Quantitative assessment of nutritional rickets by urinary phosphorus excretion

Evolutionary Perspective in Rickets and Vitamin D

Modern lifestyle limits our exposure to sunlight, which photosynthesizes vitamin D in the skin, and the incidence of nutritional rickets has been resurging. Vitamin D is one of the first hormones; it is photosynthesized in all organism from the phytoplankton to mammals. A selective sweep of the promoter of the vitamin D receptor (VDR) happened as soon as Homo sapiens migrated out of Africa; it co-adapted with skin color genes to provide adaptation to latitudes and the levels of exposure to ultraviolet (UV)B radiation along the route out of Africa. Exposure to UVB radiation balances the need for vitamin D photosynthesis and degradation of folic acid by UVB radiation. Skin color follows a latitude distribution: the darkest populations dwell in the tropical belt; and the fair-skinned populations inhabit the northern countries. Due to their greater need for calcium during their reproductive life, the skin color of women is lighter- than that of men. Vitamin D is essential for mineral homeostasis and has a wide variety of non-skeletal functions, of which the most important for natural selection is a regulatory function in the innate immune system. In the human fossil record, vitamin D deficiency coincided with bone tuberculosis. About 6,000 years ago, a diet which included cow's milk provided Neolithic humans with twice as much calcium and was more alkaline than that of its Paleolithic predecessors. Adiposity is negatively associated with the vitamin D status and obese individuals require 2-3 times more vitamin D than non-obese individuals to normalize circulating 25OHD levels. In an era of an obesity epidemic, we need more research to determine whether adiposity should be considered when determining the dietary requirements for vitamin D and calcium and the optimal serum 25OHD levels.

Parathyroid function in different stages of vitamin D deficiency rickets

Direct measurements of parathyroid activity are available in only small numbers of children with vitamin D deficiency rickets (VDR). Therefore serum immunoreactive parathyroid hormone (iPTH) and the urinary cyclic adenosine-3',5'-monophosphate excretion (UcAMP) were measured together with other important indices of calcium metabolism in 24 patients (aged 2-42 months) with VDR before vitamin D treatment. iPTH and UcAMP were significantly elevated in comparison to age-matched controls. In patients there was a highly significant positive correlation between iPTH and UcAMP and a negative relationship between both indices of parathyroid activity to serum phosphate and urine calcium, respectively, indicating that the simple measurement of serum phosphate and/or urine cAMP and Ca provides a reliable tool for the assessment of secondary hyperparathyroidism in VDR. In two patients classified as being in the early stage of VDR the parathyroid activity was not elevated despite hypocalcemia indicating relative hypoparathyroidism. Twelve patients with VDR were followed during vitamin D therapy: Within the first 2 weeks of treatment UcAMP slightly increased and thereafter decreased in most patients, but was still elevated in three patients even after 7 weeks, whereas iPTH became normal within 3 weeks of treatment. This favors the concept that vitamin D deficiency diminishes the activation of renal adenylate cyclase by PTH which is overcome by the highly increased PTH secretion in the advanced stages of rickets. The basal and calcium-stimulated serum calcitonin (CT) levels, determined in some of the patients, were normal, ruling out a significant disturbance of CT secretion in VDR.