Cardiac, bone and growth plate manifestations in hypocalcemic infants: revealing the hidden body of the vitamin D deficiency iceberg.

BACKGROUND: Whilst hypocalcemic complications from vitamin D deficiency are considered rare in high-income countries, they are highly prevalent among Black, Asian and Minority Ethnic (BAME) group with darker skin. To date, the extent of osteomalacia in such infants and their family members is unknown. Our aim was to investigate clinical, cardiac and bone histomorphometric characteristics, bone matrix mineralization in affected infants and to test family members for biochemical evidence of osteomalacia. CASE PRESENTATION: Three infants of BAME origin (aged 5-6 months) presented acutely in early-spring with cardiac arrest, respiratory arrest following seizure or severe respiratory distress, with profound hypocalcemia (serum calcium 1.22-1.96 mmol/L). All infants had dark skin and vitamin D supplementation had not been addressed during child surveillance visits. All three had severely dilated left ventricles (z-scores + 4.6 to + 6.5) with reduced ejection fraction (25-30%; normal 55-70), fractional shortening (7 to 15%; normal 29-40) and global hypokinesia, confirming hypocalcemic dilated cardiomyopathy. They all had low serum levels of 25 hydroxyvitamin D (25OHD < 15 nmol/L), and elevated parathyroid hormone (PTH; 219-482 ng/L) and alkaline phosphatase (ALP; 802-1123 IU/L), with undiagnosed rickets on radiographs. One infant died from cardiac arrest. At post-mortem examination, his growth plate showed a widened, irregular zone of hypertrophic chondrocytes. Histomorphometry and backscattered electron microscopy of a trans-iliac bone biopsy sample revealed increased osteoid thickness (+ 262% of normal) and osteoid volume/bone volume (+ 1573%), and extremely low bone mineralization density. Five of the nine tested family members had vitamin D deficiency (25OHD < 30 nmol/L), three had insufficiency (< 50 nmol/L) and 6/9 members had elevated PTH and ALP levels. CONCLUSIONS: The severe, hidden, cardiac and bone pathology described here exposes a failure of public health prevention programs, as complications from vitamin D deficiency are entirely preventable by routine supplementation. The family investigations demonstrate widespread deficiency and undiagnosed osteomalacia in ethnic risk groups and call for protective legislation.

Unexpected widespread hypophosphatemia and bone disease associated with elemental formula use in infants and children.

OBJECTIVE: Hypophosphatemia occurs with inadequate dietary intake, malabsorption, increased renal excretion, or shifts between intracellular and extracellular compartments. We noticed the common finding of amino-acid based elemental formula [EF] use in an unexpected number of cases of idiopathic hypophosphatemia occurring in infants and children evaluated for skeletal disease. We aimed to fully characterize the clinical profiles in these cases. METHODS: A retrospective chart review of children with unexplained hypophosphatemia was performed as cases accumulated from various centres in North America and Ireland. Data were analyzed to explore any relationships between feeding and biochemical or clinical features, effects of treatment, and to identify a potential mechanism. RESULTS: Fifty-one children were identified at 17 institutions with EF-associated hypophosphatemia. Most children had complex illnesses and had been solely fed Neocate® formula products for variable periods of time prior to presentation. Feeding methods varied. Hypophosphatemia was detected during evaluation of fractures or rickets. Increased alkaline phosphatase activity and appropriate renal conservation of phosphate were documented in nearly all cases. Skeletal radiographs demonstrated fractures, undermineralization, or rickets in 94% of the cases. Although the skeletal disease had often been attributed to underlying disease, most all improved with addition of supplemental phosphate or change to a different formula product. CONCLUSION: The observed biochemical profiles indicated a deficient dietary supply or severe malabsorption of phosphate, despite adequate formula composition. When transition to an alternate formula was possible, biochemical status improved shortly after introduction to the alternate formula, with eventual improvement of skeletal abnormalities. These observations strongly implicate that bioavailability of formula phosphorus may be impaired in certain clinical settings. The widespread nature of the findings lead us to strongly recommend careful monitoring of mineral metabolism in children fed EF. Transition to alternative formula use or implementation of phosphate supplementation should be performed cautiously with as severe hypocalcemia may develop.

Nutritional rickets around the world: an update.

