Stimulation of bone formation in vivo by phosphate supplementation

Effects of dietary phosphates from organic and inorganic sources on parameters of phosphorus homeostasis in healthy adult dogs

BACKGROUND

The impact of dietary phosphorus (P) excess, especially on renal and cardiovascular health, has been investigated in several species, but little is known in dogs.

OBJECTIVE

The aim of this study was to examine effects of different P sources on concentration and postprandial kinetics of selected parameters of P homeostasis in dogs.

METHODS

Eight beagles received one control diet (P 0.5% dry matter [DM]) and three high P diets (poultry meal, NaH2PO4, and KH2PO4; P 1.7% DM) for 18d. Urine samples were collected pre- and postprandially while faeces were collected quantitatively for 5d and analysed for minerals. On day 18, blood was sampled 1h pre- and 0.5, 1, 1.5, 2, 3, 5 and 7h postprandially.

RESULTS

Pi (KH2PO4, NaH2PO4) but not organic P caused an increased apparent P digestibility and significantly influenced kinetics of serum FGF23, parathyroid hormone, P, CrossLaps and bonespecific alkaline phosphatase, demonstrating a disrupted calcium (Ca) and P homeostasis with potential harm for renal, cardiovascular and skeletal health.

CONCLUSIONS

Results of feeding Pi to dogs indicate distinct disturbances of Ca and P metabolism, in contrast to organic sources. The use of Pi in food can therefore not be considered as safe. Further research, especially on dose and long-term effects, is warranted.

More Bone with Less Minerals? The Effects of Dietary Phosphorus on the Post-Cranial Skeleton in Zebrafish

Dietary phosphorus (P) is essential for bone mineralisation in vertebrates. P deficiency can cause growth retardation, osteomalacia and bone deformities, both in teleosts and in mammals. Conversely, excess P supply can trigger soft tissue calcification and bone hypermineralisation. This study uses a wide range of complementary techniques (X-rays, histology, TEM, synchrotron X-ray tomographic microscopy, nanoindentation) to describe in detail the effects of dietary P on the zebrafish skeleton, after two months of administering three different diets: 0.5% (low P, LP), 1.0% (regular P, RP), and 1.5% (high P, HP) total P content. LP zebrafish display growth retardation and hypomineralised bones, albeit without deformities. LP zebrafish increase production of non-mineralised bone matrix, and osteoblasts have enlarged endoplasmic reticulum cisternae, indicative for increased collagen synthesis. The HP diet promotes growth, high mineralisation, and stiffness but causes vertebral centra fusions. Structure and arrangement of bone matrix collagen fibres are not influenced by dietary P in all three groups. In conclusion, low dietary P content stimulates the formation of non-mineralised bone without inducing malformations. This indicates that bone formation and mineralisation are uncoupled. In contrast, high dietary P content promotes mineralisation and vertebral body fusions. This new zebrafish model is a useful tool to understand the mechanisms underlying osteomalacia and abnormal mineralisation, due to underlying variations in dietary P levels.

The effects of high phosphorus intake on calcium homeostasis

Survey data confirm that the dietary pattern of many American women who are at high risk of developing osteoporosis is typically high in phosphorus and low in calcium. The imbalance between calcium and phosphorus intake may become more pronounced with continued changes in food preferences and the growing use of phosphorus-containing food additives. Recent studies in young women have shown that a high phosphorus diet moderately low in calcium results in a mild secondary hyperparathyroidism that persists over 4 weeks. Plasma levels of calcitriol did not change despite changes in PTH and serum ionized calcium. Studies on men have shown that dietary phosphorus at levels within the normal range of intakes can affect the renal production and serum concentration of calcitriol. High phosphorus intakes for ten days reduced their plasma calcitriol levels; a 70% reduction in phosphate intake significantly increased their plasma calcitriol. Thus, several lines of evidence indicate that prolonged high phosphorus intake may impair the usual homeostatic mechanisms that come into play when dietary calcium is limited. This, in turn, could impair achievement of maximal bone mass or accelerate bone loss. Although no clinical studies have linked high phosphorus intake with lower bone mass or higher rates of bone loss in humans, this relationship has been demonstrated in animal models. For example, young beagles fed high phosphorus, moderately low calcium diets showed a significant reduction in vertebral bone mass. Current dietary patterns of high phosphorus, low calcium consumption result in persistent changes in calcium regulating hormones that are not conducive to maximizing peak bone mass during growth or slowing the rate of aging bone loss. The net effect of the present dietary pattern on bone status, particularly in teenage and young adult women, needs to be determined. Optimal nutrition early in life, which may include higher calcium and lower phosphorus intakes, together with adequate exercise, may be the most cost-effective approach to the prevention of osteoporotic fractures.

