Exercised: The Science of Physical Activity, Rest and Health

by Daniel E. Lieberman

exercise several hours a week, mostly cardio but also some weights, and keep it up as you age.

A passionate believer in the power of running to improve health and increase longevity, Fixx published The Complete Book of Running in 1977. The book was a best seller, helped ignite the running boom, and made Fixx famous.

It is largely medicinal because the absence of physical activity is unhealthy.

Incessant sitting combined with modern diets and other novelties thus contributes to evolutionary mismatches, defined as conditions that are more common and severe today than in the past because our bodies are poorly adapted to novel environmental conditions.

Exercise may not be an elixir, but by stimulating growth, maintenance, and repair, it can reduce our susceptibility to many of these mismatches.

Thus to conclude on a practical but somewhat alarming note, let’s once again use the lens of evolutionary anthropology to review briefly how and why different doses and types of physical activity affect vulnerabilities to major health conditions, physical and mental, hypothesized to be mismatches.

This calories-in-calories-out equation, however, is regulated by hormones, which in turn are strongly affected by diet and by other factors including psychosocial stress, the microbes in our gut, and, of course, physical activity.

It is untrue you can’t lose weight by exercising. Instead, weight loss from exercise is much slower and more gradual than weight loss from dieting.

Big deposits of enlarged fat cells in and around organs (so-called visceral, abdominal, or organ fat) are especially hazardous because they react sensitively to hormones and connect more directly to the bloodstream.

As far as I can tell, “fat but fit” became a controversy by sometimes being spun as proof that obesity is not a health concern for those who exercise. That is untrue. While overweight people who exercise and are physically fit lessen their risk of chronic disease, if you must choose between being fit and fat or unfit and lean, the evidence overwhelmingly indicates you should gamble on being unfit and lean.

The English physician Thomas Willis (1621–1675) coined the term “diabetes mellitus” (Latin for “honey sweetened”), what we now call diabetes, from urine that was “wonderfully sweet as if it were imbued with honey or sugar.”

By convention, someone has metabolic syndrome if they have most of the following characteristics: high levels of blood sugar, high levels of cholesterol, high blood pressure, and a large waist.

Type 2 diabetes (which differs from type 1 and gestational diabetes18) is now the fastest-growing disease in the world.

Although too much sugar in your urine or blood is a sign of type 2 diabetes, the disease’s root cause is a problem termed insulin resistance.

Among its many actions, insulin directs special molecules on the surfaces of fat and muscle cells to transport sugar from the bloodstream into those cells to be stored or burned.

Bad diets promote obesity and deluge the bloodstream with sugar and fat. Stress elevates cortisol, which releases blood sugar, causes organ fat to accumulate, and facilitates inflammation. Last but not least, persistent sedentariness contributes to metabolic syndrome by elevating blood sugar and fat levels and failing to dampen inflammation.

In addition, exercise lowers inflammation and counteracts many of the damaging effects of stress.

And most remarkably, exercise can reverse insulin resistance by restoring blocked insulin receptors and causing muscle cells to produce more of the transporter molecules that shuttle sugar out of the bloodstream.

As we have repeatedly seen, some exercise is better than none, and more is better.

In addition, many studies find that weights are also effective for restoring muscle sensitivity to insulin, lowering blood pressure, and improving cholesterol levels.

By collecting data from morgues and hospitals, Morris noticed that the drivers of London’s famous double-decker buses suffered more heart attacks than the conductors who walked up and down the aisles and stairs collecting tickets.

Plaques, however, don’t simply accumulate in arteries like crud settling in a pipe. Instead, they are dynamic, changing, growing, shifting, and sometimes breaking.

They develop when white blood cells in arteries trigger inflammation by reacting to damage usually caused by a combination of high blood pressure and so-called bad cholesterol that irritates the walls of the artery. In an effort to repair the damage, white blood cells produce a foamy mixture that incorporates cholesterol and other stuff and then hardens. As plaque accumulates, arteries stiffen and narrow, sometimes preventing enough blood from flowing to the tissues and organs that need it and further driving up blood pressure.

Plaques can also cause the artery wall to dilate, weaken, and bulge (an aneurysm) or to tear apart (a rupture), which can lead to massive bleeding (a hemorrhage).

