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The change of lifestyle in an indigenous Namibian population group (Ovahimba) is associated with alterations of glucose metabolism, metabolic parameters, cortisol homeostasis and parameters of bone ultrasound (quantitative ultrasound).
The number of patients suffering from diabetes mellitus type 2 (DM) worldwide has increased rapidly over the past few years and it is expected that the numbers will increase further: from 171 million people suffering from diabetes in 2000, to 382 million in 2013 and to 592 million people by 2035 (ID...
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| Format: | Dissertation |
| Sprache: | Englisch |
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Philipps-Universität Marburg
2014
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| Zusammenfassung: | The number of patients suffering from diabetes mellitus type 2 (DM) worldwide has increased rapidly over the past few years and it is expected that the numbers will increase further: from 171 million people suffering from diabetes in 2000, to 382 million in 2013 and to 592 million people by 2035 (IDF 2013, Hossain et al. 2007). Until some time ago diabetes, especially type 2 DM, was seen as a disease of the more affluent and therefore of the western industrialised world (King & Rewers 1991). However, this picture is changing as more and more data from the developing world become available showing an alarming rise of the prevalence of DM in these countries.
Therefore, the aim of our study was to assess the association of lifestyle changes and modification of the social environment in the face of urbanisation on the risk for a disorder of glucose metabolism in the Ovahimba people of Namibia. Secondary to this, the cortisol homeostasis of the participants was investigated with the question whether urban compared to rural participants have a higher cortisol exposure due to increased psychosocial stress. In order to assess the cardio-metabolic risk profile of the Ovahimba, the prevalence of the metabolic syndrome (MetS) was ascertained and the participants’ 10-year cardiovascular risk was calculated according to the Framingham risk score (FRS). Lastly, the acoustical properties of bone of the study cohort were examined with the question whether lifestyle changes affect bone quality. No study with the above-mentioned questions has been conducted in the Ovahimba yet; therefore another aim of our study was to establish baseline data for further research.
In order to answer the above questions, two groups of participants were formed from the Ovahimba community in Namibia: ‘Group 1’, also called ‘urban group’ consisted of participants having been living in the town of Opuwo for at least three years, thus being subjected to urbanisation and ‘westernisation’. ‘Group 2’, also called ‘rural group’ consisted of participants living a traditional lifestyle in the rural area of the Kaokoveld in north-western Namibia. Each participant underwent the oral glucose tolerance test. In addition, anthropometric measurements were taken, the body fat and lean mass measured by body impedance analysis (BIA) and the fat metabolism examined to identify cardiovascular risk factors, assess the prevalence of the MetS and calculate the 10-year cardiovascular risk. In addition, two saliva samples were collected, one at sunrise and one at sunset to examine the cortisol homeostasis and a possible association with dysglycaemia, the MetS and low quality of bone. Lastly, bone ultrasound was used to determine the acoustical properties of bone of the participants.
In the first step of statistical analysis a descriptive analysis of the data was performed as a group comparison. Nominal variables were evaluated using Fisher’s exact test. A two-sided t-test was applied to the continuous variables with the null-hypothesis of equal mean values in both groups. A prevalence of 5% was taken as the basis for the statistical power. In a second step of statistical analysis, the data were adjusted for various confounders. In addition, a Mann-Whitney-U test was applied to the data of disorder of glucose metabolism, MetS and cortisol concentration to assess a possible association of cortisol concentration and the presence or not of dysglycaemia and MetS. In a third step of statistical analysis, the Pearson’s correlation was calculated for the cortisol area under the line (AUL) and metabolic parameters as well as bone ultrasound measurements.
