CARDIOVASCULAR JOURNAL OF AFRICA • Volume 33, No 2, March/April 2022 56 AFRICA that over 60% of the pooled studies were hospital based and barely 9% had a nationwide coverage, which may explain the subtle difference in the prevalence of low HDL-C, high LDL-C and high TC of the current study. In comparison with the report of Gebreyes and colleagues,24 estimates of low HDL-C (68.5%), high LDL-C (14.1%), hypertriglyceridaemia (21.0%) and hypercholesterolaemia (5.2%) indicate a close conformity with our findings. The survey by Gebreyes and colleagues was nationwide in coverage, involving 10 260 residents of Ethiopia aged 15–69 years, and was conducted using similar WHO-STEPS instruments. The increased burden of dyslipidaemia in Nigeria and other parts of Africa is unsurprising, given the rapid rate of urbanisation of rural areas.29,30 Population growth and industrialisation, including construction of roads, houses and industries, all reduce access to land for farming activities.31 Worse still, the little available land is cultivated using mechanised methods, which encourage sedentariness, thus fueling the burden of dyslipidaemia and various other CVD risk factors in Africa. In the last decade, Nigeria has witnessed a 20.2% increase in the rate of urbanisation, from 42.6% in 2009 to 51.2% in 2019.30 Consistent with other recent evidence32,33 but contrary to the NCD survey,22 we observed a higher risk of almost all forms of dyslipidaemia among rural compared to urban dwellers. In a cross-sectional survey of 136 945 Chinese adults aged 40 years and above, the burden of elevated TC and LDL-C was significantly higher in rural compared to urban dwellers, whereas HDL-C and elevated TG were similar between the two groups.32 The change in the pattern of rural–urban disparity of dyslipidaemia may be a reflection of ongoing epidemiological shift in CVD risk factors widely observed in many developing countries, especially in Africa.34,35 This event is not surprising as over the years, rural areas have continued to experience urbanisation at an alarming rate. Therefore, the pattern of exposure to social, environmental and behavioural risk factors of CVD among rural residents has increasingly become similar, with the unhealthy risk profile noted among their urban counterparts. The risk of CVD may therefore be aggravated among people residing in rural areas due to malnutrition and poor access to healthcare in such areas. In a meta-analysis assessment of rural–urban differences in CVD risk factors among middle-income countries, unfavourable risk profiles of cigarette smoking, alcohol intake, leisure time and active travel, physical activity among rural dwellers was a major highlight of that report.36 Similar to this report, the current study shows that the behavioural risk factors of dyslipidaemia, including physical inactivity, alcohol intake and cigarette smoking are comparable between rural and urban participants. Fruit and vegetable intake are surrogate measures of nutritional status inversely related to dyslipidaemia.37,38 Although rural–urban difference in nutritional status was not assessed in the current study, evidence from large-scale observational studies across the globe,39,40 including the Prospective Urban–Rural Epidemiology study (PURE),41 indicates a lower intake of fruit and vegetables among rural residents, who tend to prioritise energy-rich foods over fruit and vegetables, as the former is a cheaper source of energy and better satisfies hunger. According to a WHO survey of 10 sub-Saharan countries,40 intake of fruit and vegetables increased with per capita gross domestic product (GDP), with lower intake in rural compared to urban areas in all countries except Mozambique. In addition to low income, the quest to improve socio-economic status, lack of storage facilities, low level of education, and cultural norms and practices are many other factors potentially responsible for lower consumption of fruit and vegetables in rural dwellers. The change in rural–urban disparity of dyslipidaemia is also evident with other risk factors of CVD. For instance, our main findings from the REMAH survey15 indicate that the rural– urban gap in the prevalence of hypertension has narrowed, two decades after a wide difference of urban compared to rural dwellers was reported.22 Worthy of discussing in this report is the variation of almost all forms of dyslipidaemia with gender, a finding which agrees with the NCD survey22 and many other surveys within Nigeria10 and across the globe.32,42 Evidence from experimental and observational studies43,44 indicates that hormonal changes typical of post-menopausal women may be the underlying mechanism responsible for increased risk of dyslipidaemia in women. Sex hormones, especially oestrogens and progestogens, are antiatherogenic factors, well-established in the regulation of lipid and lipoprotein metabolism.44 In the current study, increased risk of almost all lipid abnormalities observed in women could be the consequence of a substantial participation of post-menopausal women, who accounted for nearly 40% of the women, with an average age of 62.1 ± 9.3 years. Increase in the prevalence of all but low HDL-C with age, dramatically peaking in subjects aged 41–50 years or 51–60 years, is an occurrence that has been widely reported in many studies.10,32,45 In a sample of 11 956 rural-dwelling Chinese adults of Liaoning providence, Sun and colleagues45 observed an increase in the prevalence of all lipid abnormalities with age, which peaked in subjects aged 55–64 years for TG. Ahaneku and colleagues10 observed a similar trend in a group of apparently healthy adults from a homogenous rural community of Enugu State, south-east of Nigeria. There was an increase in mean levels of all lipid fractions, which peaked in subjects aged 45–64 years. Advancement in age is associated with several metabolic changes, including increased lipolysis in adipocytes, reduced mass of metabolically active tissues, reduced oxidative capacity of tissues and reduced hepatic or extrahepatic LDL receptors, resulting in insulin resistance, hyperinsulinaemia and altered lipid metabolism, which all promote atherogenicity.46,47 An interesting finding from our study, which partially conforms with many other studies,45,48 indicated that hypertension and obesity were associated with increased risk of all lipid abnormalities except low HDL-C, which had a contrasting picture. Additionally, increased risk of high TG was associated with diabetes, compared to their non-diabetic counterparts. Compelling evidence2,45,49 has previously shown that lipid abnormalities, including elevated LDL-C, TG and TC and low HDL-C are well-established in the pathogenesis of atherosclerosis and obesity, to which hypertension and diabetes mellitus are secondary conditions. However, it remains obscure why a lower risk of low HDL-C was observed in hypertensive individuals despite the anti-atherogenic benefits that HDL-C confers via reverse cholesterol efflux. In recent times, HDL-C has become an object of intense scrutiny and research following evidence emanating from genetic, epidemiological and large-scale clinical trial studies, which have all questioned the benefit of increasing levels of
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