why don't we just "live forever"? why do some people die sooner than others? what can we do to stay healthy and youthful, and live longer? what new technologies are being developed? will man someday be able to live forever, except for major accidents? these are the questions that originally stimulated my interest.
what does cause death, and why don't we just "live forever"?
about 100 years ago mathias schwann and theodore schweiden recognized that the cell is the
fundamental unit of all living organisms.
millions of cells make up our skin and our muscles. no matter what living organism we examine, we will always find that it is composed of cells. when cells get worn out, most of them divide and form new cells (post-fixed mitotic cells, such as nerve cells cannot). all of the basic biochemical processes take place or begin in the cell.
in youth during our growth period (up to about 18 years of age), the number of newly formed cells in our body outnumbers dying cells. in young adulthood, from about 18 to about 25 years of age, the number of newly formed cells balances the dying cells. in aging (after 25 years of age) the number of newly formed cells is less than the number of cells that die.
aging is cells dying faster than they are replaced, or losing some of their functioning.
in 1961 dr. leonard hayflick at stanford university discovered that human cells growing in a culture of energy and nutrients could reproduce themselves only a limited number of times before all of their descendants aged and died. the maximum number of times a cell can reproduce itself is now called its "hayflick limit".
each type of cell has its own hayflick limit. dr. hayflick also showed that long before cells ceased to reproduce themselves they showed certain changes in their structure and functioning, such as less ability to produce energy, less ability to make enzymes quickly enough, and more waste materials inside each cell.
so dr. hayflick concluded that these age changes in the cells play the central role in the expression of aging in the body, and result in the death of the individual well before all of its cells fail to divide.
death is caused by the loss of too many cells, or the loss of cell function. many theories have been proposed to explain how hayflick's limit is expressed in the cells in our bodies. all of them assume that aging represents a loss of control over various bodily processes, and many of them assume that the loss of control occurs at the cellular level in the dna of those cells.
the five major theories of why we age are:
1. the error hypothesis
2. the free radical theory
3. the cross-linkage theory
4. the brain hypothesis
5. the autoimmune theory
1. the error hypothesis:
the error hypothesis, or "aging by mistake", refers to the errors which may occur in the chemical reactions in producing dna, rna, or proteins, because the metabolic machinery is not 100% accurate. cell death can result from these unrepaired errors. some gerontologists attribute this error to any one, or a combination of the following: insufficient energy or nutrients, or insufficient carrying away of cell waste products.
2. the free radical theory:
the free radical theory refers to molecules, which have a strong tendency to link to other molecules, interfering with their functioning. they are produced by cells to assist in metabolism, most commonly in the "burning" of sugar. they are sometimes produced by accident if oxygen, always present in the cell and highly reactive, combines with cellular molecules. uncontrolled free radicals can cause accumulated damage to the membranes surrounding cells and to the cellular molecules of dna and rna. sufficient damage results in the eventual death of the cell.
at present the free radical theory is being hotly investigated. research on mice shows that a 40% reduction in calorie intake results in a doubling of their life span. when food is metabolized, free radicals are produced. our bodies do produce free radical "sponges" which absorb free radicals, but often not enough. the ingestion of vitamins e and c are especially good free radical absorbers.
dr. roy walford at ucla, a leading researcher in human longevity for many years and one of my idols during my schooling in gerontology, is himself practicing calorie reduction to inhibit the formation of free radicals.
3. the cross-linkage theory:
the cross-linkage theory states that the aging of living organisms is due to the occasional formation, by cross-linkage, of bridges between protein molecules in the dna which cannot be broken by the cell repair enzymes, interfering in the production of rna by dna. cross-linkages in protein and dna can be caused by many chemicals normally present in cells as a result of metabolism, and by common pollutants such as lead and tobacco smoke.4. the brain hypothesis:
the brain hypothesis, or breakdown of the brain pacemaker, refers to the theory that aging is due to a breakdown in the homeostasis of the bodily functions - especially in the control of the hypothalamus over the pituitary - which in turn causes a breakdown in control over the endocrine glands.5. the autoimmune theory:
the autoimmune theory, proposed by dr. roy walford at ucla hypothesizes that two types of white blood cells, b and t cells of the immune system weaken with age, and malfunction. b cells lose their vigor in attacking bacteria, viruses, and cancer cells, and the t cells lose their vigor in attacking cells foreign to the body, such as cancer cells and transplant cells. when b and t cells malfunction, they attack normal healthy body cells.
carl bourhenne, ma
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