Worldwide, nutritional rickets continues to be an evolving problem with several causes. This paper provides an updated literature review characterising the prevalence, aetiology, pathophysiology and treatment of nutritional rickets worldwide. A systematic review of articles on nutritional rickets from various geographical regions was undertaken. For each region, key information was extracted, including prevalence, cause of rickets specific to the region, methods of confirming the diagnosis and current treatment and preventive measures. Calcium deficiency continues to be a major cause of rickets in Africa and Asia. Vitamin D deficiency rickets is perhaps increasing in the Americas, Europe and parts of the Middle East. There continues to be a distinct presentation of calcium-predominant versus vitamin D predominant rickets, although there are overlapping features. More careful diagnosis of rickets and reporting of 25-OHD concentrations has improved accurate knowledge of rickets prevalence and better delineated the cause. Nutritional rickets continues to be an evolving and multi-factorial problem worldwide. It is on a spectrum, ranging from isolated vitamin D deficiency to isolated calcium deficiency. Specific areas which require emphasis include a consistent community approach to screening and diagnosis, vitamin D supplementation of infants and at-risk children, prevention of maternal vitamin D deficiency and the provision of calcium in areas with low calcium diets.

Disorders of mineral metabolism in the newborn.

Disorders of calcium and phosphorus homeostasis present both acute and chronic clinical consequences for newborns. The etiologies responsible range from iatrogenic, idiopathic, and inherited metabolic abnormalities. Maintenance of physiologically normal serum calcium and phosphorus requires complex interactions between the kidneys, gastrointestinal tract, and bone. Calciotropic hormones such as vitamin D and parathyroid hormone, as well as hormones controlling phosphorus homeostasis, such as fibroblast growth factor-23 (FGF-23), are essential in controlling these interactions. In newborns, calcium and phosphorus balance must necessarily be positive in order to provide the requisite building blocks for growth and maturation. Renal tubular handling of these minerals is a key control point in regulating the overall body balance in calcium and phosphorus. Adaptive changes in renal calcium and phosphorus reabsorption in newborns explain how a net positive total body balance of these minerals is achieved. Monogenetic disorders leading to abnormal renal handling of calcium and/or phosphorus have immediate clinical consequences in terms of complications associated with high or low levels of these minerals. Perhaps more importantly, chronic abnormalities of calcium and/or phosphorus, without treatment, may have serious consequences for growth and development of the growing skeleton. This article serves to review calcium and phosphorus regulation in the human body, describe differences in handling of these minerals by the newborn, and review the conditions, both acquired and congenital, that may present with abnormalities in calcium and/or phosphorus in the newborn period.

Evidence of rickets and/or scurvy in a complete Chalcolithic child skeleton from the El Portalón site (Sierra de Atapuerca, Spain).

A case of what are most likely metabolic diseases is identified in a child buried during Chalcolithic times in the El Portalón site (Sierra de Atapuerca, Burgos, Spain). The skeleton has been directly dated by C14 to between 5030 to 5020 Cal BP. Macroscopic analysis and a CT scan reveal a set of lesions both in the skull and the long bones, which indicate that this individual probably suffered from rickets and scurvy at different stages of his/her life. The lesions are bilateral and are characterized by abnormal porosity, new bone formation and deformation of long bones. The presence of non-specific stress indicators, such as enamel hypoplasias and Harris lines, allow us to establish two times of stress associated with these pathologies: one crisis during infancy (1-3 yrs) and subsequently a second crisis at the beginning of childhood (3-5 yrs). The etiology of both metabolic diseases could be associated with abnormal feeding during these stages of life and/or the living conditions of these populations, e.g., the preparation of food and/ or the existence of infections caused by the transmission of pathogens and unhealthy hygiene. Evidence of metabolic diseases during the recent European Prehistory is rather unknown and very few cases have been reported. Thus, the child from El Portalón can add relevant information about the life and health conditions of these prehistoric populations.

Amelogenesis imperfecta and other biomineralization defects in Fam20a and Fam20c null mice.

