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Why Are We Living Longer?
References
Horiuchi, S.  2000.  Greater lifetime expectations.  Nature 405: 744-745.

Tuljapurkar, S., Li, N. and Boe, C.  2000.  A universal pattern of mortality decline in the G7 countries.  Nature 405: 789-792.

What was done
Tuljapurkar et al. examined mortality over the period 1950-1994 in the G7 countries - Canada, France, Germany (excluding the former East Germany), Italy, Japan, the United Kingdom, and the United States.

What was learned
The authors found that "in every country over this period, mortality at each age has declined exponentially at a roughly constant rate."

What it means
In a commentary on the work of Tuljapurkar et al., Horiuchi notes that the average lifespan of early humans was approximately 20 years, but that in the major industrialized countries it is now about 80 years, with the bulk of this increase having come in the past 150 years.  He then notes that "it was widely expected that as life expectancy became very high and approached the 'biological limit of human longevity,' the rapid 'mortality decline' would slow down and eventually level off," but he states the now obvious fact that "such a deceleration has not occurred."

"These findings give rise to two interrelated questions," says Horiuchi: (1) "Why has mortality decline not started to slow down?" and (2) "Will it continue into the future?"

Some points to note in attempting to answer these questions are the following.  First, in Horiuchi's words, "in the second half of the nineteenth century and the first half of the twentieth century, there were large decreases in the number of deaths from infectious and parasitic diseases, and from poor nutrition and disorders associated with pregnancy and childbirth," which led to large reductions in the deaths of infants, children and young adults.  In the second half of the twentieth century, however, "mortality from degenerative diseases, most notably heart diseases and stroke, started to fall," and the reduction was most pronounced among the elderly.  Some suspected that this latter drop in mortality might have been achieved "through postponing the deaths of seriously ill people," he said; but data from the United States demonstrate that "the health of the elderly greatly improved in the 1980s and 1990s, suggesting that the extended length of life in old age is mainly due to better health rather than prolonged survival in sickness."

These observations, says Horiuchi, have led many biologists who "used to think that senescent processes might be programmed into the biological clock of the body" to shift to the view that "senescence is mainly due to the body's imperfect systems of maintenance and repair, which allow the long-term accumulation of unrepaired damage in macromolecules, cells, tissues and organs."  Finally, he notes that "the prospect of life expectancy soon exceeding 80 years is limited to these [the G7] and other countries with highly developed market economies."

To summarize to this point, it appears that in countries with highly developed market economies, such as the G7 nations, where good health care is readily available, deaths of infants, children and young adults have been dramatically reduced over the last century or so, to the point where average life expectancy is now largely determined by what happens to elderly people; and it is evident that under these circumstances, the elderly are living longer and longer with the passing of time.  It is further evident that this latter phenomenon - which is an empirical reality - is likely due to ever-improving health in older people, which in turn is likely the result of continuing improvements in their bodily systems for repairing cellular damage caused by degenerative processes associated with old age.

Since these observations are common to all G7 nations, they must be the result of an ubiquitous phenomenon that is occurring simultaneously all across the earth and increasing in impact with the passing of time.  So what are the possibilities?

We can hone in on potential candidates by considering what is responsible for most cellular damage caused by degenerative processes associated with old age.  Prominent in this regard are the oxidant by-products of human metabolism, which contribute to a number of degenerative diseases common among the elderly, including cancer, cardiovascular disease and immune system decline, among other maladies.  And what counteracts these oxidant onslaughts that wreck havoc in the elderly?  A class of compounds called antioxidants.  Hence, a good place to start our detective work would be to look for some global phenomenon that is increasing with time and could increase the amount of important antioxidants and other health-promoting substances found in our food.

The first thing that comes to our minds in this regard is the historical and ongoing rise in the air's CO2 content.  Research in this area has been very limited, however; and little is known about the effects of atmospheric CO2 enrichment on human-health-promoting substances in plants.  A few studies have observed small increases in plant vitamin A and C concentrations in response to increases in the air's CO2 concentration; and a couple have identified CO2-induced increases in plant constituents proven to be effective in fighting a number of different human cancers, as well as cardiac insufficiency.  One of these studies is highlighted in another of our Journal Reviews: CO2 Enhances Plant Production of Cancer-Fighting Substances.  As more research results of this nature are published, we will be sure to highlight them in view of the intriguing possibility that the documented increases in longevity among the elderly living in countries of developed market economies may actually be due to the historical and ongoing rise in the air's CO2 content.


Reviewed 1 July 2000