Reduced serum osteocalcin and 1,25-dihydroxyvitamin D concentrations and low bone mineral content in small for gestational age infants: evidence of decreased bone formation rates

In small for gestational age (SGA) infants, bone mineral content (BMC) is low but the reasons are unclear and the possible relationships between calcium-regulating hormones and BMC have not been studied. We hypothesized that BMC would be lower and concentrations of serum parathyroid hormone and 1,25-dihydroxyvitamin D would be higher at birth in SGA infants than in appropriate for gestational age (AGA) infants. Forty-two term SGA infants and 126 term AGA infants, matched 1:3 specifically by gestation (+/- 1 week) and birth month, were studied prospectively. The BMC of the distal one third of the radius was measured before 3 days of age by photon absorptiometry. The BMC was lower in SGA than in AGA infants. Both SGA and AGA infants had lower BMC in summer or spring than in winter; BMC differences between groups remained significant after adjustment for season (p = 0.0001). Cord serum osteocalcin and 1,25-dihydroxyvitamin D values were lower in SGA than in AGA infants. There were no differences between groups in cord serum levels of intact parathyroid hormone, 25-hydroxyvitamin D, calcium, phosphorus, and magnesium. Relationships were positive between BMC and birth weight and were inverse between BMC and intact parathyroid hormone values. We suggest that reduced uteroplacental blood flow in SGA infants may result in reduced fetal-placental production of 1,25-dihydroxyvitamin D, which results in low BMC and low serum osteocalcin values; fetal serum parathyroid hormone values may be relatively elevated because of reduced placental mineral supply.

Effects of a one-year administration of phosphate and intermittent calcitonin on bone-forming and bone-resorbing cells in involutional osteoporosis: a histomorphometric study

The bone histomorphometric effects of intermittent phosphate and calcitonin therapy during 1 year were analyzed in 15 involutional osteoporotic patients. Phosphate was administered continuously (1.5 g/day) and calcitonin was injected during 5 days every third week (50 IU/day). The bone cell response was analyzed in two separate groups, according to the amount of trabecular bone present in the iliac bone biopsy: patients with trabecular bone volume (TBV) beyond the histomorphometric spontaneous fracture threshold (0.16 mm3/mm3) (group 1; 11 patients) and patients with TBV above this threshold (group 2; 4 patients). In group 1, the treatment significantly increased TBV from 0.113 +/- 0.025 to 0.156 +/- 0.046 mm3/mm3 by thickening the existing trabeculae rather than by creating new trabeculae; stimulation of bone formation rate (+ 50%) and significant reduction in active trabecular resorption surfaces (from 0.021 +/- 0.013 to 0.010 +/- 0.006 mm2/mm2; P less than .05) may have led to positive bone balance. In group 2, TBV was not changed because of the treatment's relative inefficiency for reducing the bone-resorbing cell activity, leading to likely persistent negative bone balance. Cortical thickness did not change in either group. This study confirms the positive effectiveness of continuous treatment with phosphate and intermittent calcitonin during 1 year on bone balance in involutional osteoporosis with low amount of bone. The lack of response in patients with normal amount of bone must be verified before raising the hypothesis of different bone cell activity and before anticipating the therapeutic response according to local bone mass besides bone remodeling status in osteoporosis.

Therapy of hypercalcemia of malignancy

An appreciation of the pathogenesis of the hypercalcemia of malignancy is essential to its management. At the outset, since most patients with symptomatic hypercalcemia of malignancy are dehydrated, infusion of 2 to 3 liters of saline per day will at least partially reduce serum calcium levels. Induction of calciuresis by infusing larger volumes of saline simultaneously with parenteral administration of furosemide may reduce the serum calcium concentration to normal in the short term. Of major importance in long-term therapy, however, are drugs that inhibit bone resorption, a major cause of hypercalcemia. These include calcitonin, plicamycin, glucocorticoids, prostaglandin synthetase inhibitors, and the diphosphonates. These agents may provide long-term control of hypercalcemia in many patients. Reduction of intestinal calcium absorption by dietary means or by glucocorticoid therapy is often effective in the rare subset of patients with increased serum levels of 1,25-dihydroxyvitamin D. Oral and intravenous phosphorus therapy may be effective via unknown mechanisms in some patients. The diphosphonates, in particular, should greatly facilitate management of both acute and chronic hypercalcemia of malignancy. Daily intravenous infusion of etidronate disodium (etidronate) with saline over a period of three to six days, for example, appears to be a safe and effective means of restoring serum calcium concentrations to the normal range. Study results have shown that more than 90 percent of patients have a response to etidronate. Oral administration of the drug has been demonstrated to maintain normal serum calcium concentrations.

Effects of 1,25-dihydroxycholecalciferol on fracture healing. Calcium, phosphate, and zinc in callus and serum

The incorporation of calcium, phosphate, and zinc into the callus of closed tibial fractures was studied in adult rats fed a standard diet. Low doses (60 ng/kg per day) of 1,25(OH)2D3 5 days a week greatly increased early callus mineralization. This was not related to an increased serum calcium-phosphate molar product but rather to a decreased ratio. The incorporation of zinc into callus seemed to be correlated to the mineralization process but not to the 1,25(OH)2D3 administration as such. The question of a direct, indirect, or a complex role of 1,25(OH)2D3 in bone formation and mineralization is discussed.

