Body Measurements, Bone Mass, and Fractures

Does the East Differ from the West?

Abstract

There are population differences in hip fractures between the East and West. The hip fracture incidence in Asians is approximately 40% to 50% of that in whites in the United States. There also are differences in body measurements and bone mass between the population groups. This article discusses the role of body measurements in relation to bone mass and bone loss at different stages of life and the interrelationship of these variables with fracture risks.

Hip fractures constitute a major public health problem in Western countries. With the dramatic increase in the elderly population, hip fractures also will become an important problem in Asia. Comparisons have been made of incidence rates of hip fractures in different ethnic groups and in populations in the East and West to investigate the possible influences of genetic, environmental, and lifestyle factors on the occurrence of hip fractures. Studies have noted that black populations have lower fracture rates than do white populations.^25,41 Blacks also have higher bone mass than whites. Bone mass accounts for approximately 80% of the variance of bone strength²⁹ and is an important predictor of fractures.^3,20 Investigators have attributed the higher bone mass and lower fracture incidence in the black population to larger muscle mass and higher body weight than those in the white population.^1,36

However, comparisons of populations in the East and West have revealed a different picture. The Asian population has a lower bone mass and lower incidence of hip fractures than do whites in the United States.¹⁸ The incidence of hip fractures in the Asian population is approximately ½ of that of the US white population.^18,39 This article discusses the role of body measurements in the formation of bone mass and bone loss at different stages of life and the relation of body measurements and bone mass on osteoporotic fractures in the Western and Eastern populations.

BODY MASS AND BONE MASS

Premenopausal Women

The risk of postmenopausal osteoporosis developing in a woman depends on peak bone mass attainment and postmenopausal bone loss. The age of peak bone mass has been found to be approximately 30 years of age.^2,18 The bone mineral density of Chinese premenopausal women seems to be lower than that of US white women of similar age.¹⁸ Although the use of different bone densitometers may account for the noted differences, other anthropometric and lifestyle factors may contribute to the lower bone mass in Chinese women.

Among the anthropometric variables, body weight seems to be an important predictor of bone mineral density.^1,28 Mazess and Barden²⁷ reported that body weight is a stronger predictor of bone mineral density than are other factors, such as dietary intake and daily activity, in healthy women 20 to 39 years of age. An 11-year study in adolescents and young adults revealed that weight, exercise, and age are independent predictors of femoral neck bone mineral density.⁴⁶ The author's study on young Chinese college women 17 to 21 years of age also showed that age, weight, and physical activity are the significant determinants of bone mass (Unpublished data. Ho SC, Chan SG, Leung PC, Hon KY and Han XR: Body weight and weightbearing activity are important determinants of peak bone mass in young Chinese college women. Presented at the Seventeenth Annual meeting of the American Society for Bone and Mineral Research 1995). Body weight was found to correlate significantly with bone mineral density among Chinese women 21 to 40 years of age. When looking at the 2 age decades separately, age and weight explained 4% to 9% of the variances of spine and neck of femoral bone mineral density in women 21 to 30 years of age. These 2 variables accounted for 13% to 16% of the bone mineral density variances in the 31 to 40 year age group. In addition to body weight, lean body mass and body fatness were significantly associated with bone mineral density around or after the attainment of peak bone mass. Chinese women have a smaller body build than do white women.¹⁸ The differences in the anthropometric attributes between the Chinese and US white women could have contributed to the differences in bone mass attainment between the 2 population groups. A recent study found that the differences in bone mineral density and bone mass between Asian and white women disappear after controlling for factors such as weight and height.⁴⁰

Postmenopausal Women

Cummings et al⁴ estimated that a 50-year-old white woman has a 16% risk of having a hip fracture and 32% risk of having a vertebral fracture during her remaining life time. The amount of bone loss increases with years after menopause.⁴⁷ In addition to the changes in estrogen levels, body size has been found to be related to bone mass in postmenopausal women.^35,42,43 As in premenopausal women, body weight is a more important predictor of bone mineral density than are the other anthropometric variables.³⁸ Nordin et al³³ and others⁷ found that each 1 kg increase in body weight is related to 1 mg/m increase in vertebral mineral density in postmenopausal women. A population-based study of perimenopausal women 48 to 59 years of age found body weight, menopausal status, age, and grip strength to be independent predictors of spinal and femoral bone mineral density.²²

Despite the same initial bone mass, some peri- and postmenopausal women lose bone at a faster rate than do others. Two separate 5-year longitudinal studies of postmenopausal women found body weight, years since menopause, and serum estrone to be important predictors of rate of change of radial bone mass.^32,45 A 12-year prospective study indicated a positive relationship of weight with bone mass measurements.¹⁴ The increase in the biologically available estrogen from the conversion of adrenal androstenedione to estrone²⁴ also protects against postmenopausal bone loss.

Little data are available on factors related to postmenopausal bone loss in Asian perimenopausal women. A cross section study by Pun et al³⁷ found weight to be a significant correlate of bone mass in Chinese women 30 to 70 years of age. A population based prospective study is under way to examine the predictors of bone mass and bone loss in Chinese peri- and early postmenopausal women. Anthropometric indicators, such as body weight and body composition, will be among the factors to be examined in the study.

