| Osteoporosis in South Africa |

Category: Practitioners

Osteoporotic fracture rates in white, Asian and "mixed race" populations in this country are similar to those reported from North America and Europe. South African Blacks, however, have the lowest hip fracture prevalence in the world. No data on the incidence of vertebral fractures in this country have been published.

The two main aetiopathogenetic factors involved in the development of skeletal fracture are bone strength and increased risk of trauma and falls. Since bone mass accounts for more than 75% of the variance in bone strength, a low bone mineral density (BMD) is presently regarded as the most important risk factor for the development of osteoporosis (OP). An individual's BMD is essentially a function of peak bone mass attained during growth and subsequent age-related bone loss. The former is largely (50-75%) genetically determined, hence the recent advent of various genetic markers to assess risk of developing OP - these include polymorphisms in the genes that encode collagen, the estrogen receptor, parathyroid hormone, various local growth factors and particularly the vitamin D receptor. It has been suggested that > 30% of the inter-racial variation in BMD between African-American and Caucasians might be explained on the basis of polymorphism in the vitamin D receptor gene. Although peak BMD is largely genetically determined, diet, calcium intake, physical exercise, normal pubertal development and good general health may conribute significantly . Age related bone loss is largely a function of postmenopausal hormone deprivation and factors associated with ageing (eg. ageing osteoblasts, negative calcium balance , impaired hormone/local growth factor production). If life style factors (alcohol, smoking, lack of exercise), a poor calcium intake , a premature menopause, the use of bone toxic drugs or certain medical diseases are superimposed on this age related bone loss, clinically significant osteoporosis may ensue.

An increased propensity to fall and loss of protective responses constitute further role players in the pathogenesis of osteoporotic fractures. Recent epidemiologic data have revealed that some 50% of osteoporotic patients have muscle loss/weakness ("sarcopenia") , 30% have postural hypotension and many have poor visual acuity or problems with depth perception, cognitive dysfunction or use drugs which may predispose to falls. Ethnic differences in the predisposition to falls may further explain the incidence of fractures in populations . In fact, the skeletal protection provided by drugs such as estrogen, may be ascribed in part to their ability to improve reaction time which decreass the propensity of an individual to fall.

In North America, a lower fracture rate in African Americans (genetically quite distinct from our black populations) also exists; which is readily explined by a 10-15% higher peak BMD in this population. In addition, studies have documented a lower calcium excretion, lower biochemical parameters of bone turnover despite higher plasma parathyroid hormone (PTH) levels in black women, suggesting that blacks have decreased skeletal sensitivity to PTH. Original studies by Solomon and more recently by Daniels et al have surprisingly shown that appendicular and vertebral BMD in Pre - and postmenopausal black and white women are quite comparable, although whites may have lower femoral bone mass. Appendicular BMD has also been reported to be similar in British and Gambian subjects despite

Much lower fracture rates in the latter. Taken together these observations suggest that factors other than just bone mass contribute to the lower incidence of fractures in South African blacks. Although a lower calcium excretion and plasma 25-OH vit D have been reported in South African blacks, no biochemical evidence of a low bone turnover and/or decreased skeletal sensitivity to PTH has been found. In fact, employing quantitative bone histology, Schnitzler and co-workers found bone turnover to be higher in black than in white subjects. Histomorphometry did however suggest that blacks have thicker trabeculae.

Clearly further studies are needed to ascertain why our Black popultions are less prone to osteoporotic fractures. The availability of modern technologies in this country provides the ideal opportunity to tackle this intriguing question. Dr Magda Conradie at the Uniiversity of Stellenbosch has recently embarked on just such a study and anyone interested in partaking or or supporting such an undertaking is invited to contact her.

Published on 2005-06-08