Calcium requirements of growing rats based on bone mass, structure, or biomechanical strength are similar.

Although calcium (Ca) supplementation increases bone density, the increase is small and the effect on bone strength and fracture risk is uncertain. To investigate if bone mass, morphology, and biomechanical properties are affected by deficient to copious dietary Ca concentrations, the long bones (tibia and femur) of growing female Sprague-Dawley rats (8/group) were assessed after 13 wk of consuming 1, 2, 3, 4, 5, 6, or 7 g Ca/kg of a modified AIN-93G diet. Dietary phosphorous (P) and vitamin D remained constant at recommended concentrations. The assessment included mineralization, density, biomechanical properties of breaking by a 3-point flexure test, and morphological properties by microcomputed topography scanning of trabecular bone of the proximal tibia metaphysis. Dietary treatment did not affect food intake, weight gain, renal and muscle Ca concentrations, and bone hydroxyproline. All bone parameters measured were significantly impaired by Ca deficiency in rats fed the diet containing 1 g Ca/kg. Modest impairments occurred with some parameters (bone density, biomechanical bending moment, modulus of elasticity, and stress) in rats fed 2 g Ca/kg, but all parameters stabilized between 2 and 3 g/kg diet, with no differences between 3 and 7 g/kg. The results suggest that a threshold response in bone Ca retention or bone mass at approximately 2.5 g Ca/kg diet is associated with similar threshold responses in bone breaking strength and related biomechanics as well as trabecular structural properties. There was no evidence of a relative P deficiency or of improved or impaired bone strength and structure as Ca intakes increased beyond those needed to maximize bone density.

Calcium requirements: new estimations for men and women by cross-sectional statistical analyses of calcium balance data from metabolic studies.

BACKGROUND: Low intakes of calcium are associated with an increased risk of both osteoporosis and cardiovascular disease. OBJECTIVE: To provide new estimates of the average calcium requirement for men and women, we determined the dietary calcium intake required to maintain neutral calcium balance. DESIGN: Calcium balance data [calcium intake -(fecal calcium + urinary calcium)] were collected from 155 subjects [women: n = 73; weight: 77.1 +/- 18.5 kg; age: 47.0 +/- 18.5 y (range: 20-75 y); men: n = 82; weight: 76.6 +/- 12.5 kg; age: 28.2 +/- 7.7 y (range: 19-64 y)] who participated in 19 feeding studies conducted in a metabolic unit. Balance data from the final 6-12 d of each dietary period (minimum length:18 d) of each study (1-9 observations per subject) were analyzed. Data were excluded if individual intakes of magnesium, copper, iron, phosphorus, or zinc fell below the estimated average requirements or exceeded the 99 th percentile of usual intakes from the 1994 Continuing Survey of Food Intakes by Individuals (for iron, above the upper limit). Daily intakes of calcium ranged between 415 and 1740 mg. The relation between intake and output was examined by fitting random coefficient models. Coefficients were included to test for sex and age differences. RESULTS: The models predicted a neutral calcium balance [defined as calcium output (Y) equal to calcium intake (C)] at intakes of 741 mg/d [95% prediction interval (PI): 507, 1035; Y = 148.29 + 0.80C], 9.4 mg kg body wt(-1) d(-1) [95% PI: 6.4, 12.9; Y = 1.44 + 0.85C], or 0.28 mg kcal(-1) d(-1) [95% PI: 0.19, 0.38; Y = 0.051 + 0.816C]. Neither age nor sex affected the estimates when calcium intakes were expressed as mg/d or as mg kg body wt(-1) d(-1). CONCLUSION: The findings suggest that the calcium requirement for men and women is lower than previously estimated.

Magnesium requirements: new estimations for men and women by cross-sectional statistical analyses of metabolic magnesium balance data.

BACKGROUND: Current recommendations for magnesium requirements are based on sparse balance data. OBJECTIVE: To provide new estimates of the average magnesium requirement for men and women, we pooled magnesium data from 27 different tightly controlled balance studies conducted at the US Department of Agriculture, Agricultural Research Service, Grand Forks Human Nutrition Research Center, Grand Forks, ND. DESIGN: Magnesium balance data (magnesium intake - [fecal magnesium + urinary magnesium]) (664 data points) were collected from 243 subjects (women: n = 150; weight: 71.6 +/- 16.5 kg; age: 51.3 +/- 17.4 y; men: n = 93, weight: 76.3 +/- 12.5 kg; age: 28.1 +/- 8.1 y). Data from the last 6-14 d of each dietary period (> or =28 d) of each study were analyzed and were excluded if individual intakes of calcium, copper, iron, phosphorus, or zinc fell below respective estimated average requirements (EARs) or exceeded 99th percentiles of usual intakes of those elements (iron: above the upper limit) from the 1994 Continuing Survey of Food Intakes by Individuals. Daily intakes of magnesium ranged between 84 and 598 mg. The relation between magnesium intake and magnesium output was investigated by fitting random coefficient models. RESULTS: The models predicted neutral magnesium balance [defined as magnesium output (Y) equal to magnesium intake (M)] at magnesium intakes of 165 mg/d [95% prediction interval (PI): 113, 237 mg/d; Y = 19.8 + 0.880 M], 2.36 mg . kg(-1) . d(-1) (95% PI: 1.58, 3.38 mg . kg(-1) . d(-1); Y = 0.306 + 0.870 M), or 0.075 mg . kcal(-1) . d(-1) (95% PI: 0.05, 0.11 mg . kcal(-1) . d(-1); Y = 0.011 + 0.857 M). Neither age nor sex affected the relation between magnesium intake and output. CONCLUSION: The findings suggest a lower magnesium requirement for healthy men and women than estimated previously.

