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Human Variability in Bone Mass Measurements
Journal of Clinical Densitometry: Assessment & Management of Musculoskeletal Health, vol. 18, no. 4, 451e452, 2015 Ó Copyright 2015 by The International Society for Clinical Densitometry 1094-6950/18:451e452/$36.00 http://dx.doi.org/10.1016/j.jocd.2015.05.069
Editorial
Human Variability in Bone Mass Measurements Dorothy A. Nelson* Department of Anthropology, Sociology, Social Work, and Criminal Justice, Oakland University, Rochester, MI, USA
approach has been helpful in identifying group differences in health and disease, which has had many positive outcomes. But this approach often relies on an assumption of underlying genetic similarities, leading to generalizations of observations to groups that appear to be similar to the reference population. Genomic studies have indicated that 85%e90% of genetic variation occurs within geographically defined populations (generally consistent with races), with the residual 10%e15% occurring between groups (2). As modern populations have become more mobile, genetic admixture is becoming the norm. In fact, throughout history and prehistory, whenever different groups have come into contact, they typically exchanged genes through interbreeding. This continuous exchange of genetic material has maintained H sapiens as a single species and has made the biological concept of race largely irrelevant to our species. This is not to deny the sociocultural and political importance of the concept in modern day events. But we still have the problem of how to deal with observed population differences in BMD as measured by DXA so that we can deliver health care appropriately to individuals at risk for osteoporosis and fracture. The significance of the work by Mukwasi et al., in this issue goes beyond the recognition that the group of Zimbabwean women in their study, and other sub-Saharan African study populations, do not have patterns of BMD that are similar to wellcharacterized US populations. More importantly, they suggest that differences in body size may be a key to understanding the variability in BMD among these populations of African origin. Body size is an observable and measurable physical characteristic that is clearly the result of genetic predisposition and environmental influences, such as diet,
The literature on population differences in bone mineral density (BMD) as measured by dualenergy X-ray absorptiometry (DXA) has increased dramatically over the past several decades. The article by Mukwasi et al., published in this issue of Journal of Clinical Densitometry adds an important dimension to this burgeoning research area, which is also of great clinical interest as DXA and other bone mass measurement tools are deployed worldwide. The incidence of osteoporosis and fragility fractures is expected to increase globally in the coming decades (1), but the challenge is this: how to accurately identify significantly low BMD and increased fracture risk among individuals from populations that may not have relevant reference data. There is no easy answer, but it may be helpful to review the biological and cultural forces that result in human variability and the challenging interpretations of this phenomenon. Biologists use race to denote a subspecies of animal (in this case, the human animal) that differs in some discernible and relevant way from other such subspecific populations. Human evolution has been marked by migrations and gene flow among populations for 100s of 1000s of years, making such categories largely irrelevant to Homo sapiens. However, scientists in the 18th century (such as Carolus Linnaeus and Johann Friedrich Blumenbach) classified our species into races based largely on geographic location and, to some extent, on subjective interpretation of what an ideal human looks like. Fast forward to our century, and we still rely on 1 or more classification schemes for human variability, generally labeled race and/or ethnicity. This *Address correspondence to: Dorothy A. Nelson, PhD, Oakland University, 528 Wilson Hall, Rochester, MI 48309. E-mail: [email protected]
451
452 lifestyle, and access to health-related resources. The authors’ suggested approach to standardizing the expression of DXA data for comparative and clinical purposes has merit, given the consistent findings in this and other studies that body mass is a significant correlate of BMD as measured by DXA. But which reference data does one start with? The ideal approach, of course, is to establish relevant reference data for each population that is being measured for clinical purposes. The extent of such an effort is seemingly limitless and clearly not realistic in terms of time, effort, and cost. So it is only logical to try to fit each newly measured group into a race that has already been well characterized. We see from the current article and many others that this does not work. The Father of Taxonomy himself, Carolus Linnaeus, noted that although genus and species are ‘‘always the work of nature.the variety [is] mostly that of circumstance .’’ (3). This may have set the stage for Charles Darwin and others to identify the role of natural selection in determining how circumstance, or the environmental
Nelson context, results in biological variation. It is now incumbent on us to understand how variation in BMD reflects an individual’s or population’s genetic and environmental circumstances to provide relevant diagnostic, preventive, and treatment regimens. The optimal approach to the question that is provoked by the article this month is one that will take an international and multidisciplinary effort to identify. I challenge the authors and reviewers of Journal of Clinical Densitometry and other leading bone publications to think outside the usual box to solve this clinically important issue.
References 1. Gullberg B, Johnell O, Kanis JA. 1997 World-wide projections for hip fracture. Osteoporos Int 7:407e413. 2. Jorde LB, Wooding SP. 2004 Genetic variation, classification and ‘‘race’’. Nat Genet 36:S28eS33. 3. Linnaeus C. 1751. Philosophia botanica. Godofr Kiesewetter, Stockholm. Trans. Stafleu FA. 1971. Linnaeus and the Linnaeans: the Spreading of Their Ideas in Systematic Botany, 1735e1789. International Association for Plant Taxonomy, Utrecht, The Netherlands.
