The contribution of lifestyle to the risk of type 2 diabetes mellitus in Africans in transition in the North West Province

Abstract

Objectives: The comprehensive objective of this study was to investigate the lifestyle-related risk factors for type 2 diabetes mellitus in a sample of "apparently healthy" Africans of the North West Province of South Africa to consequently provide information for suitable prevention programmes. The hypothesis tested was that with urbanisation changes in lifestyle will increase risk factors for type 2 diabetes in Africans. The first objective was to determine the impact of urbanisation on the risk markers of glucose intolerance and type 2 diabetes and to investigate the influence of changes in lifestyle during urbanisation. The second objective was to characterise the occurrence and levels of the known risk factors for glucose intolerance and type 2 diabetes in the sample population. The third objective was to describe the unique characteristics of subjects that were identified with type 2 diabetes (fasting blood glucose concentration >7.1 mmol/1) at a relatively young age (<45 years). Background and motivation: South Africa is currently experiencing an urbanisation process coupled with westernisation that is characterised by changes in economical status, lifestyle and eating patterns. Worldwide this process is associated with an alarming increase in the incidence of diabetes mellitus and other lifestyle associated diseases (Hakeem et al, 1999). According to Steyn (1995) 28.5% of all deaths in South Africa can be attributed to chronic diseases of lifestyle and according to Mollentze et al (1993) South Africa is on the verge of a diabetes mellitus epidemic. The natural development of type 2 diabetes includes changes in glucose tolerance and insulin secretion (Bradley, 1991; Lebovitz, 1999). A degree of genetic predisposition and a wide variety of environmental factors are the common causes of most forms of type 2 diabetes (De Fronzo et al, 1997; Morwessel, 1998). Risk factors for type 2 diabetes include insulin resistance, obesity, fat distribution, changes in nutrient intakes, a decrease in physical activity levels and stress (Lebovitz, 1999). All the associated mechanisms are, however, not clear yet. It is possible that all population groups do not have the same risk factors. Therefore, it is important to study different ethnic and population groups.

The treatment of lifestyle associated diseases such as type 2 diabetes places a tremendous financial and labour burden on the already overburdened medical and health services. More information on the development and progress of these diseases and the risk factors associated with them, may result in better prevention. This may decrease the burden on the health services and increase the quality of life of potential patients. Study design: A cross sectional epidemiologic study design with a convenience - but representative sample of 1854 "apparently healthy" volunteers, 15 years and older, were used to investigate the risk factors of glucose intolerance and type 2 diabetes. Methods: This study was done as part of the TH USA (Transition and Health during Urbanisation of South Africans) project, with permission from the Ethics Committee of the PU vir CHO and in collaboration with the North West Department of Health. A representative sample of 1854 black residents of the North West Province voluntarily participated in the project, after giving informed consent. Pregnant and lactating women and subjects known to suffer from any disease or using chronic medication were excluded. These subjects were grouped according to age, gender and stratum (level of westernisation). The strata included rural people, farm workers, "squatters", urban middle class and urban upper class. All the participants underwent a glucose tolerance test. A glucose load of 50g was used. Fasting and two-hour post-load venous blood samples were taken. Serum was prepared for the determination of glucose and insulin concentrations. Glucose was determined enzymatically with the DAX Profile method and insulin with the /BL 125- insulin RIA method. The clinical profile, blood pressure, nutritional status, anthropometry, demographic information, medical history, smoking habit and alcohol use of the subjects were determined by skilled field workers and researchers, using standardised methods and tested questionnaires. The data were computerised and cleaned. Statistical analyses were performed using SPSS software and the Excel programme. Means and standard deviations were calculated and associations and relationships between risk factors and -markers were determined. Significant differences and correlations between continuous variables were calculated. Partial correlations were done to determine associations between the variables and fasting blood glucose, fasting insulin and two-hour glucose levels. Univariate analyses were used to calculate the effect of stratum on the markers of type 2 diabetes (serum glucose and - insulin levels), before and after controlling for lifestyle factors (age, smoking habit, body

mass index, waist-to-hip ratio and physical activity). The effect of stratum on these lifestyle factors was further determined with multivariate analyses. The values from the glucose tolerance test were used to divide the subjects who participated in 1996 in a normal glucose tolerance group, or in one of two glucose intolerant groups. The World Health Organisation criteria were used. The first glucose intolerant group consisted of subjects that would have been diagnosed as glucose intolerant according to the World Health Organisation. The second glucose intolerant group consisted of those subjects that had at least one value during the glucose tolerance test that fell within the limits of type 2 diabetes. Results: In general the results showed that these black subjects did not have the same patterns of risk factors or -markers for type 2 diabetes than other population groups. It further seemed that the risk factors or -markers differed between males and females. The effect of stratum (level of urbanisation) on the markers of type 2 diabetes was dependent on lifestyle factors (age, smoking habit, body mass index, waist-to-hip ratio and physical activity). In the men, physical activity was an important factor, while age, the smoking habit, waist-to-hip ratio and especially fasting insulin levels were important in females. The effect of stratum on the two-hour glucose level seemed to be dependent upon all the lifestyle factors that were tested for the males, but not for the females. This is not in accordance with the general accepted notion that changes in lifestyle during modernisation (westernisation) are, to a large extent, responsible for the development of type 2 diabetes. Type 2 diabetes mellitus was also observed in individuals younger than 45 years, especially in lean males. An analysis of the results showed that in females this phenomenon was probably the result of overweight. In the males it was not the result of overweight, but probably of an iron overload (in this case it is probably diabetes secondary to hemochromatosis and not type 2 diabetes). These findings have to be investigated in future studies and young, lean, black males with hyperglycaemia have to be tested for iron overload. Eating a typical Western diet, rich in saturated fats and animal protein did not emerge as a risk factor, probably because even the most westernised subjects, followed a prudent diet. A low intake of several micronutrients showed significant associations with glucose intolerance, suggesting that forms of malnutrition may be an important factor in these subjects. Conclusion: It was concluded that in these subjects, urbanisation and consequently westernisation, had a significant impact on the risk markers of glucose intolerance and type 2 diabetes, which were sometimes but not always, connected to the changes in certain lifestyle-related risk factors. The observed differences in the risk factors between males and females, as well as the role of iron overload in certain males, have important applications for the development of preventive and screening programmes, as well as for the diagnosis and treatment of type 2 diabetes in this population group.