When we exercise, blood pressure rises temporarily, causing the heart’s muscular chambers to adapt, mostly by becoming stronger, larger, and more elastic so it can pump more blood with each stroke.30 Just as important, arteries also adapt to exercise to keep blood pressure low, primarily by expanding, multiplying, and staying elastic.

Since Jeremy Morris’s pioneering study on London bus conductors first pointed the way, it has become indisputable that coronary artery disease is a largely preventable mismatch caused by a combination of formerly rare risk factors: high cholesterol, high blood pressure, and chronic inflammation.

These harbingers of disease, in turn, are affected by genes but are mostly caused by the same interrelated behavioral risk factors we keep encountering: smoking, obesity, bad diets, stress, and physical inactivity.

Despite having run hundreds of marathons, often for charity, Dave is the first to admit he ate untold quantities of junk food for decades.

Cholesterol. A cholesterol test usually measures the levels of three molecules in your blood.

The first is low-density lipoprotein (LDL), often termed bad cholesterol. Your liver produces these balloon-like molecules to transport fats and cholesterol throughout your bloodstream, but some LDLs have a harmful tendency to burrow into the walls of arteries, especially when blood pressure is high.

The second type of cholesterol is high-density lipoprotein (HDL), sometimes called good cholesterol, because these molecules scaven...

The third type are triglycerides, fat molecules that are floating freely in the bloodstream and a sig...

To make a long story short, diets rich in sugar and saturated fats contribute to cardiovascular disease because they promote high levels of plaque-forming LDLs. Conversely, physical activity helps prevent cardiovascular disease by lowering trigl...

Blood pressure. A blood pressure test gives you two readings: the higher (systolic) number is the pressure your heart’s main chamber overcomes when it squeezes blood throughout your body; the lower (diastolic) number is the pressur...

Blood pressures above these values are concerning because, unabated, they damage the walls of arteries, making them vulnerable to invasion by plaque-inducing LDLs. As we already saw, once plaques start to form, blood p...

Inflammation. Plaques don’t form out of the blue but instead occur when white blood cells in the bloodstream react to the inflammation caused by LDLs and high blood pressure.

Instead, it is common knowledge that to prevent hypertension, coronary artery disease, and other problems, start by not smoking and by avoiding too many processed foods rich in sugar, saturated fats, and salt.

As we have seen before, shorter doses of high-intensity cardio appear to be as good as if not more effective than lengthier doses of low-intensity cardio.

While cardio unquestionably invigorates and strengthens the cardiovascular system, lifting weights also improves cholesterol levels (raising HDLs and lowering LDLs) and lowers resting blood pressure (although not as much as cardio).

That said, doing only weights is apparently less protective than only cardio for the cardiovascular system.

In contrast, resistance athletes like football linemen develop thicker and stiffer hearts that can manage high pressures but are less able to handle the high volumes necessary for cardio exercise. Thus, athletes who exclusively weight train without also doing some cardio appear to be at as much risk as sedentary individuals of developing chronic high blood pressure and cardiovascular disease.

Bottom line: weight training isn’t bad, but don’t skip the cardio.

In addition, exercise appears to slow the rate at which the immune system deteriorates as we age.

The immune system is labyrinthine with a multitude of different components that usually work in marvelous coordination but occasionally work at odds with one another. As allergies and autoimmune diseases like lupus attest, on rare but consequential occasions, immune responses that are meant to protect us can turn against our own bodies.

It is also unclear why physical inactivity might be a mismatch for the immune system apart from the generally negative effects of sedentariness on overall health and levels of stress (which, as we have seen, depresses the immune system).

One possibility is that because heading off to the bush to hunt and gather potentially made our ancestors more likely to encounter pathogens, our immune systems evolved to compensate by ramping up our defenses when we are active.

A related possible explanation stems from the stingy way our bodies use calories. The fatigue we experience when fighting a cold is a reminder that the immune system is often energetically costly. As a result, maybe our immune s...

According to this idea, long-term physical inactivity depresses immune competence, moderate levels boost the immune system, and very high doses of physical activity temporarily compromise immune function, thus increasing vulnerability to infection, especially in unfit individuals.

Altogether, it appears that regular, moderate physical activity increases the immune system’s capacities, but how much is optimal and for which contagious diseases is poorly understood.

Because the immune system requires plentiful energy when it is fighting at full force, extreme bouts of exercise may reduce the calories needed to combat invading pathogens.