What causes aging?

The complexities of getting older make it difficult to pinpoint why one person ages well while another looks and acts older than his years. Are good health and fortitude passed down like blue eyes and blond hair? Or are they a product of the environment, including the food you eat, whether you have been exposed to harmful chemicals or infectious diseases, and how much you exercise? Both certainly play a role, but we don't yet know which has a more powerful influence.

­Genes are powerful predictors of health and longevity as well as disease and death, but they're only part of the story. If your parents and grandparents lived well into their nineties, chances are you will, too -- but not if you abuse your body along the way. (Scientists say all genetic bets are off once you've made it to age 80, however. After that, family history has little bearing on longevity.)

And if your father died young of a heart attack or your mother had breast cancer, you may be genetically predisposed to those diseases. Scientists on the Human Genome Project are continually discovering more genetic determinants of chronic and fatal diseases.

While genes partially determine who will develop chronic conditions that hasten the aging process, such as cancer and heart disease, there is no question that a healthy lifestyle is your weapon against the genes you've been dealt, or your ace in the hole if you've got good genes.

A man whose father and brothers died from heart disease in their forties and fifties may very well escape the same fate by exercising regularly and keeping his blood cholesterol levels and body weight in check. On the other hand, a man with no genetic predisposition to heart disease can certainly create heart problems by eating a high fat, artery-clogging diet and leading an entirely sedentary lifestyle.

Healthy living delays many of the body changes that aging brings. And it's never too late to start on the road to better health. Eating a nutritious diet goes a long way toward insuring good health. For instance, getting enough calcium and vitamin D at any age will retard the onset, and the progression, of osteoporosis, a bone disease that causes pain, fractures, hospitalization, and even death in the elderly.

If you're a smoker and you quit at any time, you decrease the chances of having a heart attack. And exercising or becoming more physically active improves lung function and lowers the risk for heart attack, no matter how old you are.

So what changes do your cells, tissues and body systems go through as you age? On the next page, we'll the biological process of aging.

Cells, the most basic body unit, are at the center of any discussion about aging. You have trillions of cells, and they're organized into different tissues that make up organs, such as your brain, heart, and skin.

Some cells, such as those that line the gastrointestinal tract, reproduce continuously; others, such as the cells on the inside of arteries, lie dormant but are capable of replicating in response to injury. Still others, including cells of the heart, nerves, and muscles, cannot reproduce. Some of these non-reproducing cells have short life spans and must be continually replaced by other cells in the body. (Red and white blood cells are examples.)

Others, such as heart and nerve cells, live for years or even decades. Over time, cell death outpaces cell production, leaving us with fewer cells. As a result, we are less capable of repairing wear and tear on the body, and our immune system is compromised. We become more susceptible to infections and less proficient at seeking out and destroying mutant cells that could cause cancerous tumors. In fact, many older adults succumb to conditions they could have resisted in their youth.

Though cell death is the basis for understanding the aging process, it is not the only factor. The aging process is incredibly complicated, and it's often difficult to distinguish between changes that are the result of time marching on and those that come with common medical conditions, including high blood pressure and heart disease.

Aging is the inevitable decline in the body's resiliency, which ultimately leads to dwindling powers, both mental and physical. Some aging changes affect us all. For example, diminished eyesight that necessitates reading glasses is considered normal, primarily because it affects everyone who lives long enough.

On the other hand, cataracts, which are formations on the lens of the eye that cloud your vision, can be prevented and are not considered part of the aging process, despite their prevalence in older people. To further complicate matters, organs age at different speeds. That's why a 50-year-old may hear as well as someone twenty years his junior, but may have arthritis or high blood pressure.

Theories abound about the underlying cause of aging. Some maintain that aging is preprogrammed into our cells, while others contend that aging is primarily the result of environmental damage to our cells. Although none of the theories can fully explain the process, they do help us better understand how we age. On the next page, we'll explore the most popular aging theories.

What's that sound? According to this theory, it's your biological clock, ticking away at a predetermined rate. This theory says that DNA, the cells' genetic material, holds the key to your planned demise from day one. While this aging theory appears fatalistic on the surface, remember that biology is not destiny. You can't change your genes, but you can slow the march of time with better nutrition and regular physical activity.

Your body produces hormones that help regulate myriad functions, including growth and behavior, reproduction, and immune function. In your youth, hormone production is high, but as you get older, hormone levels drop off, causing declines in the body's ability to repair itself and to keep functioning in top form.

Working cells produce waste. Over time, cells make more waste than they can possibly get rid of, which may wreak havoc on their ability to function and slowly lead to their death. Lipofuscin, or age pigment, is one of the waste products found primarily in some nerve and heart-muscle cells. Lipofuscin binds fat and proteins together in the cells. It accumulates over time and may interfere with cell function.

The protein collagen is at the heart of this theory. Collagen, akin to the body's glue, is one of the most common proteins making up the skin, bones, ligaments, and tendons. When we're young, collagen is pliable. But with age, collagen becomes more rigid, and it shrinks. That's why your skin is less elastic than before.

Aesthetics aside, cross-linking may block the transport of nutrients into cells as well as obstruct waste-product removal. Free radicals are destructive marauders roving your body, ready to pounce on healthy cells. They are produced as part of the millions of chemical reactions your body performs to sustain life.

Your body also makes them in response to environmental toxins such as excessive amounts of unprotected sunlight and cigarette smoke. Free radicals oxidize your cells (think rusting metal). As unbalanced, volatile oxygen molecules, they sacrifice healthy cells to make themselves more stable.

In doing so, free radicals destroy or alter DNA, the cell's genetic blueprint, and disrupt many other cell functions. Free radicals may kill cells as a result of their marauding, or they may give rise to mutant cells that can lead to chronic conditions including cancer and heart disease. Fortunately, the body maintains a sophisticated defense system against free radicals. Unfortunately, our defenses wane with time, and cell damage ensues.

This theory could also be called The Use It and Lose It Theory. The idea is that use, and overuse, of your organs pushes them to the brink of destruction. A poor diet, too much alcohol, and cigarette smoking are thought to accelerate natural wear and tear. With age, the body is less able to repair itself.

How does wear and tear occur? Free radicals, which inflict cellular damage, may be culpable. Similar to the wear and tear idea, this theory says you are born with a certain amount of energy. If you live "fast," you die young, because you use up your energy reserves sooner. "Laid-back people," who suffer from less stress and take life easier, would live longer should this theory prove correct.

A strong immune system is your body's most important defense against germs and toxins. White cells engulf and destroy potential pests such as bacteria and viruses. And they manufacture antibodies, the "soldiers" that patrol the bloodstream, attacking and disarming any substance they don't recognize as the body's own.

Problem is, the immune system becomes less efficient with time, and fewer antibodies are produced, increasing your infection risk. What's more, the body may turn on itself by producing antibodies that destroy its own tissue. When that happens, autoimmune disease, such as lupus and rheumatoid arthritis, is the result.

Although we still do not completely understand the process of aging, we do know quite a lot about it, as we've seen. To find out more about the aging process, check out the links on the next page.