The Wear-and-Tear Theory of Aging

The wear-and-tear theory of aging asserts that the effects of aging are caused by progressive damage to cells and body systems over time. Essentially, human bodies "wear out" due to use. Once they wear out, they can no longer function correctly.

It is sometimes called simple deterioration theory or fundamental limitation theory. It's one of many theories on aging that continue to emerge. Some view aging as a more intentional process.

This article explores the wear-and-tear theory of aging, its foundational scientific premises, and both the evidence for and against it. It also reviews different aging theories that compare with it.

An Overview of the Theories of Aging

It's important to recognize that there are several distinct theories of aging. The wear-and-tear theory is just one.

Each theory has evidence for and against it. There are two primary categories of aging: programmed theories and error theories.

Programmed theories of aging believe that aging is a normal process, just as puberty is a normal developmental process. These include:

• Phenoptosis, a view that cells are programmed to die, leading to aging and death
• Endocrine (hormone) theory, meaning that hormone activity over time causes aging
• Immunological theory (and "inflammaging"), which view aging as an immune system response

Error theories of aging believe that aging is not something that is programmed to occur, but rather aging is due to a series of "accidents." These include: 

• Rate of living theory, which holds that your body holds only so many steps or heartbeats before breaking
• Free radical theory, which links aging to the DNA damage of "free radicals" acting on cells
• Protein cross-linking theory, based on chemical changes interacting with each other in the body
• Somatic DNA damage theory, which holds that your genetic makeup controls aging until death

Wear-and-tear theory of aging falls into this latter category of error theories.

Basic Tenets of Wear-and-Tear Theory

The wear-and-tear theory basically states that bodies wear out after time. This theory can easily make sense when considering inanimate objects, from cars to clothes, that wear out and become less functional with time.

Causes of Wear-and-Tear Damage

A wide range of internal and external factors can damage body systems. Exposure to radiation, toxins, and ultraviolet light can damage genes. The effects of the body's own functioning can also cause damage. When the body metabolizes oxygen, free radicals are produced that can cause damage to cells and tissues.

There are some cellular systems that don't replace themselves throughout life, such as the nerve cells of the brain. As these cells are lost, function eventually will be lost.

Within cells that continue to divide, the DNA can sustain damage and errors can accumulate. The simple act of dividing, again and again, shortens the telomeres of the chromosomes, eventually resulting in a senescent cell that can no longer divide.

Oxidative damage in cells results in cross-linking of proteins, which prevents them from doing the jobs they are intended to do in the cells. Free radicals inside mitochondria, the powerhouses of the cells in your body, injure their cell membranes so they can't function as well.

Evidence For and Against

It might simply seem that the wear-and-tear theory "makes sense" based on observations, but it's important to compare these gut feelings with what is scientifically known about the body and aging.

Under the microscope, there are some processes that support wear-and-tear as a factor in aging, but several other findings call question to this process. There is evidence both for and against this theory.

Supporting Evidence

The wear-and-tear theory of aging fits most closely with a perceived sense of aging. In fact, regardless of chronological age, the term "aging" is used to describe the progressive deterioration of a person or object.

On a broad level, the wear-and-tear theory fits closely with one of the fundamental laws of chemistry and physics, that of entropy. This law states that all systems tend toward a state of increased entropy or progressive disorganization.

Visually, there are structural changes with age in human skin and bones. On a cellular level, there are a number of functions which decline with age. Even with a good diet, cells have a decreased ability to absorb nutrients with age.

Contrary Evidence

The strongest evidence against the wear-and-tear theory is that bodies have a tremendous ability to repair the damage. DNA is equipped with DNA-repair genes (such as tumor suppressor genes) which work to repair genetic damage.

In addition, some studies have found that the aging process may be partially or completely reversed by simply changing the microenvironment of cells or certain hormonal factors. Of course, not all damage can be repaired fully, and mistakes in repair may accumulate over time.

Another argument against the wear-and-tear theory notes that organisms in their growth phase become stronger and stronger. Rather than starting out at the peak of performance, such as a car fresh off the assembly line or a new computer right out of the box, living organisms often start life fragile.

They build strength and resilience with age. They are able to repair and replace most broken parts themselves. Finally, there are some circumstances in which wear-and-tear actually extends life expectancy.

Yet another argument comes about when looking at the biochemical nature of the body. Why do lifespans vary so much between different species of animals? Why do some whales live almost twice as long as humans?

Going against progressive deterioration and wear-and-tear: Why do salmon, after a long upstream energetic swim, spawn and then die, seemingly at the peak of physical condition?

Bottom Line on What Causes Aging

The question of why aging occurs goes back much further than Weisman's theory proposed in the 1800s. Shakespeare addressed aging in his "seven ages of man," and religious texts have done so for millennia.

While the wear-and-tear theory at first seems most natural, and makes the most sense based on observation, it becomes clear that there is much else going on in the body that cannot be explained by this theory.

A more likely explanation, according to some researchers, is that the decrease of functioning that is seen as "tear" in the "wear-and-tear" theory is really the result, not the cause, of aging. Perhaps with an increased understanding of genetics, there will be better information on just what causes bodies to age.

What Does This Mean for You?

No matter which theory of aging is correct, or if instead aging is the total sum of several of these theories, the true bottom line is that aging is universal. Certain lifestyle factors may defer one's demise to a degree, and at the very least, offer a better quality to the quantity of a lifespan.

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