A comparive study into the bone health of South African pre-pubertal children who participate in physical activites with various amounts of skeletal loading
Date
2014-08-25
Authors
Meiring, Rebecca Mary
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Abstract
Osteoporosis is a disease that may be pre-determined from the condition of bone health
during youth. In South Africa, the situation is quite unique in that the population of black
people has a reduced fracture rate compared to white people. As lifestyle and dietary
patterns change with urbanisation and there is a shift towards westernised diets and
sedentary behaviour in youth, fractures in elderly South African blacks may become more
prevalent. With these rapid lifestyle changes, it will become increasingly important to
prioritise osteoporosis and its related conditions as a major public health concern in
South Africa. Very few of the factors influencing osteoporosis have been well studied in
children of different ethnic groups. Physical activity in childhood, especially in the prepubertal
years, confers residual benefits to the adult skeleton. In this thesis, the
associations between ethnicity, history of participation in physical activity and skeletal
health were explored in a sample of pre-/early pubertal children from South Africa who
participated in four different studies. Furthermore, a novel aspect of the thesis was the
use of peripheral quantitative computed tomography (pQCT) to investigate the
mechanistic role that physical activity plays on bone health in this unique population.
First the use of an existing physical activity questionnaire for the assessment of bone
loading had to be validated in a sample of black and white boys and girls (n=38). A bone
loading algorithm was used to calculate a peak bone strain score (PBSS) from the physical
activity questionnaire. Therefore a bone specific physical activity questionnaire (B3Q) was
used in subsequent studies. The PBSS was shown to be reliable and reproducible with
significant (p<0.001) intraclass correlation coefficients. There were significant
correlations between PBSS and moderate (r=0.38; p=0.02), vigorous (r=0.36; p=0.03) and
combined moderate to vigorous intensity activity counts (r=0.38; p=0.02) as measured by
accelerometry. The ability of the PBSS algorithm to classify children into high or low
weight bearing groups was in moderate agreement with accelerometer derived
combined moderate and vigorous activity counts (κ=0.42; p=0.008). PBSS was
significantly correlated to body size adjusted bone mineral content at all sites scanned by
DXA (r=0.43-0.57; p<0.05). Positive correlations were observed between PBSS and area,
density and strength at the radius and tibia (r=0.40-0.64; p<0.05). At the radial
metaphysis, significant correlations between moderate activity (r=0.46; p=0.005) and
combined moderate and vigorous activity counts (r=0.42; p=0.01) were seen for bone
strength. No associations were seen between accelerometer measured physical activity
and bone outcomes at the tibial diaphysis. Multiple regression analysis showed that the
PBSS was a better predictor of bone mass and structure than was accelerometry.
The next study sought to determine whether children who were classified as being high
bone loaders for the past two years would present with greater bone mass and strength
regardless of their ethnicity. Sixty six children [black boys, 10.4(1.4) yrs, n=15; black girls,
10.1(1.2) yrs, n=27; white boys, 10.1(1.1) yrs, n=7; white girls, 9.6(1.3) yrs, n=17] reported
on all their physical activities over the past two years in the interviewer administered
bone specific physical activity questionnaire (B3Q). Children were classified as being
either high or low bone loaders based on the cohort’s median peak bone strain score
estimated from the B3Q. In the low bone loading group, black children had greater
femoral neck bone mineral content (BMC) (2.9 (0.08)g) than white children (2.4 (0.11)g;
p=0.05). There were no ethnic differences in the high bone loaders for femoral neck BMC.
At the cortical sites, the black low bone loaders had a greater radius area (97.3 (1.3) vs
88.8 (2.6) mm2
; p=0.05) and a greater tibia total area (475.5 (8.7) vs. 397.3 (14.0) mm2
;
p=0.001) and strength (1633.7 (60.1) vs. 1271.8 (98.6) mm3
; p=0.04) compared to the
white low bone loaders. These measures were not different between the black low and
high bone loaders or between the black and white high bone loaders. Ethnic differences
in bone area and strength apparent between children classified as having a lower bone
loading physical activity history appear to have been attenuated when children partaking
in high bone loading physical activities were compared. Greater levels of mechanical
loading seemed to have no apparent benefits in black children.
