Like a Fine Wine?

If the human body could stay at the peak of health it reaches during early adolescence, then life expectancy would be 1,200 years! However, the actual life expectancy in the United States is 77.85 years and in Canada, 80.22 years, and the world's oldest person, Jeanne Louise Calment, lived only to 122. The cause is aging: A gradual process of deterioration that affects every cell, organ, and system in the body. In addition to a general decline in body function, growing older also makes us more susceptible to serious diseases and the risk of death doubles for every eight years of age.

The Biochemistry of Aging

So what are the root causes of aging? The surprising answer is eating and breathing. In a process called metabolism, the food we eat is converted into energy that is used by the body. Oxygen and glucose are key players in this process. However, both of them can also cause biochemical havoc within our bodies.

Free Radicals

Just as a fire needs oxygen to burn, our body needs oxygen to break apart, or oxidize, the chemical bonds in food. This process releases the energy that powers our bodies, but also creates harmful products called free radicals, pesky molecules that are believed to cause much of the damage associated with aging. Free radicals are unstable because they have an unpaired electron. In order to become stable, free radicals will rip electrons loose from other molecules, thus creating damaging the other molecule and setting off a chain reaction that creates more free radicals. Free radicals damage our DNA and other important molecules, which can lead to abnormal cell function and even cancer.

Excessive oxygen creates more free radicals, and therefore more cell damage. In one dramatic example, researchers showed that lab rats lived for two to three years in a normal environment, but only survived for three days when raised in a pure oxygen environment.

Browning

Like oxygen, the simple sugar glucose can also cause dangerous side effects. Meat browns when it's cooked because the glucose in the meat combines with proteins as the meat is heated. In a similar, but much slower process, browning also occurs within our bodies. Glucose sticks to proteins, and once stuck, it acts like two-sided tape that attaches to more spots. If glucose sticks to two spots on the same protein, it forms an abnormal loop or fold; if it sticks to spots on different proteins, it binds the two proteins together. Both scenarios interfere with protein's ability to do its job, and unfortunately, the process is irreversible. Our tendons and ligaments perfectly illustrate the process of browning. Made of pure collagen protein that last throughout our lifetimes, tendons and ligaments are glistening white in children, but turn golden brown in old age.

The link between glucose and aging was first observed in untreated diabetics. With high levels of glucose in their blood, diabetics exhibited signs of advanced aging, such as the stiffening of their lungs, joints, and artery walls. These patients were also vulnerable to diseases that commonly affect the elderly, such as cataracts, strokes, and heart attacks.

One term for the protein-glucose compounds created by browning is advanced glycosylation end products, or AGEs. As you've probably guessed, AGEs accumulate with age. AGEs have been found in Alzheimer's brain lesions, and may contribute to their formation. They may also play a role in heart disease by trapping LDL cholesterol ("bad" cholesterol) in artery walls and contributing to atherosclerosis. Even worse, the processes of browning and oxidation can reinforce each other, causing more harm than either process alone.

Damage Repair

The good news is that our bodies have very efficient systems to repair the damage that's inflicted on it. Biochemists Bruce Ames once calculated that the DNA in every cell is damaged by free radicals 10,000 a day. Given that the body is made up of 75 trillion cells, that adds up to a total of 10¹⁸ bits of DNA damage each day! What's even more amazing is that each day, almost every single one of those bits of damage is repaired. Antioxidants are the body's first line of defense against free radicals, and they work by transforming free radicals into less harmful compounds and preventing damage to cells.

After cell damage has already occurred, the body has many ways of repairing or minimizing the damage. Since DNA is a double-stranded molecule, a damaged strand can be re-created accurately by using the undamaged strand as a template. When a cell component becomes damaged or unnecessary, it is removed by garbage disposal units called lysosomes and peroxisomes. Finally, if the damage is too great to fix, the cell will commit suicide in a process called apoptosis. The genes that control cell suicide are also called tumor-suppressor genes, since they kill damaged cells that are more likely to divide uncontrollably and become cancerous. Even if the tumor-suppressor genes are damaged, potentially cancerous cells can still be destroyed by a special type of immune cells called killer T cells.

