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These are just some of the fascinating findings that are emerging from the landmark Birth-to-Twenty study - Africa's largest and longest-running study of child and adolescent health and development, and one of the few large-scale longitudinal studies in the world.
It's set to have a major impact on health policies and legislation in future, and is now in its 18th year.
Health24.com caught up with one of the lead researchers – Dr Shane Norris from the University of the Witwatersrand – on the progress of the study and the preliminary findings.
Study background
The Birth-to-Twenty study kicked off at a time of demographic and health transition in South Africa – a time when urbanisation increased, the prevalence of infant mortality and infectious disease decreased, and non-infectious diseases such as cancer and heart disease became more prevalent.
The study began just after former president Nelson Mandela was released from prison in 1990. As a result, participants have often been referred to as "Mandela's children".
From April to June 1990, the births of 5 449 babies were recorded in the Johannesburg/Soweto area. However, only 3 273 families were included in the actual cohort as some of the babies didn't remain in the area long enough to participate. Participants were mainly from the black population group and the study was initially planned for 10 years, up until the year 2000.
Data was recorded at significant stages of the children's development: before birth, at the time of delivery, at six months, and when the children reached the ages of one, two, three, four/five, seven/eight and ten years.
In 2000, the decision was made to extend the study period to 20 years. At this stage, two new dimensions were added:
- bone mass acquisition in childhood and adolescence; and
- biological and social influences on adolescent development.
While the study hasn't been completed, and researchers are still in the early stages of unravelling the findings, interesting results are starting to emerge with regards to the above-mentioned categories.
Black children have stronger hips
Numerous international research studies have shown that dietary calcium intake and physical activity play a key role in the acquisition of bone mass in children and subsequent risk for osteoporosis in adult life. The Birth-to-Twenty researchers were keen to investigate whether these circumstances held true for South African children and whether ethnicity played a role.
In order to compare different ethnic groups for the bone-mass leg of the study, white children were enrolled to match the ages of the black participants who already formed part of the cohort, Norris explains. By means of a variety of tests done twice a year, ethnic differences were studied in the sample of 620 children.
The results were interesting and, in terms of fracture rates, there were significant differences: one of the key findings was that white children seemed to fracture a lot more than black children.
According to Norris, the researchers hypothesised that the bone mass of the black children studied might have been inherently higher.
It was also noted, however, that white children participated in sports more often, which could have increased their risk for fracture injuries. The researchers also picked up differences in the type of physical education offered at school – another factor that could have played a role.
When the intake of dietary calcium was studied, there were, once again, significant ethnic differences and more surprises were in store for the researchers.
The black children who participated in the study had a mean calcium intake of 415mg per day, while the white children had an intake of 855mg calcium per day. In other words, the white participants got more than double the amount calcium from the food they ate than their black counterparts. Surprisingly, however, these higher calcium intakes didn't seem to translate into greater bone mass.
But while calcium intake seemed to play a less significant role, physical activity, on the other hand, did have a positive influence, especially in the white children. The researchers found that the white kids who participated in sport exhibited greater hip bone mass than the white children who were less active. In the black group, however, exercise seemed to have less of an impact.
The researchers speculate that genes may have an important influence on hip-bone density, which would explain why the black children seem to inherently have stronger hips. According to Norris, research does in fact show that osteoporosis of the hip is less prevalent in the South African black population as opposed to the white population.
"Clearly, there's a genetic protective factor," Norris says. "What's more, the research shows that physical activity is a strong environmental mediator of bone mass, but that there's a threshold effect." This means that a child has to do a certain amount of exercise for a certain amount of time before the physical activity starts to have a positive impact on bone mass. At this stage, however, researchers don't know what this level of exercise should be.
But, Norris notes, when the two population groups of children were compared, there were no differences in bone mass at the lumbar spine. At this site, exercise seemed to have a positive effect on both black and white children. So, the research seems to indicate that exercise is probably beneficial for both black and white children living in South Africa.
Low birth weight = early puberty
Researchers around the world have known for quite some time that certain adult diseases, such as hypertension and diabetes, may have developmental origins. The Birth-to-Twenty study now adds to this growing body of research.
Startling results show that low birth weight, coupled with rapid post-natal weight gain, increases a child's risk for earlier onset of puberty – a situation which could put a child at risk for disease in later life.
The South African researchers found that the greater the difference between a child's birth weight and the child's weight at the age of two, the older the bone age of the child. A child's bone age is an indicator of physical growth, and an older bone age means that the child's skeleton has matured faster, because s/he started growing earlier than his/her peers. This translates into earlier development.
"The rationale is that some sort of problem during pregnancy, e.g. poor maternal nutrition, which results in a low birth weight, coupled with rapid weight gain in infancy predisposes the physiology of the child to earlier onset of puberty. This is especially true for girls," Norris says.
The question researchers are now trying to answer is whether the age of puberty onset influences the duration of pubertal development – an important growth stage during which changes occur that can predict health in adult life.
For example, there's some evidence to suggest that starting menstruating at an earlier age and short pubertal development duration is associated with shorter height as well as an increased risk of obesity.
Interestingly, Birth to Twenty (South Africa) also forms part of an international study with four other developing cohort studies, which has shown that a child's height at the age of two is an important predictor of adult education and income level. "This goes to show that a child's physical development also has an impact on his/her cognitive development," Norris says.
Towards a healthier future
If we put these results on the early origins of adult disease into perspective, it's clear that more can be done to ensure the long-term health of our children.
According to Norris, the message seems to be this:
- mothers should ensure healthy pregnancies by eating healthily and steering clear of cigarettes, drugs and alcohol, as this will boost their babies' chances of having normal birth weights;
- parents should optimise their babies' growth during the first two years of life – here, exclusive breastfeeding for at least the first four months, appropriate weaning foods and a nurturing environment are key.
"These steps will set up the child for the best possible outcomes later on," Norris says. "On a population level, we really want to improve birth weight and prevent stunting by age two."
The Birth-to-Twenty study is primarily funded by the South African Medical Research Council, the Wellcome Trust (UK), the University of the Witwatersrand and the Human Sciences Research Council, and this phase of the study will be completed in 2010.
- (Carine van Rooyen, Health24, August 2008)
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