The social conditions of health and disease

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Let’s think differently about how the world we live in shapes health and disease
(Source: Odra Noel, Royal Society summer science exhibition 2013)

Ageing is associated with increased likelihood of developing health problems. We all know that. Whether it’s a specific disease, such as cancer, or a general decline in physical and cognitive functioning, the accumulation of years we spend on this earth results in wear and tear to our bodies. These adverse health effects of ageing are supported by robust scientific evidence. Several cancers often develop due to multiple “hits” to our genomes by environmental factors such as smoking, chemical exposures, and stress and these add up over our lifetimes to eventually result in genetic aberrations. Another interesting example is that of telomeres – the DNA sequences at the end of our chromosomes that protect against degradation during cell division. Every time one of our cells replicates, our double DNA strands unwind and a short piece of RNA called a primer binds to each single DNA strand. The little space at the end of each DNA strand where the RNA primer sits cannot be replicated – thus, telomeres are at the end of each DNA strand rather than actual genetic material, so that they bear the brunt of this loss of replicative ability, being slowly degraded in place of actual genetic code. But, I digress – if you want to read more about telomeres, click here (1), here (2), or here (3).

However, this evidence doesn’t mean that we should necessarily accept some health conditions as inevitable with ageing. Yes, we can agree that the passing of time certainly leaves physical marks on our bodies and minds, but the question I’d like to address in this blog is how much of the health effects often observed with ageing are due to our social and material environments? How much can we attribute some of the health concerns that we typically think of as being caused by ageing, and subsequently accept as being natural or inevitable, to the exposures that older people today have faced throughout their lifetimes? Because our world is constantly changing with each generation that inhabits it, the health exposures that we encounter often in turn vary by generation. Subsequently, comparing disease rates and exposures across birth cohorts may provide us with some clues to the causes of ill health and their historical explanations.

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Source: Cancer Research UK

The most classic example of an environmental exposure (when I say “environment”, I mean literally anything outside of the body – such as a chemical agent, a lifestyle practice, or even the broader social, economic, or political environment) and its related health outcome changing over time is cigarette smoking and lung cancer. See the above graph for the changing rates of cigarette smoking over the latter half of the 20^th century for men and women in the United Kingdom – and the striking mirroring of lung cancer rates 20-30 years later. Since women began smoking later than men – for reasons including social acceptability and appearance that also related broadly to feminist movements in the mid-20^th century – you can see how the curves are different for women. As you can see, lung cancer rates are decreasing among men, while we are still under the peak of high lung cancer among women due to the above social reasons.  Clearly, this historical explanation of smoking rates explains lung cancer rates today, which are of course socially contingent and not inevitable.

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Lung cancer rates by sex in the United States, 1930-1999 (Source: CDC Introduction to Epidemiology online course)

Although we don’t need these graphs to know that smoking causes lung cancer, this type of research is useful for understanding what we don’t know about health in contemporary times. In the early 1990s, there was an epidemic of suicides among young adults in Australia. Recently published research attempted to understand whether this rise in suicide rates was because Australians born in certain years were more likely to commit suicide, or whether there was something particular about Australia in the 1990s that made people commit suicide regardless of when they were born (the authors hypothesised that high unemployment rates were the culprit), or whether people are more likely to commit suicide as young adults, regardless of when they were born or when they died (4). Using statistical methods to separate out the effects of birth year, age, and time period, the study found that men born after 1970-74 were more likely to commit suicide than those born earlier (4). Men in this age group were subject to high unemployment and unstable employment, which may in part explain the suicide rates. A definite causal link cannot be made with this research, but it adds to our knowledge about what happened and can help with future predictions about the consequences of economic uncertainty.

Yes, there are physical effects of ageing, but we have to understand that the health problems we encounter today are not inevitable and always have historical and social reasons. Looking backwards in time to understand these links can give us clues about the causes of disease as well. There are an almost endless number of questions to be asked about society and health. For example, we know that obesity rates in many countries are rising. Are humans really more sedentary than we were twenty, fifty, or one hundred years ago? What has changed in our overall social structure to cause this tipping of our individual energy systems to be, on the whole, in a surplus? Or, how has the recent global financial crisis affected health for people in different parts of the world? Epidemiology is a powerful tool that, when used creatively and carefully, can help answer these questions and add to our understanding of the crazy world we live in.

References

1. Seigel LJ. Are telomeres the key to aging and cancer? http://learn.genetics.utah.edu/content/begin/traits/telomeres/ (accessed 09 September 2013).
2. Heidinger BJ, Blount JD, Boner W, Griffiths K, Metcalfe NB, Monaghan P. Telomere length in early life predicts lifespan. PNAS 2012;109:1743-8.
3. Shammas MA. Telomeres, lifestyle, cancer, and aging. Curr Opin Clin Nutr Metab Care 2012;14:28-34.
4. Page A, Milner A, Morrell S, Taylor R. The role of under-employment and unemployment in recent birth cohort effects in Australian suicide. Soc Sci Med 2013:93:155-62.