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CARDIOVASCULAR JOURNAL OF AFRICA • Volume 31, No 3, May/June 2020
AFRICA
143
the study, and informed consent forms to take to their parents/
guardians (written in both English and Setswana). If willing
to allow their child to participate, parents/guardians were then
asked to sign the consent form. Students agreeing to participate
signed assent forms.
Ethical approval for this study was obtained from the Ministry
of Health institutional review board [HPDME: 13/18/1 Vol. X
(152)]. Permits were obtained from the Ministry of Education
and Skills Development, local authorities in Okavango and
Gaborone and from each school administration.
Information on date of birth, gender, alcohol intake and
tobacco use, and the level of physical activity was obtained using
self-administered questionnaires. Personal and family history of
heart disease, hypertension, kidney disease, diabetes mellitus,
dyslipidaemia and stroke were also documented. Height was
measured in all participants without footwear to the nearest 0.1
cm using a stadiometer. Weight was measured using a digital
scale to the nearest 0.1 kg in light clothing and without footwear.
We used WHO AnthroPlus version 1.0.4 software to calculate
body mass index (BMI) for all participants aged below 18 years.
22
BMI
z
-scores according to age, gender and height were recorded
for each participant and designated as underweight [
z
-score < –2
standard deviations (SD)]; normal weight (
z
-score –2 SD – +1
SD); overweight (
z
-score +1 SD – +2 SD); and obese (
z
-score >
+2 SD). For participants ≥ 18 years, adult BMI reference values
were used for underweight (≤ 18.5 kg/m
2
), normal weight (18.5–
24.9 kg/m
2)
, overweight (25–30 kg/m
2
) and obesity (≥ 30 kg/m
2
).
23
Waist circumference (WC) was measured to the nearest
centimetre in light clothing at the level of the umbilicus using a
non-distensible measuring tape. Using the Canadian percentile
charts for WC based on gender and age, WC > 90th percentile
was categorised as overweight for students < 18 years.
24
For
students ≥ 18 years, adult cut-offs of 94 cm and 80 cm were used
for males for females, respectively.
23
After five minutes of rest, two seated blood pressure (BP)
measurements were taken from the participants’ right arms
using portable sphygmomanometers (BPCB0A–2H, China). The
second measurement was taken after a five-minute interval and
the average of the two BP readings was recorded. An average
systolic blood pressure (SBP) or diastolic blood pressure (DBP)
≥ 95th percentile for age, gender and height was used to define
hypertension. Pre-hypertension was defined as SBP and/or DBP
≥ 90th percentile but < 95th percentile.
A repeat blood pressure measurement was done after
one week for participants whose readings were consistent
with pre-hypertension and hypertension during the initial
measurement. Participants whose average SBP and/or
DBP remained high in the second visit were categorised as
hypertensive and pre-hypertensive as appropriate.
25,26
We also
defined hypertension among participants who self-reported
current antihypertensive medication use.
Fasting blood glucose (FBG) level was measured in mmol/l
on capillary blood from a finger-prick test using the Accu-
check Performa system (Roche Diagnostics, Mannheim,
Germany) following a minimum fasting period of eight hours
in participants not known to have diabetes mellitus. Using the
American Diabetes Association diagnostic criteria, participants
were classified as having normal fasting glucose levels (< 5.6
mmol/l), impaired fasting glucose (5.6–6.9 mmol/l) or diabetes
mellitus (≥ 7.0 mmol/l).
27
Alcohol use was defined as any reported alcohol consumption
in the previous year, while cigarette smokers were current
smokers. We assessed self-reported physical exercise duration and
intensity in the previous week (both at school and during leisure
time) to three levels of physical activity: inactive, minimally
active and health-enhancing physical activity.
28
Statistical analysis
The prevalence of hypertension and selected risk factors among
adolescents is unknown in Botswana. Consequently, the sample
size was calculated from the assumption that the prevalence of
hypertension in Botswana was 20%, similar to that found in
South Africa.
29
We needed 250 participants to determine the true
prevalence of hypertension with a margin of error of ± 5%.
Data were entered and analysed using SPSS for Windows,
version 23.0 (IBM Corporation). Continuous variables (fasting
blood glucose, height, weight, WC, SBP, DBP and age) were
summarised by means (± SD). Counts and percentages
summarised categorical variables. A Pearson’s chi-squared test
was used to compare the prevalence of selected cardiovascular
risk factors (hypertension, diabetes mellitus, smoking, obesity/
overweight, level of physical activity and alcohol use) between
urban and rural students.
For univariate analysis of continuous variables (fasting
blood glucose, height, weight, WC, age), the Student’s
t
-test
was used. A
p
-value less than 0.05 was considered statistically
significant. Variables that were variables with
p
< 0.25 in the
univariate analysis were included as independent variables for
the multivariable logistic regression.
Results
A total of 252 students (132 from Shakawe senior secondary
school and 120 from St Joseph’s College) participated in the
study (Table 1). Of these, 172 (68.3%) were females, and the
mean (SD) age was 17.1 (0.9) years. Students from the rural
school were older than those from the urban school (17.5 vs
16.7 years;
p
< 0.001). None of the participants had a history of
diabetes mellitus, stroke or dyslipidaemia.
Overall, obesity or overweight was observed in 10.3% of
students (12.5% in the urban school and 8.3% in the rural
school). Female students were more likely to be overweight or
obese than male students (Table 2). Underweight was found in 25
(9.9%) students, and was more prevalent in male than in female
students. There were no urban–rural differences in the prevalence
of underweight.
None of the study participants had diabetes mellitus. Impaired
fasting glucose was found in 1.6% of participants (all females),
1.7 and 1.5% among urban and rural school participants,
respectively.
Twenty-three (9.1%) participants reported drinking alcohol.
Urban students were more likely to drink alcohol than rural
students (14.2 vs 4.5%;
p
= 0.008). Smoking was rare in both
schools. However, male students were more likely to report
cigarette smoking than female students (0.6 vs 5%;
p
= 0.019).
There were
37.7% inactive students, and inactivity was more
common in Shakawe senior secondary school students than
those at St Joseph’s College. Physical activity did not vary with
gender in the two schools.