The FAM20 family of secreted proteins consists of three members (FAM20A, FAM20B, and FAM20C) recently linked to developmental disorders suggesting roles for FAM20 proteins in modulating biomineralization processes. The authors report here findings in knockout mice having null mutations affecting each of the three FAM20 proteins. Both Fam20a and Fam20c null mice survived to adulthood and showed biomineralization defects. Fam20b (-/-) embryos showed severe stunting and increased mortality at E13.5, although early lethality precluded detailed investigations. Physiologic calcification or biomineralization of extracellular matrices is a normal process in the development and functioning of various tissues (eg, bones and teeth). The lesions that developed in teeth, bones, or blood vessels after functional deletion of either Fam20a or Fam20c support a significant role for their encoded proteins in modulating biomineralization processes. Severe amelogenesis imperfecta (AI) was present in both Fam20a and Fam20c null mice. In addition, Fam20a (-/-) mice developed disseminated calcifications of muscular arteries and intrapulmonary calcifications, similar to those of fetuin-A deficient mice, although they were normocalcemic and normophosphatemic, with normal dentin and bone. Fam20a gene expression was detected in ameloblasts, odontoblasts, and the parathyroid gland, with local and systemic effects suggesting both local and/or systemic effects for FAM20A. In contrast, Fam20c (-/-) mice lacked ectopic calcifications but were severely hypophosphatemic and developed notable lesions in both dentin and bone to accompany the AI. The bone and dentin lesions, plus the marked hypophosphatemia and elevated serum alkaline phosphatase and FGF23 levels, are indicative of autosomal recessive hypophosphatemic rickets/osteomalacia in Fam20c (-/-) mice.

Perception and knowledge of mothers on causes and treatment of rickets associated knee deformity in Ile-Ife, Osun State, Nigeria.

This descriptive cross-sectional community-based study was carried out in Ile-Ife, Nigeria to assess the knowledge of mother's on the aetiology of rickets associated knee deformities and the cultural perception of its treatment. Data collection was done using interviewer based semi structured questionnaires. A total of 464 questionnaires were administered with a response rate of 86.9%. Over half (59.8%; N=241) and 36.5% (147) of the mothers were aware of children with knee deformity and rickets, respectively. Ninety-one (22.6%) mothers had the correct knowledge and perception of the true meaning of rickets. Rickets associated knee deformity aetiology was wrongly perceived to be mainly hereditary (53.8%), cancer (50.9%) and bone infection (48.1%). Very low proportions of mothers had correct knowledge on the causes of rickets such as inadequate exposure to sunshine (21.3%) or inadequate intake of calcium (21.1%). The knowledge of aetiology of rickets was influenced by education (P<0.02), skilled occupation (P<0.0001) and the previous birth of a child with knee deformity from rickets (P<0.001). The mother's cultural perceptions of treatment for childhood rickets associated knee deformities was significantly affected by age (P<0.001), education, (P<0.001), skilled occupation (P<0.000), history of knee deformity (P<0.04) and mothers with children diagnosed to have knee deformity (P<0.004). Lack of finance, poor compliance to treatment, too long treatment periods, lack of information on where to seek for treatment and unaffordable treatment were among the important factors affecting completeness of treatment of knee deformity due to rickets. In conclusion, the awareness of mothers about rickets in Nigeria is still very low. It is a major reason for late presentation or complete failure to seek for adequate treatment of the knee deformity due to rickets. Increase and sustain public health enlightenment programmes are necessary for prevention. Health policy should incorporate free surgical fees for the established knee deformity to encourage community participation in the management of the condition.

Critical role of calbindin-D28k in calcium homeostasis revealed by mice lacking both vitamin D receptor and calbindin-D28k.

Calbindin (CaBP)-D28k and CaBP-D9k are cytosolic vitamin D-dependent calcium-binding proteins long thought to play an important role in transepithelial calcium transport. However, recent genetic studies suggest that CaBP-D28k is not essential for calcium metabolism. Genetic ablation of this gene in mice leads to no calcemic abnormalities. Genetic inactivation of the vitamin D receptor (VDR) gene leads to hypocalcemia, secondary hyperparathyroidism, rickets, and osteomalacia, accompanied by 90% reduction in renal CaBP-D9k expression but little change in CaBP-D28k. To address whether the role of CaBP-D28k in calcium homeostasis is compensated by CaBP-D9k, we generated VDR/CaBP-D28k double knockout (KO) mice, which expressed no CaBP-D28k and only 10% of CaBP-D9k in the kidney. On a regular diet, the double KO mice were more growth-retarded and 42% smaller in body weight than VDRKO mice and died prematurely at 2.5-3 months of age. Compared with VDRKO mice, the double KO mice had higher urinary calcium excretion and developed more severe secondary hyperparathyroidism and rachitic skeletal phenotype, which were manifested by larger parathyroid glands, higher serum parathyroid hormone levels, much lower bone mineral density, and more distorted growth plate with more osteoid formation in the trabecular region. On high calcium, high lactose diet, blood-ionized calcium levels were normalized in both VDRKO and the double KO mice; however, in contrast to VDRKO mice, the skeletal abnormalities were not completely corrected in the double KO mice. These results directly demonstrate that CaBP-D28k plays a critical role in maintaining calcium homeostasis and skeletal mineralization and suggest that its calcemic role can be mostly compensated by CaBP-D9k.

