Hypertension among South African children in disadvantaged areas and associations with physical activity, fitness, and cardiovascular risk markers: A cross-sectional study

Figures & data

Table 1. Descriptive statistics of all study variables, for the total sample, in February and March 2018

Continuous variables	n^a	M^j	SD^j
Age (years)	785	12.4	0.9
Socioeconomic status (0–9)	754	7.5	2.0
Systolic blood pressure (mm Hg)	785	109.8	11.3
Diastolic blood pressure (mm Hg)	785	66.6	8.1
Weight (kg)	785	43.6	11.9
Height (cm)	785	149.6	8.1
BMI (kg/m^2)	785	19.3	4.2
zBMI^b	785	0.0	1.4
Skinfolds (mm)^c	784	11.0	6.3
Total cholesterol (mmol/L)	685	3.5	0.7
HDL cholesterol (mmol/L)^d	685	1.4	0.3
LDL cholesterol (mmol/L)^e	685	1.8	0.6
HbA1c (%)^f	735	5.5	0.2
20 m shuttle run performance (laps)	753	37.3	20.8
Cardiorespiratory fitness (VO2max [ml/kg/min])	753	44.5	5.6
Grip strength (kg)	783	19.4	5.0
Normalized grip strength (kg/weight [kg]*100)	783	45.7	10.0
Number of physically active days ^g	785	4.0	2.2
Total METs^h	751	1.7	0.2
Sedentary activity (min/day)	751	659.4	86.8
Light physical activity (min/day)	751	261.7	50.7
Moderate physical activity (min/day)	751	51.0	19.3
Vigorous physical activity (min/day)	751	22.2	15.8
MVPA (min/day)^i	751	73.1	32.8
	n^a	Md^j	Interquartile range (25^th-75^th percentile)^j
Triglycerides (mmol/L)	685	0.7	0.5–0.9
Categorical variables	n	%
Sex
Girls	402	51.2
Boys	383	48.8

^aNumber of learners varies due to different numbers of missing values in specific outcome variables.

^bBMI-for-age z-scores.

^cMean of all six measurements.

^dHigh-density lipoprotein cholesterol.

^eLow-density lipoprotein cholesterol.

^fGlycosylated haemoglobin.

^gNumber of days with ≥ 60 min MVPA per day.

^hMetabolic equivalent.

ⁱModerate-to-vigorous physical activity.

^jResults for continuous variables without serious violation of normality are presented with means (M) and standard deviations (SD), whereas variables with severe violation of normal distribution are presented with medians (Md) and inter-quartile ranges (25^th – 75^th percentile).

Table 2. Differences between children classified as normotensive, prehypertensive, and hypertensive (based on systolic blood pressure), after controlling for age, sex, socioeconomic status, and zBMI scores

