Adult bone mass and physiological stress. A palaeoepidemiological study

By Lene Warner Boel


The aims of this study was in short to investigate various aspects concerning the bone health in various skeletal samples, and compare it to modern populations. Thebasis for this investigation were the Danish Medieval village populations of Nordby and Tirup. The two samples examined consist of the remains of at least 235 and 619 individuals, respectively. The cemetery of Nordby was in use from AD1000 - 1250 which was a prosperous period with population expansion in Denmark. The cemetery of Tirup represents the period from around 1150 to the first half of the 14th century where a decline in society was observed, and the general health status deteriorated. Two hundred individuals from the Terry Collection which is an American skeletal sample from the beginning of the 20th century were also investigated. Bone mineral density (BMD) was measured using Dual Energy X-ray Absorptiometry in the femurs of the 49 best preserved individuals in Nordby and equivalently femurs from 55 individuals from Tirup. These were compared with the age- and sex-matched values from modern living Danish people who shared the same genetic pool as their ancestors.

Thus, any differences in BMD between the populations can be attributed to differences in life style and in the way the skeletal samples were generated. Among males, BMD was identical or higher when compared to modern Danish men. The most likely cause is the high level of physical activity associated with rural life in Medieval times and a low level of selective mortality for BMD in males. Among females, the picture was more complex since a significantly lower BMD was observed in the years of fertility. In the older ages the level of BMD was higher than in modern Danish women. The reason for this is the increased mortality among the fertile women with low values of BMD due to a higher birth rate and prolonged periods of lactation. Only the strongest women survived to older ages. They appear to have higher levels of BMD either from the start or because they had fewer children and thus experienced less bone loss.

The increase in the incidence of osteoporosis today can therefore partly be explained by the survival to older ages of women who would have died at a younger age, had they lived in earlier centuries. For the American skeletal sample, the trend was generally the same when compared to modern Americans , but the explanation for these findings are somewhat different and more complicated. In a methodological study, a new definition of Harris lines revealed a high degree of both intra- and inter-observer reproducibility. The number of Harris lines was counted from radiographs of the distal tibia and taken as evidence for periods with severe psysiological stress in childhood. Subsequently the relationship between physiological stress, BMD, and mortality was investigated. This study can be considered as cross-sectional with the sample time being the time of death for each individual. Since the samples consisted only of dead people there are some built-in bias which has to be accounted for, e.g. the well-known selective mortality among fertile women. Despite the limitations of this study, it is possible to draw important conclusions concerning the socalled osteological paradox. In general, physiological stress in childhood seemed to be associated with BMD for women but more so for men in the Terry Collection. This illustrates the selective processes underlying the osteological paradox; among the younger, those with few Harris had a lower BMD, and among the elderly, few Harris lines were associated with an increased BMD. Evidence that low BMD is associated with increased mortality is substantial. With appropriate reservations it can be concluded that individuals of poor health died when exposed to severe stress before they developed HLs, whereas strong individuals survived and therefore express the largest number of HLs.

The heterogeneity of the distribution of HLs is illustrated; individuals with no HLs constitute a group with large variability, since it also contains individuals with excellent health. These individuals were so strong that they did not suffer from growth arrest when they were exposed to severe physiological stress, thereby having a relative increased chance of survival as compared with individuals with one or more HLs. This pattern is not found in the medieval Danish skeletons, most likely because they after all had suffered from much poorer health in general.