Preliminary observations of a form of coherence therapy for osteoporosis

Five patients suffering from osteoporosis were subjected to from three to eight cycles of a form of Coherence Therapy for Osteoporosis (1) used at this center. Assessment of the patients' trabecular bone remodeling activity as measured by histomorphometric analyses of trans-iliac crest bone biopsies showed a marked improvement.

Qualitative bone defect in uremic osteosclerosis

Osteosclerosis, an increased volume of trabecular bone, is a common but often misinterpreted feature of uremic osteodystrophy. Despite the apparent radiographic density of osteosclerotic bone, pain and fracture may be associated. If accumulated osteoid and woven bone exceed the volume of lamellar bone removed in chronic renal insufficiency, bone density may be reduced despite increased trabecular volume. Concomitant histomorphometric and photon absorption determinations of transileal bone biopsies were done to investigate the relationship between quantity and quality of bone in uremic and non-uremic osteopenic patients. In osteopenic patients with uremia, bone core density had no significant relationship to trabecular bone volume or mineralized bone volume whereas in non-uremic osteopenic patients, these parameters were directly related (r = 0.867 and r = 0.921, respectively, p less than 0.001). The bone core density in the uremic patients was negatively correlated with the total osteoid volume (r = -0.764, p less than 0.05) and positively related to the serum phosphorus concentration (r = 0.739, p less than 0.05). Serum levels of immunoreactive parathyroid hormone (iPTH) and alkaline phosphatase activity were higher in the patients with radiographic osteosclerosis than in the other uremic patients. The lack of correlation between bone volume and density indicates a qualitative defect in uremic bone. It appears that in uremia, elevated iPTH and serum phosphorus levels may augment bone formation, albeit poorly mineralized with woven architecture. While radiographic density paradoxically increases, the amount of normally mineralized bone may be reduced.

Bone response to phosphate and vitamin D metabolites in the hypophosphatemic male mouse

The hypophosphatemic male mouse (Hyp/y), the proposed model for human vitamin D-resistant rickets (VDRR), is characterized by chronic hypophosphatemia, dwarfism, and rachitic and osteomalacic bone lesions. We have reported that treatment of Hyp/y mice with phosphate salts (Pi) heals rickets but does not correct the defective endosteal bone mineralization. In an attempt to cure osteomalacia, mutant male animals were treated with Pi combined with 25-hydroxyvitamin D3 (25OHD3, 1 microgram/kg/day), 24,25-dihydroxyvitamin D3 [24,25(OH)2D3, 0.5 microgram/kg/day], or 1,25-dihydroxyvitamin D3 [1,25(OH)2D3, 0.05--0.25 microgram/kg/day] infused constantly for 3 weeks. The biochemical and skeletal effects of treatment were assessed by analytical methods and bone histomorphometry. The results show that only 1,25(OH)2D3 produced a dose-dependent elevation of serum calcium and phosphorus, and greatly improved bone mineralization at doses high enough to increase serum calcium and phosphorus concentrations within or above the normal range. Better improvement of bone mineralization was obtained when Pi was combined to 1,25(OH)2D3. In conjunction with the correction of hypocalcemia, Pi + 1,25(OH)2D3 suppressed the stimulation of bone turnover induced by Pi supplementation. The results show that, as in VDRR children, 1,25(OH)2D3 produces beneficial effects on bone lesions in Hyp/y mice, mainly through enhancement of mineral availability. However, the persistence of osteomalacia despite correction of serum mineral concentrations suggests that there is a specific bone cell resistance to mineral and/or hormonal influences in Hyp/y mice.

Healing of rickets with phosphate supplementation in the hypophosphatemic male mouse

The hypophosphatemic male mouse, an animal model for human vitamin D-resistant rickets, is characterized by low serum phosphorus concentration due to increased urinary phosphate excretion, rickets, osteomalacia, and dwarfism. Because phosphate administration can heal rickets but not osteomalacia in the human disease, we have compared the effect of phosphate supplementation on the epiphyseal and endosteal bone mineralization in the mutant animal. Phosphate was given in drinking water for 137 d and the biochemical and bone responses were assessed by analytical and histomorphometric methods. Treatment with phosphate normalized the endochondral calcification (vertebral growthplate thickness: 83 +/- 5 SD vs. controls [+/Y] 73 +/- 8 micrometers, NS), but did not correct the endosteal bone mineralization (mineralization front: 13.6 +/- 2.7 vs. +/Y 67.1 +/- 6.9% osteoid surface, P less than 0.001, endosteal mean osteoid seam thickness: 46.4 +/- 6.1 vs. +/Y 3.3 +/- 0.3 micrometers, P less than 0.001). In addition, both osteoblastic and osteoclastic recruitment and activity were stimulated, as a result of a probable increase in parathyroid hormone secretion following the phosphate induced fall in serum calcium. Our results show that in the hypophosphatemic mouse, phosphate supplementation can heal the epiphyseal, but not the endosteal defective bone mineralization. Then, the biochemical and skeletal response to phosphate therapy appear to be similar to what we have observed in the human disease, further stressing the interest of the animal model.