		Normotensive		Pre- hypertensive		Hypertensive		Model 1: Uncontrolled, based on Welch test		Model 2: Controlled for age, sex, and SES		Model 3: Controlled for age, sex, SES, and zBMI
	n^a	M	SD	M	SD	M	SD	F	η^2	F	η^2	F	η^2
Systolic blood pressure (mm Hg)	785 (646/46/93)	106.2	8.5	120.8	2.4	129.8	5.2	407.1***^k	0.510	395.5***	0.514	360.7***	0.493
Diastolic blood pressure (mm Hg)	785 (646/46/93)	65.4	7.5	69.7	7.7	73.5	8.8	49.9***	0.113	47.1***	0.112	42.0***	0.101
Weight (kg)	785 (646/46/93)	32.6	10.8	44.5	12.2	50.1	16.0	16.8***^k	0.041	19.7***	0.050	–	–
Height (cm)	785 (646/46/93)	149.2	8.1	149.5	7.1	152.3	8.2	6.1**	0.015	7.6**	0.020	–	–
BMI (kg/m^2)	785 (646/46/93)	19.0	3.8	19.7	4.1	21.5	5.8	14.7***^k	0.036	17.0***	0.044	–	–
zBMI^b	785 (646/46/93)	−0.1	1.3	0.1	1.3	0.7	1.4	13.4***	0.033	15.7***	0.040	–	–
Skinfolds (mm)^c	784 (645/46/93)	10.6	5.8	12.1	7.2	13.5	8.0	9.7***^k	0.024	12.1***	0.031	0.6	0.002
Total cholesterol (mmol/L)	685 (569/40/76)	3.5	0.7	3.6	0.6	3.7	0.7	3.0	0.009	2.6	0.008	2.7	0.008
HDL cholesterol (mmol/L)^d	685 (569/40/76)	1.4	0.3	1.4	0.4	1.4	0.4	0.2	0.001	0.3	0.001	0.4	0.001
LDL cholesterol (mmol/L)^e	685 (569/40/76)	1.8	0.5	1.9	0.6	1.9	0.6	3.3*	0.010	2.9	0.009	2.0	0.007
Triglycerides (mmol/L)^f	685 (569/40/76)	0.8	0.3	0.8	0.3	0.8	0.4	0.7	0.002	0.3	0.001	0.2	0.000
HbA1c (%)^g	735 (606/43/86)	5.5	2.4	5.5	2.4	5.5	2.5	0.0	0.000	0.2	0.000	0.1	0.000
20 m shuttle run performance (laps)	753 (621/44/88)	38.2	20.8	24.3	21.0	32.7	19.3	3.2*	0.009	3.7*	0.010	0.5	0.001
Cardiorespiratory fitness (VO2max [ml/kg/min])	753 (621/44/88)	44.8	5.6	43.7	5.6	43.3	5.4	3.0*	0.008	3.3*	0.009	0.3	0.001
Grip strength (kg)	783 (644/46/93)	19.2	4.8	19.3	4.5	21.0	5.9	5.6**	0.014	7.5**	0.020	1.3	0.004
Normalized grip strength (kg/weight [kg]*100)	783 (644/46/93)	46.0	9.8	45.3	11.6	43.3	9.8	3.1*	0.008	4.0*	0.010	–	–
Number of physically active days^h	785 (646/46/93)	4.0	2.2	4.2	2.0	3.7	2.0	1.2	0.003	1.3	0.004	1.4	0.004
Sedentary activity (min/day)^i	751 (619/43/89)	655.6	87.1	654.5	89.1	688.5	78.7	5.8**	0.015	4.4*	0.012	4.3*	0.012
Light physical activity (min/day)^i	751 (619/43/89)	261.9	52.3	266.9	48.5	257.2	38.5	0.6^k	0.002	2.2	0.006	0.8	0.002
Moderate physical activity (min/day)^i	751 (619/43/89)	51.7	19.6	49.8	16.4	46.5	17.6	2.9	0.008	2.2	0.006	0.6	0.002
Vigorous physical activity (min/day)^i	751 (619/43/89)	22.8	16.0	21.3	15.9	18.0	13.9	3.8*	0.010	3.4*	0.010	1.5	0.004
MVPA (min/day)^i,j	751 (619/43/89)	74.5	33.3	71.1	29.3	64.5	29.4	3.7*	0.010	5.2**	0.014	1.1	0.003

^aNumber of learners varies due to different numbers of missing values in specific outcome variables and reflects n before controlling for covariates. Numbers in brackets correspond to learners classified as normotensive, prehypertensive, and hypertensive (based on systolic blood pressure).

^bBMI-for-age z-scores.

^cMean of all six measurements.

^dHigh-density lipoprotein cholesterol.

^eLow-density lipoprotein cholesterol.

^fVariable was log-transformed before carrying out analyses.

^gGlycosylated haemoglobin.

^hNumber of days with ≥ 60 min MVPA per day.

ⁱControlled for daily accelerometer wear time; across all models.^jModerate-to-vigorous physical activity.^kLevene test is statistically significant (p < 0.05), indicating that variances differ between groups. *p < 0.05. **p < 0.01. ***p < 0.001.

Table 3. Differences between children classified as normotensive, prehypertensive, and hypertensive (based on diastolic blood pressure), after controlling for age, sex, socioeconomic status, and zBMI scores

