Research updates
Osteoporosis   p 10
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5 Current research on programming Link to the Medical Research Council web site
5.1 Epidemiological studies
The MRC Environmental Epidemiology Unit has studied large groups of people from their infancy to old age in longitudinal studies. Detailed records kept for over seventy years made this possible. In Hertfordshire, for instance, it was possible to trace a group of men and women born in 1920 –1930 and follow their progress to this day.

An investigation over seven decades
Carefully preserved hand-written records dating back to 1911 made this research possible. They were discovered in Hertfordshire by the Southampton group and were found to contain details of most of the babies born in the county. Births were noted by the attendant midwife. Almost all of them took place at home. The name and address of the baby, the date of birth, and the birth weight were registered. A health visitor saw the baby periodically throughout its first year and recorded its manner of feeding (whether breast fed or not), and its weight at one year. With the help of the National Health Service Central Registry at Southport, and the Hertfordshire Family Health Service Association, it was possible to trace men born during 1920 – 1930 and women born between 1923 - 1930, in the six districts of East Hertfordshire, and who still lived there. Of these, 189 women and 224 men took part in an investigation into the association between growth in infancy and bone mass in later life.
One of the most significant outcomes of this and other studies was the finding that bone mineral content (BMC) at the spine and hip is strongly associated with the weight of a subject at 1 year of age. This link was independent of adult risk factors for osteoporosis (cigarette smoking, alcohol-consumption, dietary calcium intake, physical activity and age at menopause).
5.2 Tracking

‘Tracking’ is the tendency for subjects who are small in relation to their peers to remain relatively small as they grow up, and vice versa. Growth of the skeleton is said to track from early childhood. Experiments on rats, mice, sheep and pigs have demonstrated that once this tracking is established, it is no longer possible to make animals grow faster by offering them unlimited food. Their rate of growth has become set, homeostatically controlled by feedback systems.

Before the time when tracking becomes established there is a critical window during which an adverse environment, for example malnutrition, might permanently alter the pattern of growth.

The mechanism underlying tracking is believed to be a phenomenon called programming.

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Question 9
Longitudinal studies demand great perseverance but they were essential for the research described above. Cross-sectional studies are easier and cheaper to do.

Explain what is meant by a cross-sectional study. Where in healthcare might a cross-sectional study be valuable?