While osteoporosis is a major public health concern in the developed world, little research
regarding factors influencing bone mineral accrual in children has been conducted in
developing countries. South Africa is of particular interest since the incidence of hip fractures
in South African Blacks has been reported to be amongst the lowest in the world (32; 253). In
this thesis, the association of lifestyle factors; in particular physical activity (PA), socio
economic status (SES) and dietary calcium intakes on the growing skeleton of Black and
White South African children is investigated.
After using accelerometry to validate a physical activity questionnaire (PAQ), in a
convenience sample of South African Black, White, male and female children (n=30), fitness
levels were assessed in a larger group (n=69) of similarly aged children, stratified by race and
gender. Fittest subjects had significantly greater physical activity scores (p=0.022) as reported
on the PAQ, lower body mass index’s (BMI) (p=0.001) and least percentage body fat
(p=0.001) (as assessed using Dual Energy X-ray Absorptiometry (DXA), than least fit
subjects. White males who reported to be significantly more active than all other groups on
the PAQ were significantly fitter (p<0.001) than White females and Black males and females.
The next study sought to determine whether differences observed in physical activity levels
between groups showed an association with bone mineral content (BMC), density (BMD) and
area (BA) (as assessed using DXA). PA was analyzed in terms of a metabolic (METPA;
weighted metabolic score of intensity, frequency, and duration) and a mechanical (MECHPA;sum of all ground reaction forces multiplied by duration) component for 386 children aged 9.5
(0.04) years recruited from a longitudinal birth cohort study. White children expended a
significantly greater energy score (METPA of 21.7 (2.9)) than Black children (METPA of 9.5
(0.5), p< 0.001). When children were divided into quartiles according to the amount and
intensity of sport played, the most active White children had significantly higher (p<0.05)
whole body BMD and higher hip and spine BMC and BMD after adjustment for body size
than less active children. White children in the highest MECHPA quartile also showed
significantly higher (p<0.05) whole body, hip, and spine BMC and BMD after adjustment for
body size than those children in the lowest quartile. No association between PA and bone
mass of Black children was found. No significant differences between METPA and
MECHPA quartiles and BA were observed for any group.
Given the disparate backgrounds from which many South African children come, the next
study sought to determine whether differences in socio-economic status between Black and
White South African children influence PA patterns. This study explored the relationship
between socio-economic status, PA anthropometric and body composition (via DXA)
variables in 381 children aged (9.5 (0.04) years) recruited from a longitudinal birth cohort
study . Children falling into the highest socio-economic status quartile had mothers with the
highest educational levels, generally came from dual parent homes, were most physically
active, watched less television, weighed more and had greater lean tissue than children in
lower socio-economic quartiles (p<0.001). Significantly greater levels of lean mass (p<0.001)
with increased activity level were observed after controlling for television watching time and
fat mass. There were high levels of low physical activity and high television watching time
among lower socio-economic status groups. White children were found to be more active than
Black children, more likely to be offered physical education and to participate in physical
education classes at school and watched less television than Black children.
The final study sought to investigate the association between habitual PA patterns and dietary
calcium intakes with bone mass acquisition over a one year period in 321 pre-pubertal South
African children recruited from a longitudinal birth cohort study. Data were analyzed by
regressing change in BMC and BA from age nine to ten years, against BA (for BMC), height and body weight. The residuals were saved and called residualized BMCGAIN and BAGAIN.
Residualized values provide a good indication of weight, height and BA-matched
accumulation rates. White children had significantly higher PA levels and calcium intakes
than Black children. Most active White males had significantly higher residualized
BMCGAIN and BAGAIN at the whole body, hip and spine but not at the radius, than those
who were less active. Most active White females had significantly higher residualized
BAGAIN at all sites except the radius than less-active girls. No such effects were seen in
Black children. There was no interactive effect on residualized BMCGAIN or BAGAIN for
calcium intake and PA in boys or Black girls, but an interactive and possible synergistic effect
of calcium and physical activity was observed at the spine, radius and hip in White girls. In
this population, PA has an osteogenic association with White children, but not Black children,
which may be explained by the lower levels of PA in the Black children. Despite this, Black
children had significantly greater bone mass at the hip and spine (girls only) (p< 0.001) even
after adjustment for body size.In conclusion, differences between White and Black children’s PA levels were observed, with
White children reporting higher PA levels and exhibiting higher fitness levels than Black
children. Physical fitness correlated well with self reported physical activity levels on the
PAQ and objectively measured body composition. Socio-economic status differences between
White and Black children are highly related to differences in physical activity patterns and
body composition profiles. Bone mass and area gain is accentuated in pre- and early-pubertal
children with highest levels of habitual physical activity. Limited evidence of an effect of
dietary calcium intakes on bone mass in boys and Black girls was found. The role of exercise
in increasing bone mass may become increasingly critical as a protective mechanism against
osteoporosis in both South African race groups, especially because the genetic benefit
exhibited by Black children to higher bone mass may be weakened with time, as
environmental influences become stronger.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/7072 |
| Date | 06 July 2009 |
| Creators | McVeigh, Joanne Alexandra |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
Page generated in 0.0032 seconds