Curtiss D. Hunt

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 (≥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.

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 μmol (42 ± 6.5 μg)/L milk] and 4 [3.24 ± 0.6 μmol (35 ± 6.5 μg)/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.

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, Results: A mixed-model analysis of variance indicated that boron concentrations were stable in full-term (30 and 28 μg/L milk; P = 0.5) but not in preterm (37 and 27 μg/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 μg/L, P < 0.0001; PRT: 542 to 425, P = 0.05), iron (FT: 355 to 225 μg/L, P = 0.0003; PRT: 406 to 287, P = 0.002), selenium (FT: 26.9 to 18.6 μg/L, P < 0.0001; PRT: 28.7 to 20.4, P < 0.0001), and zinc (FT: 4060 to 1190 μg/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.

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; 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) μg/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.