Aging is commonly defined as the accumulation of diverse harmfull changes occurring in cells and tissues with advancing age that are responsible for the increased risk of disease and death (Harman 2003). The observation that most of the animals living in a natural environment rarely show that accumulation of diverse harmful changes occurring in cells and tissues simply because they die earlier for predation, disease, starvation, or drought indicates that aging is a phenomenon unique to humans.
“In other words, the advancing knowledge of hygiene and biomedicine has led us to discover the aging process, something that was teleologically not intended for us to be experienced (Hayflick 2000)”
Life expectancy is defined as the average total number of years that a human expects to live. Differently, life span is the maximum number of years that a human can live. While the human life span has substantially remained unchanged for the past 100,000 years at ~125 years, life expectancy has sensibly increased (~27 years during the last century), especially in Western Countries. The lengthening of life expectancy is mainly due to the elimination of most infectious diseases occurring in youth, better hygiene, and the adoption of antibiotics and vaccines. What is needed to increase lifespan is a topic of intensifying research.
The major theories of aging (the free radical theory, the immunologic theory, the inflammation theory and the mitochondrial theory) are all specific of a particular cause of aging. They provide useful and important insights for the understanding of physiological changes occurring with aging. But the search for a single cause of aging (such as a single gene or the decline of a body system) has recently been replaced by the view of aging as an extremely complex, multifactorial process. It seems that several processes simultaneously interact and operate at different levels of functions in the body. Therefore, different theories of aging should be considered as complementary in order to explain some or all the features of the normal aging process.
The most influential of current theories, the free radical/oxidation theory of aging is supported by a large and growing body of evidence. Free radicals are atoms or molecules that contain at least one unpaired electron. This makes the molecules chemically unstable and allows them to react easily with other compounds in the body. In so doing, free radicals and other reactive oxidants can cause extensive damage to cells and tissues, impairing the immune system and leading to infections and various degenerative disorders, such as cardiovascular disease. Perhaps worst of all, they can damage the DNA in our cells and put us at risk for cancer. Many researchers believe that the havoc that free radicals and other reactive oxidants wreak on our bodies is the basis for the aging process.
Lately however new areas of focus have started to gain traction. In particular around cell metabolism. For example the findings around Nicotinamide Adenine Dinucleotide (NAD+) which is a coenzyme found in all living cells. It serves both as a critical coenzyme for enzymes that fuel reduction-oxidation reactions, carrying electrons from one reaction to another, and as a cosubstrate for other enzymes such as the sirtuins and polymerases. These cellular NAD+ concentrations reduce during aging. Supplementation is shown to improve health span in mouse models of muscle aging and cognitive decline. The mechanism of action is not fully clear, but it may involve activation of sirtuin NAD-dependent protein deacetylases, along with enhanced mitochondrial function. Other sirtuin activators also improve health span and slightly extend life span in mice. It would appear science is coming closer to influencing the aging process and related illnesses.
In the side bar of this page you can find a description of the leading aging theories.
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