		Normotensive		Pre- hypertensive		Hypertensive		Model 1: Uncontrolled, based on Welch test		Model 2: Controlled for age, sex, and SES		Model 3: Controlled for age, sex, SES, and zBMI
	n^a	M	SD	M	SD	M	SD	F	η^2	F	η^2	F	η^2
Systolic blood pressure (mm Hg)	785 (646/46/93)	108.2	10.8	115.0	9.3	120.8	9.9	54.6***	0.123	49.5***	0.117	45.7***	0.109
Diastolic blood pressure (mm Hg)	785 (646/46/93)	64.3	6.0	75.5	1.4	81.7	6.2	354.3***^k	0.475	336.2***	0.473	327.2***	0.468
Weight (kg)	785 (646/46/93)	43.0	11.1	44.4	12.7	48.8	16.1	8.7***^k	0.022	7.2**	0.019	–	–
Height (cm)	785 (646/46/93)	149.3	8.0	149.1	9.0	151.6	8.3	2.8	0.007	2.4	0.006	–	–
BMI (kg/m^2)	785 (646/46/93)	19.1	3.9	19.8	4.8	21.0	5.7	7.5***^k	0.019	6.1**	0.016	–	–
zBMI^b	785 (646/46/93)	−0.1	1.3	0.2	1.4	0.4	1.5	4.7**	0.012	4.1*	0.011	–	–
Skinfolds (mm)^c	784 (645/46/93)	10.7	5.8	11.7	8.1	13.3	8.1	6.0**^k	0.015	4.8**	0.013	0.9	0.002
Total cholesterol (mmol/L)	685 (569/40/76)	3.5	0.7	3.4	0.7	3.7	0.7	4.3**	0.012	3.9*	0.012	3.8*	0.012
HDL cholesterol (mmol/L)^d	685 (569/40/76)	1.4	0.3	1.4	0.4	1.4	0.4	0.2	0.001	0.3	0.001	0.5	0.001
LDL cholesterol (mmol/L)^e	685 (569/40/76)	1.8	0.5	1.6	0.6	1.9	0.6	4.5**	0.014	4.4*	0.014	4.0*	0.013
Triglycerides (mmol/L)^f	685 (569/40/76)	0.8	0.3	0.8	0.4	0.9	0.6	4.6**^k	0.013	4.5*	0.014	3.8*	0.012
HbA1c (%)^g	735 (606/43/86)	5.5	0.2	5.6	0.3	5.5	0.2	8.1***^k	0.022	6.9**	0.019	7.2**	0.020
20 m shuttle run performance (laps)	753 (621/44/88)	37.9	21.1	35.6	17.0	33.4	19.2	1.7	0.004	0.9	0.003	0.2	0.001
Cardiorespiratory fitness (VO2max [ml/kg/min])	753 (621/44/88)	44.7	5.6	44.3	4.4	43.1	5.6	2.7	0.007	1.5	0.004	0.4	0.001
Grip strength (kg)	783 (644/46/93)	19.4	4.8	19.4	5.9	19.7	5.6	0.1	0.000	0.2	0.001	0.2	0.001
Normalized grip strength (kg/weight [kg]*100)	783 (644/46/93)	46.2	9.8	44.8	10.8	41.9	10.1	6.6**	0.017	5.3**	0.014	–	–
Number of physically active days^h	785 (646/46/93)	4.0	2.1	4.0	2.2	4.0	2.3	0.0	0.000	0.1	0.000	0.1	0.000
Sedentary activity (min/day)^i	751 (619/43/89)	660.4	84.1	634.2	104.2	664.8	97.8	1.9	0.005	4.9**	0.013	1.6	0.005
Light physical activity (min/day)^i	751 (619/43/89)	263.4	50.7	274.8	39.1	239.8	51.0	8.9***	0.023	5.0**	0.014	6.6**	0.018
Moderate physical activity (min/day)^i	751 (619/43/89)	51.5	19.5	52.7	18.4	45.4	17.4	3.5*	0.009	1.0	0.003	1.1	0.003
Vigorous physical activity (min/day)^i	751 (619/43/89)	22.6	16.2	23.7	12.4	17.6	12.7	3.7*	0.010	1.3	0.004	0.8	0.002
MVPA (min/day)^i,j	751 (619/43/89)	74.1	33.4	76.4	29.0	63.0	27.9	4.1*	0.011	1.4	0.004	1.1	0.003

^aNumber of learners varies due to different numbers of missing values in specific outcome variables and reflects n before controlling for covariates. Numbers in brackets correspond to learners classified as normotensive, prehypertensive, and hypertensive (based on systolic blood pressure).

^bBMI-for-age z-scores.

^cMean of all six measurements.

^dHigh-density lipoprotein cholesterol.

^eLow-density lipoprotein cholesterol.

^fVariable was log-transformed before carrying out analyses.

^gGlycosylated haemoglobin.

^hNumber of days with ≥ 60 min MVPA per day.

ⁱControlled for daily accelerometer wear time; across all models. ^jModerate-to-vigorous physical activity.

^kLevene test is statistically significant (p < 0.05), indicating that variances differ between groups. *p < 0.05. **p < 0.01. ***p < 0.001.

Table 4. Differences between children classified as normotensive, and hypertensive (based on combined systolic and diastolic blood pressure), after controlling for age, sex, socioeconomic status, and zBMI scores