Elderly Men and Women

A large number of cross section studies suggested that rates of bone loss decrease in the elderly, but recent population-based studies, including more subjects of very old age, indicated a continuous or accelerated bone loss after the age of 70 years.^13,21 A study in the United Kingdom showed that approximately ⅓ of the decline in bone mineral density with age could be explained by the associated age related decline in weight.²⁶ A study of 348 apparently healthy women 70 years of age and older showed that risk factors for low bone mineral density are low body weight, high number of years since menopause, and impaired mobility.³⁴ The authors' cross section study on the Chinese elderly population aged 70 years and older has revealed body weight to be the strongest predictor of the femoral neck bone mineral density. Multiple regression analysis revealed that body weight, gait time, gender, alcohol consumption, and age caused approximately 48% of the femoral neck bone mineral density variance. Weight was the most significant predictor, with a partial correlation coefficient of 0.46. Age became nonsignificant after adjustment for the other 4 variables.¹⁷

It is suggested that the effect of weight on trabecular bone density is related to the imposed load as a function of body weight. An increased body weight also is a combination of increased adipose tissue or muscle mass or both. The association of increased adipose tissue with increased bone mineral density in women could be attributable partly to the increased production of estrogen and thus the promotion of bone mineralization. The mechanical stress exerted by muscle on bone also may influence bone mass.⁸ Body measurements probably exert different influences at different sites, depending on the bone types and the various degrees of mechanical force exerted. Thus, it is not surprising to find inconsistent results in the relations of body measurements and bone mass at other bone sites, such as that of the forearm.^12,19,44

BODY MEASUREMENTS AND HIP FRACTURES

Femoral neck fractures are relatively rare before the sixth decade of life but increase exponentially with age, and almost 50% occur in people 80 years of age and older.¹⁶ Many of the known risk factors for bone loss are related to hip fractures. Anthropometric measurements, such as body weight, have been found to be strong predictors of hip fracture and of bone mass at weightbearing sites in different population groups.^14,22 Body size might influence fracture risk in several ways. Within a given population, greater body mass might correlate with bone mass and possibly mechanical strength or force on different bone sites. Many studies have found a consistent inverse relation between body mass index and hip fracture and are indicative of a threshold value with excess risk present in persons with a body mass index below 22.^9,30,32,45

In addition to body mass index, other anthropometric indicators may have an independent relationship to the risk of hip fractures.⁹ Height has been found to be related to the risk of hip fracture. Hayes et al¹⁵ has estimated that each 9-cm increase in women's height would increase the risk by 50%. Cummings and Nevitt⁶ also observed a 1.5-fold increase in the risk of hip fracture for each standard deviation or approximately a 6-cm increase in height. A recent large prospective study of Norwegian women confirms such an association.³⁰

Attention recently has been drawn to the relation between hip geometry and fracture risks. Faulkner et al¹⁰ noted an inverse association between hip axial length and hip fracture risks. Cummings et al⁵ found that Asian women had a hip axial length approximately 1.2 standard deviations shorter than that of white women. A cross section study of Chinese subjects 70 years of age and older found a significant positive correlation between hip axial length and body height (r = 0.627, p = 0.0001). Asian women have a 40% to 50% lower risk of hip fracture than do North American white women.^16,31 The increased force of impact during a fall because of taller body build and longer hip axial length in the North American population may partially explain the differences in hip fracture incidences between the 2 population groups.⁶ Other hip geometry, such as smaller femoral neck angle, also may account for the lower risk of structural failure in the femoral neck in Asian women.³¹

FALLS

In addition to age-related decline in bone mass, the higher incidence of hip fractures in the elderly is related partly to the rising incidence of falls with age. In the United States, 30% of elderly citizens fall each year.^4,6 Although only a small proportion of falls results in fractures, approximately 90% of hip fractures are related to falls.¹¹ Body stature and biomechanics of the fall have been suggested to play an important role in the occurrence of hip fractures. Higher body weight is associated with bigger muscle mass and fat area around the femoral neck region and acts as a local shock absorber of the energy of impact. The padding around the hip is related to decreased fracture risk if a fall occurs.²³ Thus, body mass could protect against fractures by increasing the bone mass and decreasing the probability of fractures after a fall. However, taller and heavier subjects may have an increased risk of fractures because of the greater impact of force during a fall.¹⁵

Available data on falls in the Asian elderly are limited. Cross section studies showed a lower occurrence of falls compared with that in Western countries (Unpublished data. Ho SC, Woo J, Chan SG, Yuen YK and Sham A: Risk factors for falls in a Chinese old-old cohort. Presented at the Thirteenth Annual Meeting of the American College of Epidemiology 1994), but no prospective data are available. The Asian population is shorter and lighter than the white population.^18,39 The shorter stature and closer proximity of hip to the ground may reduce the impact of force during a fall. However, lower body and fat mass may reduce the padding around the hip area. Conversely, although higher body weight in whites confers a lower risk of hip fracture, the greater height and body weight increase the impact of a fall and the risks of fractures.

IMPLICATIONS

Bone mass increases or decreases in response to changes in the level of its functional stimulus. Thus, an individual's bone mass is partly a result of this stimulus. There is increasing evidence that anthropometric characteristics are among the variables to be considered in the formation and maintenance of bone mass and in the prevention of bone loss and fractures. Osteoporosis and fracture risk increase with age. A general decline in body mass with age has been noted in the elderly population. Maintenance of body weight together with preservation of muscle strength by maintaining physical activity in the elderly are among the modifiable factors in the prevention of osteoporotic fractures.