Objective—To evaluate the diagnostic value of serum
concentrations of total magnesium (tMg) and ionized
magnesium (iMg), concentrations of magnesium
(Mg) in muscle, intracellular Mg (icMg) concentrations,
urinary Mg excretion (EMg), Mg clearance (CMg),
and fractional clearance of Mg (FCMg) in horses fed
diets with Mg content above and below National
Research Council recommendations.
Animals—9 young female horses.
Procedures—6 horses were fed a reduced-Mg diet
for 29 days followed by an Mg-supplemented diet for
24 days. Control horses (n = 3) were fed grass hay
exclusively. Blood, urine, and tissue samples were
collected, and an Mg retention test was performed
before and after restriction and supplementation of
Mg intake. Serum tMg, serum iMg, muscle Mg,
icMg, and urine Mg concentrations were measured,
and 24-hour EMg, CMg, and FCMg were calculated.
Results—Reductions in urinary 24-hour EMg, CMg, and
FCMg were evident after 13 days of feeding a reduced-Mg diet. Serum tMg and iMg concentrations, muscle
Mg content, and results of the Mg retention test
were not affected by feeding the Mg-deficient diet.
Spot urine sample FCMg accurately reflected FCMg calculated
from 6- and 24-hour pooled urine samples.
Mean ± SD FCtMg of horses eating grass hay was 29
± 8%, whereas mean FCtMg for horses fed a reduced-Mg diet for 29 days was 6 ± 3%.
Conclusions and Clinical Relevance—The 24-hour
EMg was the most sensitive indicator of reduced Mg
intake in horses. Spot sample FCMg can be conveniently
used to identify horses consuming a diet deficient
in Mg. (Am J Vet Res 2004;65:422–430)