Seasonal Differences in Mineral Homeostasis and Bone Metabolism in Response to Oral Phosphate Loading in Older Northern Chinese Adults

Vitamin D deficiency in older people has been ­rec­ognized as an important factor influencing skeletal ­integrity. This is because it induces secondary hyper­parathyroidism which in turn promotes an increase in bone remodeling rate and bone loss.1,2 However, we have previously shown that vitamin D status of older people in China is very low in winter (40–50% <25 nmol 25-hydroxyvitamin D (25(OH)D)/L), and plasma parathyroid hormone (PTH) is elevated, but the incidence of osteoporotic fracture is low in this population.3–6 Resistance to the bone resorbing effects and racial differences in PTH dynamics have been reported.7 We therefore explored ethnic differences in bone and mineral metabolism as challenged by 5 days of oral phosphate (P) loading to stimulate PTH secretion in older British, Chinese, and Gambian adults in summer and winter.8 In summary, ethnic differences in bone and mineral metabolism were found, in particular, in the rate of renal clearance of phosphate, which was more rapid in the Chinese subjects than in their British and Gambian counterparts. No evidence was found for resistance to the resorbing effects of PTH. Seasonal differences in vitamin D status and bone and mineral metabolisms as challenged by P-loading were found only in the Chinese subjects and are reported here.