Season of birth and lifespan

OVER THE last fifty years, there has been a notable reduction in old-age mortality. Longevity of humans has risen substantially all over the world. The situation in India is striking; the average lifespan of an Indian has risen from less than 50 years to over 62 years within the last three generations. Factors such as increased immunity to pathogens, better nutrition, better public health and sanitation, and the discovery and use of effective drugs and vaccines are responsible for this increase in lifespan. The situation in Western Europe, US and Japan is even more striking, where people live up to their seventies, and are touching the 80-year mark. Clearly the world of tomorrow will be one of geriatrics. People above 60 years are expected to form almost half the population of the world. (My cousin Saraswati Pitchai refers to her colony in Indira Nagar, Chennai as "Sunset Boulevard". Every city in India, I suspect, has such boulevards). Adjustments need to be made in society in order to accommodate this demographic shift. Insurance companies are already recomputing their payment tariffs. Architects and building contractors are redrawing the interiors and accesses of housing colonies in order to suit the needs and conveniences of the greying occupants. Employees are rewriting rules of employment, retirement age and pension benefits.

It is not clear from the point of human biology as to how long a healthy human being can live. One can reframe this question from the engineering angle, considering the human body as a machine, taking into account the wear and tear of the parts, control of diseases and increased methods of protecting the body from them, methods of protection against interpersonal violence and so forth. What is the natural lifespan of the human body?

Natural lifespan of the body

In asking this question, it is important to emphasise the word natural. Surgeons today can replace worn out organs such as kidney, heart and liver.The number and diversity of such replaceable transplantable or implantable body parts will grow in time. With the new technology of generating tissues from stem cells, organ building and replacement may become possible in a short while. But these are aids, add-ons and new parts for old, somewhat like the famous axe of George Washington. (Visitors to his home are shown, as the joke goes, an axe that is claimedto be the one that the truthful little George used to cut down the cherry tree about three hundred years ago. When pushed, the tour guide tells you - "well, yeah, the handle has been changed but twice and the blade only once"). The lifespan of a natural, no- parts-replaced, human body is thought to be around 100-120 years. Claims made by some, soon after the human genome was mapped and sequenced, that men may live up to 600 years, are now regarded as wild and unsubstantiable. The current consensus on the matter, as the scientific debate covered in The Washington Post last fortnight, puts the natural longevity of humans to be no more than 120 years.

Why do we not live up to the programmed six score years? Wear and tear, disease, violence and accident take their toll. This is why persons such as Yayati, Bhishma and Methuselah are exceptions that are admired with a tinge of wistfulness, as also Markandeya who won the boon of being forever sixteen years old. It is worth recalling too that hardly 1000 generations ago, our ancestor cousins the Neanderthals did not get to live beyond 40 years of age. Their men did not live to see their hair turn grey and their women did not live to know menopause.

Live to 3, live to 73

Thanks to greater understanding of body physiology, nutrition and health, diseases and toxins and methods to protect and preserve bodily health, infant mortality has dropped substantially. So has old-age mortality. There is a direct connection between the two. Dr. Amartya Sen pointed out in his 1987 book Standard of Living that life expectancy at birth is a critical index of human health. A healthy infant has the chance of growing into a healthy adult. As the scientist- technologist Dr. S. Varadarajan succinctly remarked once- "if you live up to 3 you can live up to 73". The biologist Dr. James Watson made the same point when he said four years ago in Hyderabad: "children are pretty much indestructible". He meant thereby that the growing human body learns to defend its health in a variety of ways, and that it takes quite an effort to disable and destroy it.

Analysis of demographic features has pointed to the possibility that factors in early life affect mortality in late life. The British group of Dr. DJP Barker has shown associations between different measures of children's living conditions at birth and during the first years of life (pre- and post-natal life) and the prevalence of different diseases in adult life, especially heart disease. They suggest that nutrition in foetal and early life in crucial for latter day health and fitness. Environmental conditions in the early postnatal period seem to have a say in adult health and mortality. The Norwegian group of Forsdahl and Waaler have done similar studies but suggest that more data (longitudinal and covering all socioeconomic groups) would be worthwhile. The question yet to be sharply answered is, as title of the 1994 paper by the Oslo health scientists R. Nordhagen and L. S. Bakketeig asks: "In the beginning was the child. Is health and course of life determined already in utero and during early childhood?" The group of James Vaupel at the Max Planck Institute for Demographic Studies at Rostock, Germany also believe that further data needs to be collected before the Barker ideas can be totally accepted. They believe that both genetic and non-genetic factors account for increases in human lifespan.