		Normotensive		Hypertensive		Model 1: Uncontrolled, based on Welch test		Model 2: Controlled for age, sex, and SES		Model 3: Controlled for age, sex, SES, and zBMI
	n^a	M	SD	M	SD	F	η^2	F	η^2	F	η^2
Systolic blood pressure (mm Hg)	785 (646/46/93)	106.6	8.9	124.5	9.0	467.3***	0.374	441.3***	0.371	391.3***	0.345
Diastolic blood pressure (mm Hg)	785 (646/46/93)	64.5	6.4	76.0	8.5	331.7***^k	0.298	318.3***	0.298	302.0***	0.289
Weight (kg)	785 (646/46/93)	42.5	10.6	49.0	15.3	35.9***^k	0.044	37.0***	0.047	–	–
Height (cm)	785 (646/46/93)	149.0	8.1	151.9	8.1	14.2***	0.018	14.0***	0.000	–	–
BMI (kg/m^2)	785 (646/46/93)	19.0	3.7	21.1	5.5	30.1***^k	0.037	31.8***	0.041	–	–
zBMI^b	785 (646/46/93)	−0.1	1.3	0.5	1.4	24.9***	0.031	28.6***	0.037	–	–
Skinfolds (mm)^c	784 (645/46/93)	10.6	5.8	13.1	7.8	18.3***^k	0.023	20.4***	0.027	0.1	0.000
Total cholesterol (mmol/L)	685 (569/40/76)	3.5	0.7	3.6	0.7	3.5*	0.005	3.7*	0.005	4.0*	0.006
HDL cholesterol (mmol/L)^d	685 (569/40/76)	1.4	0.3	1.4	0.4	0.7	0.001	0.8	0.001	0.0	0.000
LDL cholesterol (mmol/L)^e	685 (569/40/76)	1.8	0.5	1.9	0.6	3.5 ^k	0.005	3.9*	0.006	2.7	0.004
Triglycerides (mmol/L)^f	685 (569/40/76)	0.8	0.3	0.9	.05	4.6*^k	0.007	4.2*	0.006	2.6	0.004
HbA1c (%)^g	735 (606/43/86)	5.5	0.2	5.5	0.2	0.7	0.001	0.5	0.001	1.0	0.001
20 m shuttle run performance (laps)	753 (621/44/88)	38.1	21.0	33.3	19.3	5.9*	0.008	6.4*	0.009	0.5	0.001
Cardiorespiratory fitness (VO2max [ml/kg/min])	753 (621/44/88)	44.8	5.6	43.3	5.4	8.2**	0.011	7.2**	0.010	0.7	0.001
Grip strength (kg)	783 (644/46/93)	19.2	4.8	20.4	5.7	7.3**	0.009	9.6**	0.013	0.7	0.001
Normalized grip strength (kg/weight [kg]*100)	783 (644/46/93)	46.2	9.9	43.2	10.0	10.8**	0.014	11.7**	0.015	–	–
Number of physically active days^h	785 (646/46/93)	4.0	2.2	3.9	2.2	0.6	0.001	1.0	0.001	0.8	0.001
Sedentary activity (min/day)^i	751 (619/43/89)	655.4	86.3	677.6	86.8	7.3**	0.010	13.2***	0.018	4.3*	0.006
Light physical activity (min/day)^i	751 (619/43/89)	264.2	51.1	250.1	46.9	8.7**	0.012	7.5**	0.010	6.2*	0.009
Moderate physical activity (min/day)^i	751 (619/43/89)	52.0	19.4	46.1	17.8	10.5**	0.014	7.6**	0.010	3.7*	0.005
Vigorous physical activity (min/day)^i	751 (619/43/89)	23.1	16.1	18.0	13.3	11.5**	0.015	10.8**	0.015	4.2*	0.006
MVPA (min/day)^i	751 (619/43/89)	75.1	33.3	64.2	28.9	12.5***	0.016	10.9**	0.015	4.4*	0.006

^aNumber of learners varies due to different numbers of missing values in specific outcome variables and reflects n before controlling for covariates. Numbers in brackets correspond to learners classified as normotensive, prehypertensive and hypertensive (based on systolic blood pressure).

^bBMI-for-age z-scores.

^cMean of all six measurements.

^dHigh-density lipoprotein cholesterol.

^eLow-density lipoprotein cholesterol.

^fVariable was log-transformed before carrying out analyses.

^gGlycosylated haemoglobin.

^hNumber of days with ≥ 60 min MVPA per day.

ⁱControlled for daily accelerometer wear time; across all models. ^jModerate-to-vigorous physical activity.

^kLevene test is statistically significant (p < 0.05), indicating that variances differ between groups. *p < 0.05. **p < 0.01. ***p < 0.001.

Figure 1. Percentage of children classified as normal, overweight, or obese, stratified by blood pressure status (normotensive vs hypertensive) (n = 785)

Figure 2. Percentage of children below or above physical activity recommendations (≥ 60 min MVPA per day), stratified by blood pressure status (normotensive vs hypertensive) (n = 751)

Figure 3. Association between normal, overweight, or obese and blood pressure status (normotensive vs hypertensive) among children classified below or above physical activity recommendation (≥ 60 min MVPA per day) (n = 751)