Natural killer activity in the experimental privational rickets.

To study the 'in vivo' importance of vitamin D on the natural killer (NK) activity, rats were submitted to privational rickets induced by a diet deficient in vitamin D and phosphorus (D-P-). Thirty days after the beginning of treatment the animals showed low body weight, changes in the bone development, and decreased levels of 25-hydroxyvitamin D(3) (25-OH D(3)). NK activity, evaluated using a cytotoxicity assay against 51Cr-labeled Yac.1 target cells, was not modified by the rickets-inducing treatment during the first 30 days. Following a long-term treatment (60 days) the rachitic rats (D-P-) exhibited higher NK activity than control animals (D+P+) (P<0.05). On the other hand, D-P+ animals showed higher cytotoxic activity than D-P- and D+P+ groups. Feed replacement to the rachitic rats by a complete diet (D-P-/D+P+) led to a partial recuperation of growth, bone development, and 25-OH D(3) serum levels. The NK activity was also influenced by vitamin D intake, decreasing after treatment.

Static and dynamic bone changes in hospitalized patients suffering from rickets--a histomorphometric study.

AIMS: The aim of this study was to assess static and dynamic bone changes in patients suffering from rickets. METHODS AND RESULTS: Transcortical iliac crest biopsies of 15 hospitalized children with rickets were taken after labelling new bone formation with two cycles of tetracycline administration 10 days apart. Undecalcified sections were prepared, appropriately stained and histomorphometric analysis performed. Static and dynamic bone changes were measured including the volume of bone and osteoid, trabecular and cortical bone dimensions and resorptive and mineralization activities. The results were compared with normal values. The nature of the mineralization fronts was noted. Trabecular osteoid volumes of all but one patient was above the normal range of 1.9% (+/-0.4%). This patient suffered rickets associated with the Kwashiorkor-Marasmus syndrome. Tetracycline labelling was found to be more sensitive than subjective evaluation of the nature of the mineralization fronts. Despite a balanced hospital diet, a bone formation rate of zero was found in three cases, indicating a need for vitamin D and mineral supplementation. Seven cases had decreased mineralization lag times, indicating response to the balanced diet. CONCLUSIONS: This study showed that histometric analysis of labelled bone biopsies is a helpful adjunct to the diagnosis but particularly assessment of response to management of deficiency states in children.

Unusual metaphyseal disturbance in two kittens.

This report describes the presenting features, radiographic changes, biochemical alterations and clinical progress of two kittens, from separate litters, which were found to have a growth plate disturbance initially diagnosed and treated as vitamin D3-dependent rickets, but subsequently suspected to be a metaphyseal chondrodysplasia.

Histological study of tibial dyschondroplasia-like lesion from light-type chicks fed cysteine-supplemented diets.

Tibial dyschondroplasia (TD)-like lesions were induced in New Hampshire x Single-Comb White Leghorn cross chicks (NH x SCWL) by feeding excess levels of cysteine. The chicks fed a corn/soybean-meal-based diet had no spontaneous TD. The chicks fed the basal diet supplemented with 1.5% or 3.0% cysteine showed more than 50% TD-like lesions at 10 days of age; in another experiment, 1.6% cysteine induced TD (15%) at 3 weeks of age. A low incidence of rachitic-like lesions was found in Expt. 2 but was not associated with the dietary treatments. The TD-like lesion had a normal upper growth plate and a large mass of cartilage extending into the metaphysis; in contrast, the rachitic-like lesion had a widened upper growth plate. Microscopic examinations of the growth plates showed a widened prehypertrophic zone for the TD-like lesion and a widened proliferative zone for the rachitic-like lesion compared with the normal growth plate.

Effect of testosterone on bone mineralization of the hypophosphataemic rat.