Journal of Clinical Densitometry: Assessment & Management of Musculoskeletal Health
Volume 18, 2015
Editorial
Human Variability in Bone Mass Measurements Dorothy A. Nelson* Department of Anthropology, Sociology, Social Work, and Criminal Justice, Oakland University, Rochester, MI, USA
approach has been helpful in identifying group differences in health and disease, which has had many positive outcomes. But this approach often relies on an assumption of underlying genetic similarities, leading to generalizations of observations to groups that appear to be similar to the reference population. Genomic studies have indicated that 85%e90% of genetic variation occurs within geographically defined populations (generally consistent with races), with the residual 10%e15% occurring between groups (2). As modern populations have become more mobile, genetic admixture is becoming the norm. In fact, throughout history and prehistory, whenever different groups have come into contact, they typically exchanged genes through interbreeding. This continuous exchange of genetic material has maintained H sapiens as a single species and has made the biological concept of race largely irrelevant to our species. This is not to deny the sociocultural and political importance of the concept in modern day events. But we still have the problem of how to deal with observed population differences in BMD as measured by DXA so that we can deliver health care appropriately to individuals at risk for osteoporosis and fracture. The significance of the work by Mukwasi et al., in this issue goes beyond the recognition that the group of Zimbabwean women in their study, and other sub-Saharan African study populations, do not have patterns of BMD that are similar to wellcharacterized US populations. More importantly, they suggest that differences in body size may be a key to understanding the variability in BMD among these populations of African origin. Body size is an observable and measurable physical characteristic that is clearly the result of genetic predisposition and environmental influences, such as diet,
The literature on population differences in bone mineral density (BMD) as measured by dualenergy X-ray absorptiometry (DXA) has increased dramatically over the past several decades. The article by Mukwasi et al., published in this issue of Journal of Clinical Densitometry adds an important dimension to this burgeoning research area, which is also of great clinical interest as DXA and other bone mass measurement tools are deployed worldwide. The incidence of osteoporosis and fragility fractures is expected to increase globally in the coming decades (1), but the challenge is this: how to accurately identify significantly low BMD and increased fracture risk among individuals from populations that may not have relevant reference data. There is no easy answer, but it may be helpful to review the biological and cultural forces that result in human variability and the challenging interpretations of this phenomenon. Biologists use race to denote a subspecies of animal (in this case, the human animal) that differs in some discernible and relevant way from other such subspecific populations. Human evolution has been marked by migrations and gene flow among populations for 100s of 1000s of years, making such categories largely irrelevant to Homo sapiens. However, scientists in the 18th century (such as Carolus Linnaeus and Johann Friedrich Blumenbach) classified our species into races based largely on geographic location and, to some extent, on subjective interpretation of what an ideal human looks like. Fast forward to our century, and we still rely on 1 or more classification schemes for human variability, generally labeled race and/or ethnicity. This *Address correspondence to: Dorothy A. Nelson, PhD, Oakland University, 528 Wilson Hall, Rochester, MI 48309. E-mail: [email protected]
451
452 lifestyle, and access to health-related resources. The authors’ suggested approach to standardizing the expression of DXA data for comparative and clinical purposes has merit, given the consistent findings in this and other studies that body mass is a significant correlate of BMD as measured by DXA. But which reference data does one start with? The ideal approach, of course, is to establish relevant reference data for each population that is being measured for clinical purposes. The extent of such an effort is seemingly limitless and clearly not realistic in terms of time, effort, and cost. So it is only logical to try to fit each newly measured group into a race that has already been well characterized. We see from the current article and many others that this does not work. The Father of Taxonomy himself, Carolus Linnaeus, noted that although genus and species are ‘‘always the work of nature.the variety [is] mostly that of circumstance .’’ (3). This may have set the stage for Charles Darwin and others to identify the role of natural selection in determining how circumstance, or the environmental
Nelson context, results in biological variation. It is now incumbent on us to understand how variation in BMD reflects an individual’s or population’s genetic and environmental circumstances to provide relevant diagnostic, preventive, and treatment regimens. The optimal approach to the question that is provoked by the article this month is one that will take an international and multidisciplinary effort to identify. I challenge the authors and reviewers of Journal of Clinical Densitometry and other leading bone publications to think outside the usual box to solve this clinically important issue.
References 1. Gullberg B, Johnell O, Kanis JA. 1997 World-wide projections for hip fracture. Osteoporos Int 7:407e413. 2. Jorde LB, Wooding SP. 2004 Genetic variation, classification and ‘‘race’’. Nat Genet 36:S28eS33. 3. Linnaeus C. 1751. Philosophia botanica. Godofr Kiesewetter, Stockholm. Trans. Stafleu FA. 1971. Linnaeus and the Linnaeans: the Spreading of Their Ideas in Systematic Botany, 1735e1789. International Association for Plant Taxonomy, Utrecht, The Netherlands.
Journal of Clinical Densitometry: Assessment & Management of Musculoskeletal Health
Volume 18, 2015