URINARY CALCIUM, SODIUM AND BONE MASS OF YOUNG FEMALES

Calcium is an important determinant of peak bone mass in young adults because of its influence on skeletal development during growth. Attainment of maximum peak bone mass requires optimal positive balance between calcium intake and obligatory losses of calcium, primarily in urine and feces. Urinary excretion is an important determinant of calcium retention in the body. Accordingly, the purpose of this study was to evaluate the influence of various nutrients on urinary calcium excretion, and to assess their impact on bone mass of young females, aged 8-13 y, during early puberty. The study was conducted in 381 healthy white females in pubertal stage 2. From each participant we collected basic anthropometric measurements, a 3-d food record, blood, a 24-h urine sample, and bone mass measurements of the total body and forearm by dual X-ray absorptiometry. Urinary sodium was found to be one of the most important determinants of urinary calcium excretion: [urinary calcium (mmol/d) = 0.01154 X urinary sodium (mmol/d) + 0.823], whereas calcium intake had relatively little impact: [urinary calcium (mmol/d) = 0.02252 X calcium intake (mmol/d) + 1.5261]. Urinary calcium was much higher at a calcium intake of approximate to 37.5 mmol/d (1500 mg/d), supporting the notion that calcium is a threshold nutrient. Calcium intake had a significant positive influence on the bone mineral content and density of the whole body and radius shaft whereas urinary calcium had a negative influence, presumably by reducing calcium accretion into the skeleton. Low calcium intake and relatively high obligatory calcium loss in the urine, potentiated by sodium intake during rapid growth, may reduce calcium retention in the skeleton with a concomitant reduction in peak bone mass. The results of this study suggest that the calcium requirement for adolescents may be influenced by sodium intake.