological response to stress. This suppression has big health impacts. Chronic inflammation and stress are indiscriminate killers, increasing the risks for heart disease, cancer, diabetes, mental illness and other maladies.

Research by David Raichlen at the University of Southern California, Gene Alexander at the University of Arizona and others is revealing how exercise keeps our brains fit too. Aerobic activity increases blood flow to the brain and causes the release of molecules that stimulate the generation of new brain cells and keep old ones healthy. Running, cycling and walking also challenge the brain to coordinate myriad signals involved in balance, navigation and movement, helping to maintain our cognitive reserve. Again, this is particularly important as we age because it helps ward off dementia and other forms of cognitive decline.

“Being active doesn’t change the number of calories you spend each day, it changes how you spend them”

Counter-intuitively, one thing that exercise doesn’t do very well is increase our daily energy expenditure. Research from my lab, done with Raichlen and others, reveals that Hadza hunter-gatherers in Tanzania burn the same number of calories a day as adults in the US and Europe, despite being five to 10 times as active. It isn’t that exercise is less energetically expensive for the Hadza (we checked). Instead, their bodies have adjusted to their physically active lifestyle by spending less energy on other tasks, which keeps their total daily calorie expenditure in check. The same seems to be true for people everywhere: being physically active doesn’t change the number of calories your body spends each day, it changes how you spend them.

This may be bad news for people relying on exercise to lose weight, but I believe it helps us understand why activity is so important in the modern world. I argue that this “metabolic management” underpins the suppressive effects of regular exercise. In our typically sedentary lives, the body has an abundance of calories at its disposal. As a result, physiological activities such as inflammation and the fight-or-flight response, which are normally short-lived and sporadic, are always on, raging in the background. Similarly, our reproductive systems produce an overabundance of sex hormones – twice the levels we see in populations like the Hadza. Exercise helps us regulate these and other overzealous activities. By forcing our bodies to economise, it helps prevent many of the diseases that haunt the developed world.

As with all good things, there is a dark side to this. Taken too far, the suppressive effects of exercise can cut into essential functions. This might explain the curious finding, reported in many large studies, that extreme exercisers have slightly higher mortality rates than people who work out a couple of times a week. We also know that the rigorous regimes of elite athletes can lead to overtraining syndrome, a constellation of problems including reduced immunity and fertility. White blood cell counts crash. Colds last longer. Libido drops. Women stop ovulating. Exercise stops being healthful and starts being harmful.

So how much exercise do we need to get to reap the crucial health benefits without feeling the downside?

Because our body’s response to exercise evolved to meet the physical demands of hunting and gathering, perhaps populations who still forage for their food should be our guide? In communities like the Hadza, adults get about 2 hours a day of moderate-and-vigorous physical activity – meaning anything more strenuous than a casual stroll. Most of this comes in the form of hard walking: moving fast over hilly terrain, while scouring the landscape for food. There are plenty of other activities, though. Women often spend an hour or more digging starchy wild tubers from rocky ground. Men climb trees and chop into branches to expose bees’ nests and take honey. Kids drag firewood or haul buckets of water back to camp. Other indigenous communities have similar workloads.

lift
In the modern world, we rarely reach the activity levels of hunter-gatherers

It is unlikely you would care to trade lifestyles with these hunter-gatherers. Their limited access to medicine means that children die far too often from curable, acute infections, skewing average life expectancy sharply downwards. But when it comes to the health conditions that those in the developed world are most likely to die from, hunter-gatherers are paragons of public health. Men and women in these communities regularly live into their 60s and 70s without any sign of the problems we often see as the inevitable consequences of ageing. They have the healthiest hearts on the planet, never develop diabetes, and stay strong and spry into old age. They are getting the daily dose of exercise that humans evolved to require, and the health benefits are apparent.

Even serious athletes might find it useful to gauge their exercise dosage by hunter-gatherer standards. My friend Dom aims for a very Hadza-like 2 hours of running per day to stay sharp for ultramarathons. Much more than that and he begins to sense the telltale signs of overtraining. Olympic-level athletes often log far longer training hours: swimmers might do 5 to 6 hours a day during intensive workouts. But it is telling that such workloads, and the arms race to pack in ever more training, can push some athletes towards the temptation of performance-enhancing drugs. Steroids and similar drugs mimic the hormones our bodies suppress when exercise cuts too deep. Athletes dope to ward off the effects of overtraining so they can push themselves past their evolved boundaries.

Post haste

For the rest of us, growing soft and sluggish in our hedonistic zoos, 2 hours’ exercise each day might seem like a lot. But people who manage it do get huge benefits. A study of postal workers in Glasgow, UK, found that those who clocked more than 15,000 steps a day carrying the mail, which equates to about 2 hours of brisk walking, had cardio-metabolic health on a par with hunter-gatherers – and this in a city with the lowest life expectancy in the country. A much larger study in the US followed 4840 adults to see whether physical activity reduced the risk of dying over the subsequent five to eight years. No surprise, it found that more active people had lower mortality rates. Just 25 minutes of moderate-and-vigorous activity a day reduced the risk of dying within this timeframe by 25 per cent compared with the least active people. And more was better. Adults who were active for 100 minutes or more each day had the lowest mortality rates: 80 per cent lower than the couch potatoes.

These and other similar studies suggest that current public health guidelines set the bar too low. In the US, the Centers for Disease Control and Prevention recommends at least 25 minutes of moderate-and-vigorous exercise a day. (Some 90 per cent of people there fail to achieve this.) The 10,000-step target pursued by fitness-tracker enthusiasts – originally a marketing ploy dreamed up by a Japanese manufacturer of pedometers in 1965 – gives a comparable amount of exercise. This is because many of those steps won’t count as moderate-and-vigorous physical activity (see “How many steps?”). These targets are a good start – even low intensity steps at least get you moving – but we should strive for more. Benefits continue to accrue with more exercise, and the optimal dose seems to be closer to the levels we see with the Hadza. Higher exercise workloads may be particularly important for people who spend their days at a computer. A recent study of nearly 150,000 Australian adults found that it took over an hour a day of vigorous exercise to cancel out the ill-health effects of sitting during work hours.

But if 15,000 steps a day/2 hours’ brisk walking is a distant goal for you, don’t be discouraged. A little of this medicine is still far better than none. Studies consistently show that even modest amounts of exercise confer huge health benefits compared with a slothful existence. For the most sedentary among us, an extra 30 minutes a day of activity that elevates our heart rate would halve our mortality rate, adding high quality healthy years to our lives.

An evolutionary perspective suggests that most of us could do with more exercise. It is a powerful drug, but we shouldn’t be afraid to self-medicate liberally. The only dangerous dosages are “none” and “life isn’t fun anymore”. If you find a way to stay active that tickles your brain’s reward centres, you are doing it right. The best dose of exercise is the one that gets you coming back for more.

How many steps?

The optimum amount of exercise you should get each day is equivalent to about 15,000 steps, taken at a brisk walk or faster. Steps that fall below this “moderate-and-vigorous” activity level will count for less. Here’s a rough guide to how that stacks up

STEPS (moderate and vigorous)

50: climbing four flights of stairs

150: running the length of a football pitch

1250: 1 kilometre brisk walk

1500: approximate daily average for US and UK adults

3000: US recommended daily minimum

3000: approximate equivalent to 10,000 steps on a fitness tracker

7500: 1 hour of running, dancing or cycling

15,000: daily round of a postal worker in Glasgow

19,000: UK recommended weekly target for adults

200,000+: ultramarathon

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