Boron concentrations in milk from mothers of full-term and premature infants.

BACKGROUND: Boron is a bioactive element that satisfies several of the criteria for essentiality in humans. OBJECTIVE: The objective was to establish the profile of boron metabolism in human milk. DESIGN: Lactating mothers of premature (PRT; n = 10, <2000 g birth weight, <37 wk gestation) and full-term (FT; n = 10, >2500 g, 39-41 wk gestation) infants living in St John's, Canada, collected milk samples once a week for 12 wk. Samples were analyzed for boron, copper, iron, selenium, and zinc by atomic emission or absorption spectrometry after wet-ash digestion with nitric acid and hydrogen peroxide in polytetrafluoroethylene tubes. RESULTS: A mixed-model analysis of variance indicated that boron concentrations were stable in full-term (30 and 28 mug/L milk; P = 0.5) but not in preterm (37 and 27 mug/L; P = 0.01) milk between weeks 1 and 12, respectively. As expected, there were reductions in the concentrations of copper (FT: 651 to 360 mug/L, P < 0.0001; PRT: 542 to 425, P = 0.05), iron (FT: 355 to 225 mug/L, P = 0.0003; PRT: 406 to 287, P = 0.002), selenium (FT: 26.9 to 18.6 mug/L, P < 0.0001; PRT: 28.7 to 20.4, P < 0.0001), and zinc (FT: 4060 to 1190 mug/L, P < 0.0001; PRT: 5970 to 1270, P < 0.0001) over time. CONCLUSIONS: The stable milk boron concentrations over time suggest that boron may be under homeostatic control. The patterns of change in copper, iron, selenium, and zinc concentrations in milk differ from those of boron. Further research is needed to elucidate the mechanism of milk boron secretion.

Transmembrane partitioning of boron and other elements in RAW 264.7 and HL60 cell cultures.

The trace element boron is essential for all higher plants and is beneficial or has been established as essential for several animal models of human nutrition. To help identify the biomolecules that require boron for function in humans, we determined whether intracellular boron is retained against a concentration gradient. Cells (Abelson leukemia virus BALB murine monocyte-macrophage RAW 264.7 [RAW] and HL60) and supplemented media (Dulbecco's modified essential media [+ 10% fetal calf serum] and Iscove's modified Dulbecco's medium [+ 5% fetal calf serum], respectively) were analyzed for mineral concentrations after culture and subculture. Special corrections were made for trapped extracellular media in cell pellets and endocytosed media. For RAW cells, the partitioning coefficients (PC; intracellular/extracellular ratios) were, in rank order, as follows: Mn, 110; Fe, 67; P, 65; Zn, 32; K, 15; Cu, 7.1; Mg, 4.3; B, 1.7; Ca, 0.4; Na, 0.3. For HL60 cells, the partitioning coefficients were, in rank order, as follows: Mn, 212; Zn, 211; P, 123; K, 21; Fe, 16; Mg, 11; B, 1.7; Ca, 0.8; Na, 0.3. Trapped extracellular media was estimated to be 6.7 +/- 0.8%; trapped extracellular and endocytosed media together was 24.8 +/- 0.3% of the mass within the isolated cell pellets. The partitioning coefficients indicate a positive gradient for intracellular accumulation of boron, zinc, phosphorus, manganese, magnesium, potassium, iron, and copper in RAW264.7 and HL60 cells. Specifically, the data indicate the existence of a selective boron-binding molecular species within the cell or the existence of a boron-specific membrane transporter.

Dietary boron: progress in establishing essential roles in human physiology.

This review summarizes the progress made in establishing essential roles for boron in human physiology and assesses that progress in view of criteria for essentiality of elements. The evidence to date suggests that humans and at least some higher animals may use boron to support normal biological functions. These include roles in calcium metabolism, bone growth and maintenance, insulin metabolism, and completion of the life cycle. The biochemical mechanisms responsible for these effects are poorly understood but the nature of boron biochemistry suggests further characterization of the cell signaling molecules capable of complexing with boron. Such characterization may provide insights into the biochemical function(s) of boron in humans.

