IJE Advance Access published online on July 16, 2008
International Journal of Epidemiology, doi:10.1093/ije/dyn152
Letter to the Editor |
Cities, urbanization and health
*Corresponding author. ARCHEPI Research Programme, Institute of Health and Society, Medical School, Framlington Place, Newcastle NE2 4HH, UK. E-mail: n.c.unwin{at}ncl.ac.uk
Leon1 presents a good summary of the recent history of urbanization and its effects on health and life expectancy, and we are pleased to see that he highlighted several misconceptions. We agree that data on the differences in adult mortality between urban and rural areas in low income countries are sparse, particularly so for non-communicable diseases. We would like to draw attention to some work that we have contributed to in Tanzania and Cameroon that does allow some comparisons between urban and rural areas. This work included running a demographic surveillance system for over 10 years in one urban and two rural areas of Tanzania, the Adult Morbidity and Mortality Project (AMMP), the presence of which facilitated epidemiological studies, including a small cohort study of rural to urban migrants. In Cameroon there have been surveys examining and comparing the prevalence of diabetes, hypertension and other cardiovascular risk factors in urban and rural areas.
AMMP conducted annual enumeration rounds of between 100 000 and 160 000 people in the two rural areas of Tanzania (Morogoro and Hai, respectively) and 6-monthly enumeration of approximately 65 000 in an urban area (Dar es Salaam).2,3 Independently, deaths were followed-up with verbal autopsy to determine the probable cause of death. This approach has been validated and shown to provide robust estimates of cause-specific mortality at the community-level.2 Outputs from this system included age-, sex- and cause-specific mortality rates and trends in mortality.
Table 1 highlights aspects of the mortality data from AMMP. Life expectancy at birth was greatest in the relatively better off rural area, and similar in the urban area and poor rural area. In adult men, however, death rates (summarized as probability of death between 15 and 59 years), were highest in the poor rural area. There were some notable differences in cause-specific mortality between the areas. Deaths from injuries were highest in the rural areas, maternal mortality in the urban area was between that in the two rural areas and death rates from stroke highest in the urban centre. Two prevalence surveys, one carried out in the AMMP area of Dar es Salaam and the other in Hai,4,5 demonstrated a significantly higher prevalence in the urban area in those aged 15 and above of: overweight and obesity (32 vs 4% in men, 44 vs 21% in women); diabetes, based on fasting glucose or on treatment (5.9 vs 1.7% in men, 5.7 vs 1.1% in women) and hypertension, based on blood pressure 
160/95 or on treatment (37 vs 26% in men, 39 vs 27% in women).
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It seems clear then that urban living in Tanzania is associated with a much higher risk of cardiovascular disease and diabetes. However, data from a cohort study we conducted of 209 adult rural to urban migrants from the rural Morogoro area to urban Dar es Salaam, with 209 age-, sex- and village-matched non-migrants suggests that this statement requires qualification.6 Over the first 6 months following migration both blood pressure and triglyceride levels fell. One plausible explanation for this counter-intuitive finding is that the diet in the urban area is more varied than in the rural area, with greater availability of fresh fruit, vegetables and animal protein. Our relatively crude food frequency data support this, and detailed nutrition data using biomarkers from South Africa in rural, periurban and urban populations also support this.7 However, against this background of a more varied diet, the rural to urban migrants in our study had lower levels of physical activity and showed a steady increase in weight of around a mean of 2 kg over the first year. The hypothesis that length of exposure to an urban environment, and not simply current urban residence, is a key determinant of associated cardiovascular risk is supported by work from Cameroon. In two cross-sectional surveys of adults, one rural and one urban, respondents were asked to indicate how much of their life had been spent in rural and urban areas. This was done using a chart on which place of residence was recorded across the life course. This study found that length of urban residence was related to body mass index, fasting glucose and diabetes independently of current place of residence (although this was not the case for blood pressure).8
In summary, we have drawn attention to data from rural and urban Tanzania and Cameroon that defy simple generalizations about the health impact of urban compared with rural living, and in general support the more complex picture painted by Leon. Arguably, greater urbanization in low income countries is an essential component of economic development9 and from this perspective is both inevitable and desirable. A better understanding of the complex relationships between urban living and health will provide a firmer basis for measures designed to improve health and prevent disease in the rapidly expanding urban centres of low and middle income countries.
References
1 Leon D. Cities, urbanization and health. Int J Epidemiol (2008) 37:4–8.
2 Setel PW, Unwin N, Alberti K, Hemed Y. Cause-specific adult mortality: evidence from community-based surveillance selected sites, Tanzania, 1992–1998. MMWR (2000) 49:416–19.[Medline]
3 Kitange HM, Machibya H, Black J, et al. Outlook for survivors of childhood in sub-Saharan Africa: adult mortality in Tanzania. Adult morbidity and mortality project. Br Med J (1996) 312:216–20.
4 Aspray TJ, Mugusi F, Rashid S, et al. Rural and urban differences in diabetes prevalence in Tanzania: the role of obesity, physical inactivity and urban living. Trans R Soc Trop Med Hyg (2000) 94:637–44.[CrossRef][Web of Science][Medline]
5 Edwards R, Unwin N, Mugusi F, et al. Hypertension prevalence and care in an urban and rural area of Tanzania. J Hypertens (2000) 18:145–52.[CrossRef][Web of Science][Medline]
6 Unwin N, McLarty D, Machibya H, et al. Changes in blood pressure and lipids associated with rural to urban migration in Tanzania. J Hum Hypertens (2006) 20:704–6.[CrossRef][Web of Science][Medline]
7 Vorster HH, Venter CS, Wissing MP, Margetts BM. The nutrition and health transition in the North West Province of South Africa: a review of the THUSA (Transition and Health during Urbanisation of South Africans) study. Public Health Nutr (2005) 8:480–90.[Web of Science][Medline]
8 Sobngwi E, Mbanya J-C, Unwin NC, et al. Exposure over the life course to an urban environment and its relation with obesity, diabetes, and hypertension in rural and urban Cameroon. Int J Epidemiol (2004) 33:769–76.
9 Sachs J. The limits of convergence: nature, nurture and growth. Economist (1997) 343:19–24.
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