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General: There are several theories as to why the body loses functioning as it ages. Researchers believe that several factors may be involved. Cross-linking theory: The cross-linking theory of aging is based on the observation that as individuals age, the proteins, DNA, and other molecules in the body develop inappropriate attachments, or cross-links, to one another. These unnecessary links decrease the mobility of protein and other important molecules in the body. When proteins become damaged or are no longer needed, an enzyme called protease destroys them. However, cross-linkages prevent protease from doing its job. As a result, dysfunctional and unneeded proteins remain in the body and can cause damage. For instance, cross-linking of the skin protein called collagen has been show to be partly responsible for wrinkled skin. Cross-linking proteins in the lens of the eye has also been shown to cause age-related cataracts. In addition, it has been suggested, but not proven, that cross-linking of proteins in the walls of arteries or the kidneys may be partly responsible for hardening of the arteries (atherosclerosis) and age-related kidney dysfunction. Although many scientists agree that cross-linking plays a role in aging, there is not enough scientific evidence to confirm that this process is a primary cause of aging. Free radical theory: The most common theory of aging is called the free radical theory. Many researchers believe that molecules, called free radicals, damage the body's tissues. Free radicals are produced when the body fights against infections. Although free radicals are needed for the body to produce energy, maintain immunity, transmit nerve signals, produce hormones, and contract muscles, they may also contribute to the process of aging. Free radical damage begins at birth and continues through adulthood. However, when individuals are young, the effects are minimal because the body has many different ways to repair and replace cells to maintain proper functioning. As individuals age, the damage caused by free radicals increases. Studies have shown that free radicals attack the structures of the body cells and create substances called lipofuscins. When lipofuscins build up in the body, they show up as darks spots on the skin, which are commonly called age spots. Lipofuscins also interfere with the body's ability to repair damage cells and reproduce new ones. As a result, lipofuscins lead to decreased energy levels and they prevent the body from building muscle mass. They also destroy enzymes that are needed for daily functioning. Studies have also shown that free radicals attack substances in the body called elastin and collagen. These substances help keep the skin smooth, moist, and flexible. As a result, free radicals may cause changes in the appearance of the skin, such as folds or wrinkles. Genetic theory: It has been suggested that an individual's genetic makeup regulates the rate at which he/she ages. This is called the telomerase theory of aging. However, it does not necessarily mean that patients will live to be the same age as their parents. This is because many other factors, including diet and lifestyle, may influence a person's internal biological clock. Telomeres are proteins on the ends chromosomes that carry genetic information. Every time a cell divides in the body the telomeres shorten. The shortening of telomeres is believed to cause cellular damage because the cell is unable to make a correct copy of itself. Over time, this process leads to cellular dysfunction, aging, and death. Wear and tear theory: According to the wear and tear theory of aging, tissues and organs eventually die after years of damage. An individual's genetic makeup (DNA) sustains repeated damage from toxins, radiation, and ultraviolet light throughout the course of a lifetime. Although the body can repair DNA damage, not all of those repairs are accurate or complete. As a result, damage slowly accumulates over time. For instance, one study evaluated the effect of a lifetime of exposure to stress hormones, such as cortisol. The body produces cortisol in response to physical and emotional stress. Researchers found that the amount of cortisol in the body rises with age. Although cortisol levels decline at night in younger adults, the levels do not fall as far in older adults. The researchers concluded that the increased levels of cortisol might be the result of wear and tear of lifelong exposures to stress.
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