African Americans, 25-hydroxyvitamin D, and osteoporosis: a paradox

Abstract

African Americans have lower serum 25-hydroxyvitamin D concentrations and a lower risk of fragility fractures than do other populations. I review the evidence on factors other than vitamin D that might explain this paradox and the calcium economy in different life stages. Researchers are actively trying to explain this genetically programmed advantage. Factors that could protect African Americans against fracture include their higher peak bone mass, increased obesity rates, greater muscle mass, lower bone turnover rates, and advantageous femur geometry. In addition, bone histomorphometry in young adults shows longer periods of bone formation. Although African Americans fall as frequently as do whites, the direction of their falls and their manner of breaking falls could protect them from fractures. African American girls accrue more calcium than do white girls during adolescence as the result of increased calcium absorption and superior renal calcium conservation. In adulthood, higher parathyroid hormone concentrations do not result in increased bone loss in African Americans because of their skeletal resistance to parathyroid hormone, and their superior renal conservation of calcium persists. These advantages diminish in the elderly, in whom further increases in parathyroid hormone result in increased bone turnover and bone loss. Ultimately, I explain the paradox by multiple factors associated with fracture risk and calcium economy in African Americans. Despite African Americans' reduced risk of osteoporotic fractures, such fractures remain an important public health problem for this population that vitamin D intervention studies have not addressed.

FIGURE 1

Serum 25-hydroxyvitamin D [25(OH)D] distribution in all African American and white participants in the third National Health and Nutrition Examination Survey (with sampling weights), by age.

FIGURE 2

Three-dimensional scatter plot of 4-wk serum 25-hydroxyvitamin D [25(OH)D] response change above baseline expressed as a function of both basic skin lightness (L*) and ultraviolet-B dose rate. The surface is a hyperboloid, plotting equation 1, and was fitted to the data by least-squares regression methods. Reproduced with permission from Elsevier (6).

FIGURE 3

Total femur bone mineral density (BMD) in African American and white females, by age, in the BMD Child Study and the third National Health and Nutrition Examination Survey. Reproduced with permission from the Journal of Clinical Endocrinology and Metabolism (19). Copyright 2007, the Endocrine Society.

FIGURE 4

Mean calcium retention and 95% CIs for regression lines across different calcium intakes, by race. The darker shading represents African American girls (●, 84 observations in 55 girls) and the lighter shading represents white girls (○, 98 observations for 66 girls). Reproduced with permission from the American Journal of Clinical Nutrition (50). Copyright 2007, the American Society for Nutrition.

FIGURE 5

Box and whisker plot of the percentage change in total-body bone mineral content (BMC) from baseline. Reproduced with permission from the Archives of Internal Medicine (52). Copyright 2005, American Medical Association. All rights reserved.