CLINICAL CASES
459
[51.3]
Why can excessive ingestion of phosphate-containing soft drink in oth-
erwise well-nourished individuals lead to decreased bone density?
A. Increased levels of blood phosphate ion interact with sensors on the
membrane of parathyroid cells to stimulate parathyroid hormone
release.
B. Phosphate ion binds the active site on calcium transporters in the
intestine, inhibiting their ability to transport calcium.
C. Phosphate ion depletes cellular levels of ATP resulting in inhibition
of Ca2+-ATPase calcium transporters.
D. Phosphate ion spontaneously forms an insoluble precipitate with
calcium ion, decreasing its absorption in the intestine.
E. Phosphate ion is excessively incorporated into bone, weakening its
structure.
A nsw ers
[51.1]
B. Hyperparathyroidism is the likely cause of all of the patient’s
symptoms. Increased parathyroid hormone leads to bone demineral-
ization, increased calcium uptake from the intestine, increased blood
levels of calcium, decreased calcium ion excretion by the kidney, and
increased phosphate excretion in the urine. Increased blood calcium
levels caused renal stones, while bone demineralization progressed to
osteopenia. The patient’s intake of calcium and vitamin D are not
excessive. Calcitonin acts to decrease bone demineralization. Muscle
weakness and depression reflect the widespread role of calcium ion
in many physiologic processes.
[51.2]
C. The major action of vitamin D is to increase absorption of calcium
from the small intestine. Deficiency of the vitamin leads to low blood
calcium levels, stimulation of parathyroid hormone secretion, and
acting synergistically, promotion of bone demineralization. Renal
excretion of calcium is decreased by hypocalcemia but elevated
parathyroid hormone levels promote renal excretion of phosphate, to
prevent excessive accumulation of this product of bone demineraliza-
tion. Although lack of exercise decreases bone density, it does not
lead to rickets if vitamin D is sufficient.
[51.3]
D. Precipitate formation effectively decreases available calcium ion.
Because of widespread consumption of soft drinks rather than water
by school age children aided by school vending machines, this con-
sequence is of concern.
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