Clinical observations in patients with X-linked hypophosphataemic rickets, that bone changes can be corrected during puberty, suggest that androgen can participate actively in the process of bone mineralization. In the present study we investigated the role of testosterone on the bone mineralization of male rats placed on a low phosphorus and vitamin D diet and kept in complete darkness after weaning. After 15 days the animals presented hypophosphataemia, rickets and osteomalacia, as assessed by histomorphometry of the tibia and seventh caudal vertebra calcification fronts respectively. Testosterone propionate administration for five days, while the animals were kept on the same rachitogenic diet, induced an improvement in the bone mineralization process of the hypophosphataemic rat independently of serum phosphate levels. Testosterone-treated rats were cured of rickets but not of osteomalacia, despite the reduction in osteoid seam area.

The effects of hypocalcemia/hypophosphatemia on porcine bone and dental hard tissues in an inherited form of type 1 pseudo-vitamin D deficiency rickets.

The effects of Vitamin D deficiency rickets on the formation of mineralized dental tissues were studied in a breed of pigs which had moderate and marked hypocalcemia because of an inherited defect in the renal production of the biologically-active Vitamin D metabolites. Affected piglets developed classical symptoms of rickets which were fatal unless Vitamin D supplements were given. The dissected mandibles of homozygous (rickets) and heterozygous (normal) pigs were photographed and radiographed. Compared with those of normal pigs, the mandibles of homozygous pigs demonstrated slowed development/eruption of permanent teeth, under-mineralized bone, underdeveloped dentin (enlarged pulp chambers), interglobular dentin, and enamel hypoplasia. Enamel defects in rachitic pig teeth were difficult to observe radiographically, but could be detected visually and by SDS-PAGE analysis of the enamel protein components of developing and maturing enamel. There was significant retention of amelogenins in the enamel maturation zones of developing molars taken from rachitic pigs, but there was no obvious difference in the patterns of amelogenin processing.

Osteomalacia in hereditary hypophosphatemic rickets with hypercalciuria: a correlative clinical-histomorphometric study.

We characterized the bone disease of transilial biopsy specimens from children with hereditary hypophosphatemic rickets with hypercalciuria (HHRH) and genetically related asymptomatic hypercalciuric subjects. All HHRH patients showed irregular mineralization fronts, markedly elevated osteoid surface and seam width, increased number of osteoid lamellae, and prolonged mineralization lag time. These findings are consistent with a mineralization defect and indicate unambiguously that the bone disease in HHRH is osteomalacia. The only abnormality seen in the asymptomatic hypercalciuric subjects was slightly extended osteoid surface. Parametric and nonparametric statistical analyses performed on a pooled sample of HHRH patients and asymptomatic hypercalciuric subjects revealed a very high inverse correlation and a tight linear relationship between serum phosphorus and osteoid parameters. Serum 1,25-dihydroxyvitamin D, which is low in other forms of hereditary hypophosphatemia and osteomalacia, is elevated in HHRH and correlated positively with osteoid parameters and the mineralization lag time. Serum alkaline phosphatase showed similar relationships. These results as well as the clinical, biochemical, and radiological remission of bone disease consequent to phosphate therapy strongly suggest that in HHRH 1) hypophosphatemia alone is sufficient to cause osteomalacia; and 2) the elevation of 1,25-dihydroxyvitamin D reflects the degree of the primary renal phosphate leak, but is not involved in the pathogenesis of the bone disease.

Hypercalciuric hypophosphatemic rickets, mineral balance, bone histomorphometry, and therapeutic implications of hypercalciuria.

A 14-year-old Turkish boy had severe rickets that had been clinically evident since he was 2 years of age. When he was 5 years of age, he had normal serum calcium and phosphorus levels and increased alkaline phosphatase activity. Treatment with modest dosages of vitamin D (5000 U/d for 3 weeks) resulted in hypercalcemia. At 10 years of age, high-dose vitamin D (40,000 U/d) plus phosphorus (1.1 g/d) therapy for 20 days resulted in symptomatic nephrolithiasis. When, 14 years of age, he had normocalcemia, hypophosphatemia, increased alkaline phosphatase activity, and normal circulating parathyroid hormone concentration. Levels of 25-hydroxyvitamin D were normal but those of 1,25-dihydroxyvitamin D were markedly increased. Rickets and osteopenia were evident on radiographs, and osteomalacia was present on trabecular bone obtained at biopsy. Balance study results showed increased intestinal absorption of calcium and phosphorus, hypercalciuria, and increased urinary phosphorus excretion. This patient manifests an unusual form of hypophosphatemic rickets in which hypercalciuria is a cardinal feature. In contrast with most varieties of hypophosphatemia, this disorder is characterized by appropriately increased production of 1,25-dihydroxyvitamin D in response to hypophosphatemia. It is recommended that urinary calcium excretion be assessed in all patients with hypophosphatemic rickets so that appropriate therapy will be instituted.