The analysis showed significant differences in all anthropometrical data (age, height, weight, hip and waist circumference (WC), body mass index (BMI), systolic and diastolic blood pressure (SBP, DBP) before and after exercise and heart rate after exercise) except for ‘sex’, ‘diastolic BP after exercise’ and ‘heart rate at rest’. The characteristics of glucose metabolism showed significant differences of the fasting glucose (FG) and 2-hours glucose, but not of the HbA1c. The analysis of the primary question ‘disorder of glucose metabolism: yes/no’ presented a significant difference between the urban and the rural group (Group 1 28.3% vs. Group 2 12.7%), but this significance was not present in the individual components (diabetes mellitus type 2 (DM) 3.3% vs. 0.0%, impaired glucose tolerance (IGT) 18.3% vs. 7.9%, impaired fasting glucose (IFG) 6.7% vs. 4.8%). The investigation of the fat metabolism and BIA measurements showed significant differences for triglycerides, HDL-Chol, LDL-Chol before adjustment and lean body mass after adjustment. A significant difference was seen in the mean saliva cortisol concentrations at sunrise and sunset, the mean absolute decline of cortisol concentration and the cortisol AUL. The relative decline was not significantly different. The prevalence of the MetS was significantly higher in the urban group (Group 1 31.7% vs. Group 2 7.9%). Looking at the Framingham risk score, Group 1 showed a 10-year cardiovascular risk of 5.3 ± 5.3% and Group 2 of 5.5 ± 7.9%, with no significant difference. The analysis of the bone ultrasound measurements only showed two significant differences between the urban and the rural group: the SOS data after adjustment for ‘sex’, ‘age’ and ‘height’ and the SOS and SI data after adjustment for ‘sex’, ‘age’, ‘height’ and ‘weight’. The mean Z-score of the study cohort was +1.6 ± 1.6 standard deviations. There was only a weak correlation between milk consumption and walking time and bone ultrasound measurements.
The interpretation of the results of our study showed that the Ovahimba of Namibia are not spared the adverse effects of urbanisation and westernisation. The significant difference in the presence of a disorder of glucose metabolism between the urban and rural group supports the hypothesis that urbanisation concurrent with a change of lifestyle was associated with an increased risk of dysglycaemia in our study cohort.
Research has shown that psychosocial stress, a condition often associated with urbanisation leads to an alteration of the cortisol homeostasis with increased cortisol exposure. The urban participants had significantly higher mean concentrations at sunrise and sunset and a higher cortisol AUL, supporting our hypothesis that urban participants will present higher cortisol exposure due to increased psychosocial stress.
Another adverse effect of urbanisation is the high prevalence of metabolic and cardiovascular disorders. Our study population presented a prevalence of the MetS of 31.7% in the urban and 7.9% in the rural group, showing a significant difference. The calculation of the FRS showed that of the total study cohort, 48.0% were at low risk, 43.9% at moderate risk, 5.7% at high risk and 1.6% at very high risk of incurring a cardiovascular event in the next 10 years. This assessment tool showed no significant difference between the two study groups.
The acoustical properties of bone were measured to establish baseline data for future studies and to assess the impact of urbanisation on bone quality. Our hypothesis that urban participants will have a lesser bone quality could partially be verified. There were two measurements of bone ultrasound that showed a statistically significant difference between the urban and the rural group. Firstly, the SOS measurement after adjustment for ‘sex’, ‘age’ and ‘height’ and ‘sex’, ‘age’, ‘height’ and ‘weight’ (ps+a+h = 0.004, ps+a+h+w < 0.001); and secondly the SI measurement after adjustment for ‘sex’, ‘age’, ‘height’ and ‘weight’ (ps+a+h+w = 0.025). This significant difference in the SOS and SI measurement after adjustment could be the first indication that a change of lifestyle affects bone ultrasound measurements in the Ovahimba. The mean SI - Z-score of the whole cohort was 1.6 standard deviations above that for a person of the same age, gender and ethnicity. This indicates that the Ovahimba have greater bone strength compared to the Afro-Americans.
In conclusion, our study has shown that urbanisation is associated with an increased risk for a disorder of glucose metabolism in Ovahimba people. We could furthermore show that urbanisation is associated with an increased cortisol exposure, alterations of metabolic parameters including the metabolic syndrome and parameters of bone quality.
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| DOI: | 10.17192/z2014.0543 |