The genetic factors would involve looking for longevity genes, disease susceptibility genes and so on, while the nongenetic factors would comprise food and nutrition, and environmental conditions. For example, the long lightless Arctic winters induce a state of morbidity and depression (termed Seasonal Affective Disorder or SAD) among some people living in Scandinavia.Infection in the tropics poses a constant health challenge to the people living there, debilitating many individuals and affecting their long term health and life span. Long term exposure to sunlight causes cataract of the eye lens, and some forms of skin cancer.

Birth month and lifespan

Dr. Vaupel and his colleague Gabriele Doblhammer have revisited this issue in a paper in the February 27, 2001 issue of The Proceeding of the National Academy of Sciences U.S. provocatively titled Life span depends on month of birth. Now, before you readers think that this in any way related to astrology and horoscopes, let us point out the rationale. They conjectured that the month of birth may be an indicator for environmental factors that are linked to the seasons of the year. As they point out, "If this conjecture is true, then the patterns of two geographically close population should resemble each other, and the pattern in the Northern Hemisphere should be mirrored in the Southern Hemisphere. Furthermore, lifespans of people who were born in the Northern Hemisphere but who died in the Southern Hemisphere should resemble the pattern of the Northern Hemisphere".

Against this background, Doblhammer and Vaupel decided to analyse the birth and death records kept in the appropriate government offices in Austria, Denmark and Australia. The choice of the countries was evidently based on the fact that they have four well-defined three-month- long seasons, have high degrees of public hygiene so that infectious diseases do not affect the population, and have well documented records of births and deaths. Australia has the convenient feature of having a large number of people originally born and raised post natally in Europe who have come and settled down as citizens. The investigaters obtained the population register data from Denmark, which followed every person living there from 1968 to the present. For Austria and Australia, they use information from death certificates for all deaths that occurred between 1988 and 1996, and 1993-97 respectively. They could also obtain data on as many as 43000 people born in Britain who died in Australia. The mortality follow up of all Danes who were at least 50 years old on 1.4.1968 was done over a 32 week period of the year 1998. Similar follow up of 50+ year-old- citizens was done with the other country data as well.

Was born in Autumn, lived a bit longer?

The results they deduced from the data are worthy of note. They found that the month of birth and remaining life expectancy at age 50 are related. In the Northern Hemisphere countries of Austria and Denmark, people born in autumn (October, November, December) live longer than those born in spring (April - June or the second quarter of the year). In Denmark, the average life expectancy at age 50 is a further 27.52 years (total life span of 77.52 years). Of these, analysis showed that autumn- born people lived about 0.31 years (114 days) longer than the spring-born people. In Austria, the average life expectancy at age 50 is +27.70 years (total lifespan of 77.7 years) and here the autumn- born people live a full 0.6 years (or 7 months or 219 days) longer than those born between weeks 14 and 26 of the year. The pattern in the Southern Hemispheric nation of Australia was the exact opposite or mirror image. The mean age at death of people born in Australia in the second quarter of the year is 78.00 years, while for those born in the fourth quarter it is 77.65 years. This difference of 0.35 years (about 4 months or 128 days) is statistically significant. Note that the seasons in the Southern Hemisphere are half a year out of phase from those in the North; when it is spring in Europe, it is fall down under.

What about people born and raised in their early years in the North, who moved and settled in Australia? For those born between March and May, the lifespan of such (British) immigrants is 0.26 years (3 months or 95 days) lower than that of native Australians; British immigrants born, between November and January lived 0.36 years (131 days or 4 months) longer than their Australian-born counterparts. British immigrants to Australia are in this respect similar to the Danes and Austrians born and living in the North.

In none of these four populations was there a difference in the monthly pattern between men and women. So this trend is sex- independent. There must be other factors to explain this relationship between the month or season of birth and remaining life expectancy at age 50. Doblhammer and Vaupel have attempted to test four hypotheses that might help explain the connection. The first hypothesis relates to the possibility that people born in April are older than those born in November when the high mortality due to winter hits them. Analysis of the seasonal distribution of deaths, taken from the Danish data as test, showed that differences by season of death cancel each other, whereas the differences by month of birth remain. So we need to look at other hypotheses for explanation.