Familial hypophosphatemic rickets: bone mass measurements in children following therapy with calcitriol and supplemental phosphate.

Familial hypophosphatemic rickets is characterized by defective skeletal mineralization resulting in abnormal growth and development. The pathologic and radiologic correlates of this syndrome have been given some investigation, but the effect of this mineralization defect on bone mineral density has not been adequately assessed. We measured axial and appendicular bone mineral in 17 children (mean age 5.59 +/- 4.87) with familial hypophosphatemia at baseline and at 6-month intervals after initiation of therapy with vitamin D3 (calcitriol) and phosphate supplementation. Noninvasive quantitative techniques included single photon absorptiometry (SPA) of the radius, combined cortical thickness (CCT) of the second metacarpal, and quantitative computed tomography (QCT) of vertebral trabecular bone. Thoraco-lumbar and hand/wrist radiographs were qualitatively assessed for the prevalence and severity of osteosclerosis, rickets, and other parameters indicative of metabolic bone disease as well as skeletal age. Quantitative determinations of bone mineral by each technique were compared with normal values for age and sex, and individual standardized scores (z-scores) were calculated at each measurement interval. Standard scores were also calculated for bone age-adjusted mineral values. At baseline, spinal trabecular bone by QCT was not significantly different from normal values; however, measurements of peripheral cortical bone by either SPA or CCT were significantly lower than values for normal children of the same age and sex (P = 0.05 and P = 0.01, respectively). Following therapy with calcitriol and phosphate, peripheral bone mass was not shown to improve significantly when contiguous standard scores were compared even when values were adjusted for bone age.(ABSTRACT TRUNCATED AT 250 WORDS)

A comparative study of the calcification-promoting action of 1,25 (OH)2D3 and calcitonin on the growth cartilage of rats with 1-hydroxyethylidene-1, 1-biphosphonic acid (HEBP)-induced rickets.

When HEBP (1-hydroxyethylidene-1, 1-biphosphonic acid) was administered to young rats in large doses over a short period rickets was consistently produced. When HEBP was administered concomitantly with 1,25 (OH)2D3 or calcitonin (CT), calcification appeared in the growth-plate cartilage where there had been an increase in thickness due to the inhibition of calcification. This experiment was done in an attempt to clarify differences in the calcification-promoting mechanisms of 1,25 (OH)2D3 and CT. The serum alkaline phosphatase level was reduced in rats with an accelerated calcification following the administration of 1,25 (OH)2D3, but there was no reduction in the serum alkaline phosphatase level in rats in which the calcification was accelerated by the administration of CT. The mode of appearance of calcification in the growth-plate cartilage by 1,25 (OH)2D3 or CT differed, depending on the time of administration. These results suggest that mechanisms involved in the enhancement of calcification by 1,25 (OH)2D3 and CT differ in cases where rickets are induced by HEBP.

Response of rachitic rat bones to 1,25-dihydroxyvitamin D3: biphasic effects on mineralization and lack of effect on bone resorption.

Rachitic rats, maintained on a diet adequate in Ca and P, were treated daily with varying amounts of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3]. At low levels (1 ng/day) 1,25-(OH)2D3 sustained a healing response equivalent to that of 25-hydroxyvitamin D3 (100 ng/day) or the parent vitamin. Above 5 ng/day administration of 1,25-(OH)2D3 resulted in an accumulation of osteoid, giving a histological appearance similar to vitamin D deficiency. The effects of 1,25-(OH)2D3 on bone did not correlate with changes in plasma Ca or inorganic phosphorus; doses that were effective in raising bone ash and reducing the amount of osteoid failed to normalize plasma Ca, whilst the amount of sterol required to normalize plasma Ca was excessive in terms of the effect on bone. 1,25-(OH)2D3 did not stimulate any of the histological parameters of bone resorption. We conclude that 1,25-(OH)2D3 can effectively heal the bone lesions of vitamin D deficiency, but that, at high concentrations, the sterol can inhibit mineralization. Furthermore, these results question the accepted role of 1,25-(OH)2D3 as a regulator of bone resorption in vivo.