I had a discussion with a colleague at a forum I
attended. He claimed it will be possible
to extend human life expectancy so that we die at 120 or even 150. Length of life is tricky because there are
several life expectancies. Our present
mean life expectancy in the US for a newborn is about 85 years. When I was born in 1931 it was about 65
years. When my father was born in 1901
it was about 55 years. For a baby born
in 1776 it was about 35 years. Most of
that gain came from a reduction in infant mortality. In 1776 about half of all babies died in
their first year of life. Pneumonia,
gastrointestinal infections, and tuberculosis were the most common. By 1860 public health measures were becoming standard.
After the germ theory of 1880 became
widespread, pasteurization of milk, chlorination of water, and vaccinations
became common. Infants and children
lived. If we excluded infant mortality the
differences in survival to old age are not profoundly different from biblical
times (about 70 years) for most of human history until the twentieth
century. In that century antibiotics, antiseptic
surgery, and numerous medical processes (hormones, blood transfusions, prescription
medications for high blood pressure) could extend length of life. That is why a
baby today can live to about 85 years. The maximum length of a human life,
based on birth certificate evidence, is about 122 years.
We have no problem on the causes of aging of the cars we
drive. Every part of a car experiences wear and tear with usage, no matter how
often we bring our cars in for servicing.
If we are foolish and don’t routinely service our cars they break down
much faster. It could be problems with the engine, the transmission, the radiator,
the electrical system, the body frame, or a rusting through or crystallizing of
the metal components. No one would look for a single cause of what makes an old
car on its last years of usefulness. Yet
many people like to seek a single cause of aging in all of cellular life. At least eight major reasons have been
identified on why we get old and die.
First, our DNA in our chromosomes undergoes breakage from background
radiation, chemical agents we consume or inhale, or the by-products of our metabolism
in our cells. Chromosome breakage in
dividing cells often causes cell death. Second in importance, our DNA is
vulnerable to gene mutations arising from the same type of agents when they
alter chemical components of the DNA in our genes. We have repair enzymes to stop most of this
damage which occurs every day of our lives, but as we age cells get less
efficient and the damage accelerates. Third the tips of our chromosomes are capped
by telomeres. Each time a cell divides
the telomeres shorten. Human cells in tissue cell cultures can only divide
about 30 times and they stop. The one
major exception to this is certain cancer cells which can keep on growing
decades after the death of the person whose cells are in tissue culture. Fourth, genes are temporarily turned on or
off by coating with proteins or attachments of methyl groups. This is called epigenetics. As we age
different chromosomes get coated (methylated) and thus functions can shut down
in those cells. Fifth, our mitochondria
are found in all our cells and they do the real breathing, taking oxygen
brought from our lungs to our cells and they convert digested food into carbon
dioxide and water and produce abundant energy molecules for other activities
and generate the heat of our bodies. They
have their own DNA and the oxidative processes can severely damage their
genes. As we age our mitochondria are
fewer in number and less efficient, which is one reason old people feel colder
and have icy hands and feet. Sixth our
proteins as they are synthesized by our genes undergo folding. In older people that process of folding
becomes less efficient and the impaired folded proteins form tangles and this
can lead to Alzheimer syndrome in some and diabetes in others as important
proteins in the brain neurons or the pancreas are misfolded. Seventh stem cells are used to regenerate red
blood cells, white blood cells, the lining of our intestines, and several other
issues that are subject to mechanical wear and tear. As we age stem cells lose that capacity and
become fewer in number so we wrinkle and our bones deform with aging. The eighth reason why we age involves the communications
from cell to cell throughout our bodies.
There is “cross-talk” locally by diffusion and cross talk at greater
distances, especially by hormones. These
communications break down and become unreliable as our bodies age.
While some have argued that we are programmed to die, I
favor the interpretation that , like old cars, we just wear out from these
eight known processes of aging. I am
skeptical we can extend life to 120 years for the majority of adults without
considerable years of ill health. All
eight of these processes need to be addressed and just having the right diet
and the right exercise and right attitude is no guarantee for indefinite life
extension anymore than the well pampered car can last forever if driven to work
every day. So far, I am 83. The oldest person on my father’s side lived to
be 93. None of my maternal grandparents descendants
has lived to be 90. I’m not
complaining. Those 83 years have been
filled with adventure, love, learning, and creativity. I have also been lucky. I have never been hospitalized as an inpatient
for a major illness. But I know I am old.
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