The second hypothesis assumes that there may be socioeconomic differences in the seasonal distribution of births. For example, would couples prefer certain seasons to have children and would the intensity of the preference vary between social or economic groups? Using education as an indicator of social group, the data from Austria were analysed. Interestingly, it is seen that the spring peak in births is stronger among adults with high or medium education, whereas autumn births seem more in number among those with just basic education. It is important to add here that these data were of people who were aged 50+ and classified education- wise. It does not classify their parents on the basis of their education. Nevertheless, this spring-born for highly educated and fall-born for basic educated seems true elsewhere too. It was suggested as early as in 1941 that among the higher occupational classes, comparatively more births occur in spring and summer. And an analysis of the 1971 census of UK reveals that non-manual workers tended to be born in spring and manual workers in autumn and winter! We need to look into this apparent correlation and tease out the causative factors.

The third assumes that selective survival in the first year is the causal mechanism that explains the relation between month of birth and lifespan. It specifies that autumn- born infants suffer higher mortality in the first year. Such a specification would mean that the weaker ones do not survive beyond infancy, leaving the more robust ones alive and to experience lower mortality at later years. As a support to this argument, the authors point out that around the turn of the century (between 1865-1930), babies born in Vienna during September and November had higher birth weights, presumably because of better maternal and foetal nutrition and lived a healthier lives than those born in other months. It is also known that low birth weight is associated with increased cholesterol levels, higher systolic blood pressure and decreased lung function at adult ages. Seasonal differences in gestational age and birth weight are also attributed to the seasonal incidence of weight are also attributed to the seasonal incidence of infectious diseases of the mother during her third trimester of pregnancy.

The fourth hypothesis is related to the above, suggesting that weakness of the baby in utero or in the first year of life makes him or her susceptible to diseases in adult life. This would be in part genetic and in part environmental- due to nutritional and epigenetic factors. Here too, a connection with birth weight is reasonable to expect.

Based on these, Doblhammer and Vaupel have been able to rationalise why there has been a correlation between the birth month and lifespan of over a million Austrians, six hundred thousand Austrians and a quarter million Australians, all born in the early decades following the First World War.Their result that among these people, those born in autumn live a bit longer than those born in spring.

No longer true in the West

It is important to point out here that when such an analysis was done on more recent populations (the word used in this context is cohorts - "of the same field or farm"), this connection between birthmonth and lifespan became less pronounced. This is obviously because of substantial improvements in maternal and infant health. The pattern that obtained in the early years of 1900 is slowly vanishing with time, largely because of better sanitation, individual and public health measures. Thus, while the connection between season of birth and longevity will blur even more in coming years in the developed world, it would be of interest to look for such connections and locate causal factors in other parts of the globe. There are attendant problems in this regard. One is the paucity of data regarding birth dates and death records. The second is the situation that obtains in the developing world, which is largely in the tropics, namely infectious diseases. While some of these are seasonal, others are perennial. And then there are places where seasons are not sharply defined (remember the saying that Madras has three seasons - hot, hotter and hottest?). Then again, as we conquer one infection, others loom large and snatch precious lives from the cradle and the couch. AIDS is today the deadliest of them. Imagine an Africa where every 1000 children born, 300 die before they turn five years of age, and 700 die before they turn sixty. The life expectancy in Sierra Leone is 33 yrs, Niger 37 and Malawi and Zambia 38, while it is over 75 in Israel, Canada, Western Europe or Japan. (The situation was not so bleak in these African countries 50 years ago. The people there were a healthy and sturdy lot. It is the wars, the pestilence, the HIV). It hardly matters which month a child is born in any more. As Catherine Michard, Christopher Murray and Barry Bloom write in the February 2001 issue of JAMA in their article Burden of Disease - Implications for Future Research, the challenge for research in the 21st century is to maintain and improve life expectancy and the quality of life that was achieved for most of the world's population during the 20th century.

D. Balasubramanian, L.V.Prasad Eye Institute,Hyderabad - 500 034

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