Isotope concentrations from 24-h urine and 3-h serum samples can be used to measure intestinal magnesium absorption in postmenopausal women.

Studies suggest a link between magnesium status and osteoporosis. One barrier to more conclusive research on the potential relation is measuring intestinal magnesium absorption (MgA), which requires the use of stable isotopes and a ≥6-d stool or 3-d urine collection. We evaluated alternative methods of measuring MgA. We administered 2 stable magnesium isotopes to 15 postmenopausal women (cohort 1) aged 62 ± 8 y with a dietary magnesium intake of 345 ± 72 mg/d. Participants fasted from 1200 h to 0700 h and then consumed breakfast with ∼23 mg of oral ²6Mg and ∼11 mg of i.v. ²5Mg. We measured magnesium isotope concentrations in 72-h urine, spot urine (36, 48, 60, and 72 h), and spot serum (1, 3, and 5 h) samples collected after isotope dosing. We calculated MgA using the dose-corrected fraction of isotope concentrations from the 72-h urine collection. We validated new methods in 10 postmenopausal women (cohort 2) aged 59 ± 5 y with a dietary magnesium intake of 325 ± 122 mg/d. In cohort 1, MgA based on the 72-h urine collection was 0.28 ± 0.08. The 72-h MgA correlated most highly with 0-24 h urine MgA value alone (ρ = 0.95, P < 0.001) or the mean of the 0-24 h urine and the 3-h (ρ = 0.93, P < 0.001) or 5-h (ρ = 0.96, P < 0.001) serum MgA values. In cohort 2, Bland-Altman bias was lowest (-0.003, P = 0.82) using means of the 0-24 h urine and 3-h serum MgA values. We conclude that means of 0-24 h urine and 3-h serum MgA provide a reasonable estimate of 72-h MgA. However, if researchers seek to identify small changes in MgA, we recommend a 3-d urine or extended stool collection.

Can a questionnaire predict vitamin D status in postmenopausal women?

OBJECTIVE: Our objective was to determine whether a questionnaire can identify individuals with vitamin D insufficiency (VDI). DESIGN: Women completed the Vitamin D & Sun (VIDSUN) questionnaire and we measured their serum 25-hydrocyvitamin D (25(OH)D) levels. We assessed the sensitivity and specificity of the questionnaire to identify VDI (25(OH)D level <50 nmol/l). SETTING: Clinical Research Unit, University of Wisconsin-Madison. SUBJECTS: Postmenopausal women. RESULTS: We recruited 609 postmenopausal women with a mean age of 61 (sd 6 years), of whom 113 (19%) had VDI. Women with VDI were more likely to be black (17% v. 2%, P < 0.001), heavier (BMI 33.0 (sd 7) kg/m2 v. 29.0 (sd 7) kg/m2, P < 0.001) and less likely to tan in the past year (49% v. 72%, P < 0.001), use sunscreen (57% v. 72%, P < 0.001) or report sun exposure in the last 3 months. They consumed less vitamin D from supplements (2.15 (sd 5.24) µg/d (86 (sd 210) IU/d) v. 4.55 (sd 8.48) µg/d (188 (sd 344) IU/d), P = 0.003). In logistic regression models, black race, BMI, suntan within the past year, sun exposure in the past 3 months, sunscreen use and supplemental vitamin D intake were the most useful questions to identify VDI. From these six items, a composite score of ≤ 2.25 demonstrated ≥89% sensitivity but ≤35% specificity for VDI. CONCLUSIONS: The VIDSUN questionnaire provides an initial tool to identify postmenopausal women at high or low risk of VDI. Existing studies suggest that inclusion of physical activity and TAG levels might improve the performance of the VIDSUN questionnaire.