Cross-sectional studies in black and white pre-pubertal children have observed significant
ethnic differences in structural bone outcomes as measured by pQCT but there are a
limited number of intervention studies that have been conducted in black children. The
cortical bone of black and white children may respond differently to mechanical forces,
yet no physical activity interventions and their effects on bone structure in black children
have been done. The aim of the third study was to determine whether a weight-bearing
physical activity intervention improves measures of bone mass, structure and strength in
pre-pubertal black children. Children (9.7 ± 1.1 years) were randomised into an exercise
(EX; n=12) and control (CON; n=11) group. The EX children performed a 20-week weightbearing
exercise program performed twice a week for 45 minutes per session, while CON
children continued their regular activities. Changes in tibial trabecular volumetric bone
density, area and strength were greater in the EX than the CON group (all p<0.01). At the
cortical site of the tibia, the change in bone density was greater in the EX group than the
CON group (all p<0.05). The greater change in tibial periosteal circumference in the EX
groups also resulted in a greater change in cortical thickness of the tibia compared to the
CON group (p<0.05).
The final study assessed whether rates of bone accrual differed over one year between
high and low bone loaders and also between black and white South African children.
Forty seven children (18 boys, 29 girls) were followed up after one year. High bone
loaders tended to have greater baseline BMC at all sites measured by DXA but the
difference was only significant at the femoral neck (p=0.03). At the follow up visit,
femoral neck BMC remained significantly higher in the high compared to the low bone
loaders (p=0.003). Bone strength index (BSI) at the follow up visit was significantly greater
in the high bone loaders compared to the low bone loaders (p=0.05). Although there was
a trend for the high bone loaders to have greater indices of density and area at the 65%
tibia compared to the low bone loaders, this was not significantly different at baseline or
at follow up. High bone loaders had greater relative changes in whole body BMC
(p=0.002), tibial cortical area (p=0.03), cortical density (p=0.04) and cortical thickness
(p=0.03) compared to low bone loaders. There were no significant differences in DXA
bone outcomes between black and white children at baseline and follow up. At baseline,
total density at the 4% radius was greater in black than in white children (p<0.001) but
total density at the follow up visit was not significantly different between black and white
children (p=0.06). Trabecular density was greater in the black than in the white children
at baseline (p=0.01) as well as at follow up (p=0.04). BSI at baseline was greater in the
black than in the white children (p=0.05) but this significance disappeared at follow up.
Similar to the 4% radius, cortical density at baseline was significantly greater in the black
compared to the white children at the 65% radius (p=0.01) and at the 65% tibia (p=0.04).
In conclusion, the PBSS algorithm from the B
3Q can be used to reliably and accurately
collect data on previous participation in weight bearing exercise and is able to classify
children as being either high or low bone loaders. It appears that in order for White
children to reach the same bone mass/health levels as Black children, they may need to
participate in higher levels of weight-bearing physical activity. Ethnic differences in bone
area and strength apparent between children classified as having a lower bone loading
physical activity history appear to have been attenuated when children partaking in high
bone loading physical activities were compared. The associations may indicate that a
strong environmental influence (i.e. high participation in physical activity) may offer
similar or even superior benefits to bone over genetic (ethnic) influences. The use of
pQCT appears to be sufficiently sensitive in detecting bone structural changes in response
to mechanical loading interventions. As such, pQCT measures were able to determine the
efficacy of a weight bearing physical activity intervention on trabecular and cortical sites
in black children, and, similar to what has previously been observed in white and Asian
children, our knowledge on the attainment of bone in response to an exercise
intervention in black children is deepened. Moreover, the bone accrual that occurs in a
population of black and white children from a low-middle income country may also differ
between ethnicities and may reflect an environmental influence that modifies existing
paradigms on physical activity and bone health in children. The promotion of weightbearing
physical activity should occur in all youth, to oppose the possible lifestyle induced
risks for developing osteoporosis in adulthood.