Why do black South Africans have a low hip fracture rate despite low calcium intake?

After Nelson Mandela was released from prison February 11, 1990, all children born in the great Johannesburg area were enrolled in a 20-year longitudinal study. Officially known as “Birth to Twenty,” the study and its 3,273 youth, are colloquially referred to as “Mandela’s Children.” It’s the largest and longest running study of child and adolescent health and development in Africa, and one of the few large-scale longitudinal studies in the world.

One of the main aims of the study is to follow bone health in growing children, specifically the differences in bone mass acquisition between black and white children and the factors that influence this.

Besides its obvious importance in adult health and possibly measuring the change from “third world” population to “developed world” population, there is a situation in South Africa that’s very counterintuitive, and possibly unique: “Black South African adults have among the lowest hip fracture rates in the world,” according to the study’s lead author, Joanne A. McVeigh.

“Yet our study found that, as children, blacks have significantly lower physical activity levels and calcium intakes than age- and gender-matched white children,” McVeigh adds. McVeigh is presenting the research at the 35th Congress of the International Union of Physiological Sciences in San Diego, March 31 - April 5, 2005.

*Paper presentation: “Physical activity and bone mass accumulation patterns differ in black and white South African children,” by Joanne A. McVeigh, Shane A. Norris and John M. Pettifor, MRC Mineral Metabolism Research Unit, Department of Paediatrics, and School of Physiology, University of Witwatersrand Medical School, Johannesburg. 12:30 p.m.-3 p.m. Sunday April 3, Physiology session/abstract: 347.8; board #A64. On view 7:30 a.m. - 4 p.m.

9- and 10-year-olds compared for exercise level, bone mineral content

Since it’s accepted that physical activity has an osteogenic effect on bone mass, the relationship between physical activity, or exercise, and bone mineral contect (BMC) levels in black and white South African children was compared across the ages of 9 and 10 – halfway through the 20-year study.

White children’s mean exercise scores didn’t differ between ages 9 and 10, however black children’s level significantly increased – by about 50%. “We expected that an increased exercise level would be associated with greater BMC, especially at the hip,” McVeigh said. “However, both group’s BMC gains over that year were similar at the hip.”

Indeed, over the same year, white kids showed a significantly greater change in height, whole body and spine BMC. Significant positive correlations between exercise and BMC accumulation were found for white children at the whole body, hip and spine, but not for black children.

“Nevertheless, after controlling for body weight and bone area, black children remained with the advantage of a significantly greater hip BMC both at ages 9 and 10,” McVeigh reported. This, despite the fact the researchers found that overall white children are much more physically active and have significantly higher calcium intakes than black children.

“However, when we compared bone mass within different quartiles of activity, we found that the most active white children did indeed have better bone mass than the most active black children, but black children have a lower and narrower range of physical activity. Physical activity appears to be the most important modifiable factor influencing white children’s bone health,” she noted.

Paradox indicates need for re-thinking osteoporosis development

“The results of our study raise an apparent paradox, which has implications on our thinking about osteoporosis, the roots of which may well lie in the childhood years. It’s obvious that intervention is becoming increasingly important during the growing years as this is the period when physical activity and nutrition have the most impact. It is possible that with the black South African population starting to adopt more ‘Western’ lifestyles, we may see an increase in fracture rates as their genetic advantage is outweighed by lifestyle influences. It will be important to follow up on these children and our study will continue to do this, throughout their pubertal years and until they turn 20. Only time will tell if these trends will persist.”

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