The selenium metabolite methylselenol inhibits the migration and invasion potential of HT1080 tumor cells.

There is increasing evidence for the efficacy of certain forms of selenium as cancer-chemopreventive compounds. Methylselenol has been hypothesized to be a critical selenium metabolite for anticancer activity in vivo. To determine whether tumor cell migration, invasion, and cell cycle characteristics are inhibited by methylselenol, we exposed HT1080 cells to methylselenol. Methylselenol was generated with seleno-L-methionine (a substrate for methioninase). Submicromolar methylselenol exposure led to an increase in the G1 and G2 fractions with a concomitant drop in the S-phase, indicating slower cell growth. Furthermore, methylselenol inhibited the migration and invasion rate of the tumor cells by up to 53 and 76%, respectively, when compared with the control tumor cells. Although all cells had increased matrix metalloproteinase (MMP) enzyme activities of pro-MMP-2 and pro-MMP-9, the active form of MMP-2 was decreased in HT1080 cells cultured with methylselenol. In addition, methylselenol increased the protein levels of antimetastasic tissue inhibitor metalloproteinase (TIMP)-1 and TIMP-2. Collectively, these results demonstrate that submicromolar concentrations of methylselenol increase both prometastasis MMP-2 and MMP-9 and antimetastasis TIMP-1 and TIMP-2 expression. The apparent net effect of these changes is the inhibition of pro-MMP-2 activation and carcinogenic potential or activity.

Boron concentrations in milk from mothers of exclusively breast-fed healthy full-term infants are stable during the first four months of lactation.

Because boron is a bioactive element that satisfies several of the criteria for essentiality in humans, the aim of the present work was to determine the profile of boron metabolism in human milk during the first 4 mo of lactation. The concentration of boron and other minerals was determined in archived milk collected (1980-84) 1 time/mo for 4 mo from lactating mothers of full-term, exclusively breast-fed infants living in Houston, TX. A linear model (treating month as a continuous variable) indicated that B concentrations were stable (P = 0.14) between mo 1 [3.88 +/- 0.6 mumol (42 +/- 6.5 microg)/L milk] and 4 [3.24 +/- 0.6 micromol (35 +/- 6.5 microg)/L milk, mean +/- SEM]. Mg concentrations increased slightly over time (1.18 +/- 0.09 to 1.36 +/- 0.09 mmol/L, P < 0.0001), whereas Ca concentrations decreased slightly (7.01 +/- 0.29 to 6.68 +/- 0.29 mmol/L milk, P < 0.02) and Zn decreased substantially (0.04 +/- 0.004 to 0.02 +/- 0.004 mmol/L milk, P < 0.0001). Similarities in findings reported here and earlier (from samples collected in St. John's, Newfoundland) provide further evidence that boron may be metabolically regulated. Future investigations of boron regulatory mechanisms should focus on metabolism of bone as the major storage site of B and kidney excretion, the major excretory route for B.

Dietary boron decreases peak pancreatic in situ insulin release in chicks and plasma insulin concentrations in rats regardless of vitamin D or magnesium status.

Because dietary boron deprivation induces hyperinsulinemia in vitamin D-deprived rats, the influence of dietary boron on insulin metabolism as modified by nutritional stressors was examined in two animal models. Male weanling Sprague-Dawley rats were assigned to each of four (Experiment 1) or 8 (Experiment 2) dietary groups for 35 d: the basal diet (< 0.2 mg B; <1.0 mg Mg/kg) was supplemented with boron (as orthoboric acid) to contain <0.2 or 2.0 (a physiologic amount) mg B/kg; with magnesium (as magnesium acetate), at 100 (inadequate) or 360-400 (adequate) mg/kg; and with cholecalciferol [vitamin D-3; 25 microg/kg for study length (Experiment 2), or, depleted for 16-17 d then repleted until end of experiment (Experiments 1 and 2)]. In the rat model, boron reduced plasma insulin (Experiment 1, P < 0.002; Experiment 2, P < 0.03), but did not change glucose concentrations regardless of vitamin D-3 or magnesium status. Cockerels (1 d old) were fed a ground corn, high protein casein and corn oil-based basal diet (low boron; 0.3 mg B/kg) supplemented with boron as orthoboric acid to contain 0.3 or 1.65 mg/kg (a physiologic amount) and vitamin D-3 at 3.13 (inadequate) or 15.60 (adequate) microg/kg. In the chick model, boron decreased (P < 0.045) in situ peak pancreatic insulin release at 26-37 d of age regardless of vitamin D-3 nutriture. These results suggest that physiologic amounts of boron may help reduce the amount of insulin required to maintain plasma glucose.