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Low body mass index and the associations with cardiovascular function in Africans : the PURE study
Venter, Herman Louwrens
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Cardiovascular disease is known as one of the leading causes of mortality worldwide, where low income countries or developing countries have the highest prevalence of cardiovascular disease. One of the main reasons for this statistics is acculturation that leads to changes in behavioral lifestyle and malnutrition within these countries. Low body mass index was found to be an independent risk factor for cardiovascular disease in several studies. From literature it is found that body mass index is lower than the ideal body mass index and is associated with cardiovascular disease. According to Higashi (2003) a body mass index of 22.2 kg/m2 is associated with the lowest morbidity. If body mass index decreases to lower values than the ideal body mass index, a J–curve will be evident suggesting higher prevalence of cardiovascular disease associated with low body mass index. These findings imply that not only high body mass index but also a low body mass index may be a risk factor for cardiovascular disease, morbidity and mortality. Whether low body mass index is associated with cardiovascular risk in an African population remains unclear. Objective: The aim of this study was to investigate the possible associations of low body mass index with variables of cardiovascular function in Africans, with a low socio–economic status. Methodology: This prospective cohort study (N= 2 010) is part of the Prospective Urban and Rural Epidemiology study (PURE) conducted in the North–West Province of South Africa in 2005, where the health transition in urban and rural subjects was investigated within an apparently low socio–economic status group. Our crosssectional PURE sub–study included 496 African people from rural and urban settings, (men, N= 252 and women, N= 244) aged between 35–65 years and body mass index lower than 25 kg/m2. Subjects were sub–divided into two groups. The first group consisted of Africans with a low body mass index smaller or equal to 20 kg/m2 (men; N= 152, women; N= 94) whilst the second group consisted of Africans with a normal body mass index larger than 20 kg/m2 and smaller or equal to 25 kg/m2 (men; N= 100, women; N= 150). Systolic blood pressure and diastolic blood pressure measurements were obtained with the validated OMRON HEM–757 device. The pulse wave velocity was measured using the Complior SP device. Blood was drawn by a registered nurse from the antebrachial vein using a sterile winged infusion set and syringes. Analyses for cholesterol, high density lipoprotein, triglycerides, gamma–glutamyl transferase and high sensitive C–reactive protein were completed utilizing the Konelab 20i. Data analyses were performed using the Statistica 10 program. Statistical analyses were executed to determine significant differences between age, body mass index and lifestyle factors as well as cardiovascular related variables in the different groups. T–tests were used to determine significant differences between independent groups. ANCOVA tests were used to determine BMI group differences independent of age, smoking and alcohol consumption. Partial correlations, which were adjusted for age, smoking and alcohol consumption, determined associations between the BMI groups and cardiovascular variables. Results: Our results indicated significantly higher mean values for the African men, with low body mass index, for cardiovascular variables (Diastolic blood pressure, 88.0 ± standard deviation (SD) 13.4 mmHg; mean arterial pressure, 103.8 ± SD 14.4 mmHg and carotid–radial pulse wave velocity, 12.6 ± SD 2.47 m/s) compared to the normal body mass index group (Diastolic blood pressure, 84.2 ± SD 12.2 mmHg; mean arterial pressure, 100.0 ± SD 13.2 mmHg and carotid–radial pulse wave velocity, 11.6 ± SD 2.00 m/s). The African women with low body mass index had a significant difference for carotid–radial pulse wave velocity (11.3 ± SD 2.43 m/s) compared to the normal body mass index group (10.6 ± SD 2.10 m/s). In African men, after the variables were adjusted for age, smoking and alcohol consumption, we revealed that diastolic blood pressure (88.0 with confidence interval (CI) [86.0–90.0] mmHg) and carotid–radial pulse wave velocity (12.5 with CI [12.1–12.9] m/s) remained significant higher in the low body mass index group. Additionally, carotidradial pulse wave velocity was negatively associated with body mass index in African men. In the low body mass index group, Pearson and partial correlations of r= –0.204; p= 0.012 and r= –0.200; p= 0.020 were found respectively in carotid–radial pulse wave velocity. Furthermore, in our unadjusted scatter plot with body mass index versus pulse wave velocity this negative trend of increasing carotid–radial pulse wave velocity with decreasing body mass index was noticeable in both African men and women. Even when carotid–radial pulse wave velocity was adjusted for age, smoking, alcohol consumption, mean arterial pressure and heart rate, a J–curve between carotid–radial pulse wave velocity and body mass index was still evident. Conclusion: A detrimental effect of low body mass index is evident on cardiovascular function in Africans. If body mass index decreases from the optimum value of 22.2 kg/m2 to lower values, a J–curve is evident between body mass index and cardiovascular variables suggesting higher prevalence of cardiovascular disease associated with low body mass index. In our sub–study the carotid–radial pulse wave velocity increases significantly in African men with low body mass index, thus supporting the theory that stiffening of the arteries is evident in Africans with a low body mass index. Low body mass index may contribute to the high prevalence of cardiovascular disease mortality within developing countries and therefore, increase the risk for cardiovascular disease.
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