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

Author
item HANSEN, KAREN - University Of Wisconsin
item NABAK, ANDREA - University Of Wisconsin
item JOHNSON, RACHAEL - University Of Wisconsin
item MARVDASHTI, SHEEVA - University Of Wisconsin
item KEULER, NICHOLAS - University Of Wisconsin
item SHAFER, MARTIN - University Of Wisconsin
item ABRAMS, STEVEN - Children'S Nutrition Research Center (CNRC)

Submitted to: Journal of Nutrition
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/8/2014
Publication Date: 4/2/2014
Citation: Hansen, K.E., Nabak, A.C., Johnson, R.E., Marvdashti, S., Keuler, N.S., Shafer, M.M., Abrams, S.A. 2014. Isotope concentrations from 24-h urine and 3-h serum samples can be used to measure intestinal magnesium absorption in postmenopausal women. Journal of Nutrition. 144(4):533-537.

Interpretive Summary: Magnesium is very important in human nutrition but it is difficult to use magnesium stable isotopes to measure how much magnesium in the diet is absorbed. We collaborated with scientists in Wisconsin to test a simplified way to measure the absorption of dietary magnesium. We found that just collecting a single blood sample after giving stable isotopes was almost as good as collecting urine for three days in older women. This finding will help us further use this test to understand dietary magnesium requirements which is critical for the best bone health in children and for preventing osteoporosis in later life.

Technical Abstract: 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 (p = 0.95, P < 0.001) or the mean of the 0-24 h urine and the 3-h (p = 0.93, P < 0.001) or 5-h (p = 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.