Relationships between physical activity, cardiorespiratory fitness and sedentary behaviour, and risk factors for cardiovascular disease and type 2 diabetes, in black South African women
Non-communicable diseases (NCDs), including cardiovascular disease (CVD) and type 2 diabetes (T2D), constitute the second highest cause of mortality in South Africa (SA) and seem to be exacerbated by the high prevalence of obesity, particularly amongst black SA women. Although the aetiology of obesity is complex, common antecedents for its development include a sedentary lifestyle and poor nutrition. The overall aim of this thesis was to examine the association between physical activity (PA) and risk factors for CVD and T2D in a sample of apparently healthy black SA women. The aims of this thesis were addressed in two separate studies with the following objectives: Study 1: i) to compare body composition and metabolic risk factors for CVD and T2D between active and inactive groups classified according to international PA recommendations for health (Part 1, crosssectional analysis) and ii) to determine whether PA level predicts changes in body composition and metabolic risk factors for CVD and T2D over a 5.5-year follow-up period (Part 2, longitudinal analysis); Study 2: to examine the independent effects of PA, cardiorespiratory fitness (CRF) and sedentary time on body composition and metabolic risk factors for CVD and T2D (cross-sectional analysis). In part 1 of study 1, a sample of 240 apparently healthy black SA women(26±7 years) underwent the following measurements in 2005/6: PA (Global PhysicalActivity Questionnaire (GPAQ)), body composition (dual-energy x-ray absorptiometry and computerised tomography), blood pressure, fasting glucose, insulin and lipid concentrations. Thereafter (part 2), a sub-sample of women (n=57) underwent follow-up testing after a 5.5-year follow period (2010/11), which included additional measurements of objective PA (accelerometry) and CRF (VO2max, ml/kg/min) measured during a submaximal step-test. Study 2 included women from the follow-up subsample and 19 additional women (n=76). Cross-sectional comparisons of objective PA, CRF and sedentary time with body composition and metabolic risk factors for CVD and T2D were examined. Study 1: Using the GPAQ, the majority (61%) of women were sufficiently active, meeting the guidelines for moderate- to vigorous-intensity physical activity (MVPA) according to international criteria. Women who were active had significantly lower body weight (p<0.001), measures of body fat (BMI, fat mass, %body fat, waist circumference, central and appendicular fat mass, p<0.001), and measures of insulin resistance (fasting serum insulin, p=0.010 and HOMA-IR, p=0.010, respectively), and higher high-density lipoprotein cholesterol (HDL-C, p=0.041) compared to the inactive group. At follow-up, bodyweight increased from 82.0±19.6 kg to 89.5±19.2 kg (p<0.001) in the active group, and from 91.0±15.6 kg to 98.3±13.2 kg (p<0.001) in the inactive group, whereas serum lipid concentrations remained unchanged (p>0.05), and diastolic blood pressure decreased significantly in those who were active (78±7 vs. 74±14 mmHg, p=0.039). Study 2: Using accelerometry as an objective measure of PA, more than half (51.3%) of the women met international MVPA criteria and the goal of ≥10 000 steps per day (55.3%). Greater light PA and steps per day, but not MVPA, were associated with lower trunk (central) fat mass (r=-0.25, p=0.03, r=-0.31, p=0.01 and r=-0.09, p=0.42, respectively). Conversely, greater sedentary time was associated with higher TG and TG/HDL-C (r=0.36, p=0.01 and r=0.34, p=0.04, respectively), and these relationships were independent of body fat. In addition, higher CRF was associated with reduced body fat% (r=-0.34, p=0.02) and central fat mass (r=-0.31, p=0.03), as well as reduced insulin resistance (HOMA-IR; r=-0.41, p=0.01). These associations were independent of body fat and PA, but not VAT. CRF was inversely associated with sedentary time (r=-0.31, p=0.03) and not with any of the PA variables (p>0.05). Both PA and CRF level were associated with reduced total and central fat mass, and reduced metabolic risk for CVD and T2D amongst a sample of apparently healthy black SA women. Promotion of increasing daily PA, including light-intensity and MVPA, whilst reducing sedentary time, and increasing CRF should be encouraged to reduce levels of obesity and risk factors for CVD and T2D.
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