Signs of Aging

Breathing and eating are essential to life, but they also cause constant and unavoidable damage to our bodies. Over time, our repair system gradually fails to keep up, and the end result is aging.

Even at the cellular level, aging eventually leaves its mark. Compared to young cells, older cells will have more DNA damage and mutations. Another sign of cellular aging is a high amount of lipofuscin, a waste product that's left over after lysosomes have destroyed unwanted cell parts.

Even brand new cells from an old person and a young one bear a subtle difference – cells from an old person have slightly shorter chromosomes. This is because each chromosome is "capped" at its end by a long section of DNA called telomeres. Since a cell can't copy its DNA all the way to the end when it divides, telomeres ensure that none of the important DNA is lost. A little bit of the telomeres are lost every time a cell splits into two, and when telomeres become too short, cell division is forced to stop altogether.

To prevent the species from dying out, a baby must start his or her life with full-size telomeres. This is made possible by an enzyme called telomerase, which allows sperm and egg cells to divide without telomere shortening. By adding telomerase to the nutrient mixture, scientists have been able to create more "youthful" cells which lived longer and underwent many more cell divisions. However, don't expect telomerase to become the elixir of youth. Improper activation of the enzyme has been linked to cancer, since telomerase can help rogue cells to divide quickly and uncontrollably.

Moving onto a bigger scale, we can also see the effects of aging on every organ and system in the body. Our sense of taste diminishes as our number of taste buds decrease; our hearing is lost as the hair cells in our inner ears die off; and our vision worsens as the lenses of our eyes gradually stiffen.

When most cells die, a neighboring cell divides and easily replaces the dead cell. However, nerve and muscle cells lose the ability to divide around the time of our birth, which has long-lasting implications for our muscular and nervous systems.

To move a muscle, both muscle fibers and nerves must work together. Since both are irreplaceable, our maximum potential muscle strength and endurance starts to decline after age 30. However, diet and exercise can have a phenomenal impact on our actual muscle strength at any age, which is why some people are more fit in later life than they were in their youth.

The heart is made almost entirely of muscle, and therefore very vulnerable to damage. Throughout our lives, our arteries may slowly become stiff and clogged up, leading to cardiovascular diseases such as heart attack and stroke. Heart attacks occur when the coronary arteries, which supply blood to the heart itself, become blocked. Similarly, strokes occur when the blood supply to a part of the brain is disrupted, which can cause cell death and permanent brain damage.

Even for those who never suffer a stroke, brain cells are inevitably lost as we grow older. Fortunately, the brain has a built-in reserve capacity – in other words, we have more brain cells than we'll ever use. When nerve cells die, new connections are made among the surviving cells, thus taking over the functions once performed by the dead cells. The creation of new connections between nerve cells is also essential for learning, and fortunately, it's an ability we never lose. So although we may need more time to make a decision or develop a spotty short-term memory as we age, a healthy brain will always be able to learn, adapt, and translate all of our accumulated experiences into wisdom.

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For countless millennia, humanity has tried – and failed – to remain young forever. We can improve our physical condition and prevent disease by exercising, eating well, and maintaining a supportive network of friends and family, but that does not prevent us from growing old. Aging is the price we must pay for living, since the fundamental processes of life are what eventually wear us down.

However, age does not have to limit how we lead our lives. Astronaut John Glenn went into outer space at the age 76. Claude Monet produced his finest paintings at the age of 85. And Tina Turner continues to rock audiences worldwide at the age of 67. Comedian Jack Benny summed up the situation nicely with these words: "Age is a question of mind over matter. If you don't mind, it doesn't matter."

Source: Incredible Voyage: Exploring the Human Body (1998), published by the National Geographic Society. "Aging" chapter written by Steven N. Austad.

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