Exercised : Why Something We Never Evolved to Do Is Healthy and Rewarding (9781524746995) by Lieberman Daniel

Author:Lieberman, Daniel [Lieberman, Daniel]
Language: eng
Format: epub
Publisher: Random House Digital
Published: 2021-01-05T00:00:00+00:00

The Essence of Senescence

Sometimes when I look in the mirror, I don’t recognize the gray-haired fellow with a receding hairline who stares back. Happily, I don’t yet feel as old as I look. Aging is inexorable, but senescence, the deterioration of function associated with advancing years, correlates much less strongly with age. Instead, senescence is also influenced strongly by environmental factors like diet, physical activity, or radiation, and thus can be slowed, sometimes prevented, and even partly reversed. The distinction between aging and senescence may seem obvious, but the two processes are frequently confused. Many conditions occur more commonly with advancing age, but only some are actually caused by age. Menopause, for example, is a normal consequence of aging that happens when a woman’s ovaries run out of eggs. In contrast, type 2 diabetes occurs among some older people for reasons not intrinsic to the aging process itself but instead from factors like obesity and physical inactivity whose damaging effects accumulate with age.

Stated differently, some aspects of senescence are neither inevitable nor universal.27 As we age, not all of us will get high blood pressure, dementia, or incontinence. In addition, some species seem immune to senescence. Bowhead whales, giant tortoises, lobsters, tortoises, and some clams can live and reproduce after hundreds of years. (The world record is a clam named Ming who researchers dredged up and, in the cruelest of ironies, then killed to determine it was 507 years old.)28 How and why do certain animals and humans, including those who exercise, tend to senesce more slowly?

At a mechanistic level, we senesce from a multitude of nasty processes that damage cells, tissues, and organs. One worrying source of wear and tear arises from the chemical reactions that keep us alive. The oxygen we breathe generates energy in cells but leaves behind unstable oxygen molecules with free, unpaired electrons. These reactive oxygen species (charmingly also called free radicals) steal electrons indiscriminately from other molecules, thereby “oxidizing” them. That theft sets off a slow chain reaction by creating other unstable, electron-hungry molecules obliged to steal electrons from yet more molecules. Oxidation burns things gradually and steadily. Just as oxidation causes metal to rust and apple flesh to brown, it damages cells throughout the body by zapping DNA, scarring the walls of arteries, inactivating enzymes, and mangling proteins. Paradoxically, the more oxygen we use, the more we generate reactive oxygen species, so theoretically vigorous physical activities that consume lots of oxygen should accelerate senescence.

A related driver of senescence is mitochondrial dysfunction. Mitochondria are the tiny power plants in cells that burn fuel with oxygen to generate energy (ATP). Cells in energy-hungry organs like muscles, the liver, and the brain can have thousands of mitochondria. Because mitochondria have their own DNA, they also play a role in regulating cell function, and they produce proteins that help protect against diseases like diabetes and cancer.29 Mitochondria, however, burn oxygen, creating reactive oxygen species that, unchecked, cause self-inflicted damage. When mitochondria cease to function properly or dwindle in number, they cause senescence and illness.

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