Ideology with evidence: global warming, maps and ethics.

The world is getting fatter and apart from the usual reasons for being worried about this—vast increases in diabetes and cardiovascular diseases—Edwards and Roberts have given us a new one. They have calculated the energy costs of food production and transportation arising in two hypothetical populations, in which one has a 3.5% prevalence of obesity (reflecting UK in 1970) and the other of 40% (UK in 2010). The authors claim an increase in greenhouse gas emissions of between 0.4 and 1.0 Giga tonnes of carbon dioxide equivalents that was readily translated for environmentally naı̈ve readers by The Sun— ‘FATTIES CAUSE GLOBAL WARMING’ as a front page headline. It would be tempting to suggest that air companies should levy an additional carbon tax on obese passengers but the underlying message is the need to reduce the population distribution of energy consumption downwards—something for which we all need to take responsibility—rather than target those at the top end of the distribution. Powles provides a commentary that highlights the need for considering what we eat as an integral part of policy on climate change but notes the reluctance of affluent Western politicians and the public to consider a halving of their meat consumption in order to move towards ‘contraction and convergence’ policies for global control of green house gas emissions. Obesity is associated with several cancers but whether this is cause or effect remains debatable because of confounding and reverse causality. In this issue, Brennan et al. have used a Mendelian randomization approach to examine the question. The recent discovery of the FTO gene has opened up new opportunities for studying the effect of obesity on cancers (and other diseases) as it is both a common polymorphic genetic variant and is also strongly associated with obesity. Comparisons of cancer cases and controls by FTO genotype provides an estimate of the effect of obesity (as FTO risk alleles are associated with a 3 kg increase in body weight) on cancer risk that is unconfounded by age, smoking and other factors and not affected by reverse causality. Although the findings reported here suggest a protective effect of higher body mass index for lung cancer and an increased risk of renal cancer among overweight people, estimates are imprecise requiring much larger sample sizes and replication. Having seen some of the beautiful geographic information system (GIS) maps created by Tanser et al. at the Africa Centre in Kwa Zulu Natal on a visit there a few years ago, I was interested to read their paper on spatial clustering of HIV infection in rural South Africa. One of the early myths about the Africa Centre was that it was responsible for a cluster of HIV infection on its doorstep created by the social changes associated with the need to accommodate large numbers of new migrant staff in an underresourced rural area. Using sophisticated GIS methods (i.e. I do not understand them at all) they demonstrate markedly increased HIV prevalence in clusters E-mail: [email protected] Figure 1 Headline in The Sun, Tuesday 21 April 2009. The Sun 04/2009—reprinted with permission Published by Oxford University Press on behalf of the International Epidemiological Association