| Summary: | The present study sought to define a possible role of phosphorus-stimulated
ammoniagenesis, previously observed in vitro and in
experimental animals, in the hypocalciuric effect of phosphorus in young
men. It was hypothesized that titrating some of the hydrogen ions
destined for excretion with ammonia would be beneficial to kidney
calcium reabsorption because ammonium ion unlike hydrogen ion does
not increase urinary calcium loss. To test this hypothesis, seven young
men (22 - 31 years old and average weight of 70 kg) were fed a single
menu providing 800 mg phosphorus, 1200 mg calcium, 11.7 MJ (~ 2800
kcal) and 14.5 g nitrogen for 10 days. In a subsequent 10-day period,
dietary phosphorus was doubled to 1600 mg by the addition of cheddar
cheese and milk to the menu. Dietary nitrogen, calcium and vitamin D
were equalized for the two periods. Three 24-hour urine samples and a
fasting blood sample were collected at the end of each experimental
period. Comparison of period 2 with 1 showed urinary phosphorus
excretion + 82%, urinary calcium excretion - 38%, serum phosphorus
+ 6.2%, serum calcium - 3.8%, urinary sulfate and creatinine excretion
no change. Urinary ammonia nitrogen excretion increased in all seven
subjects with an average increase of 13%. Furthermore each individual
increase in ammonia nitrogen excretion was found to be directly
correlated with urinary phosphorus excretion (r = 0.76, P < 0.05).
Urinary urea nitrogen excretion was found to vary inversely with urinary
ammonia nitrogen excretion with an average decrease of 12% and
urinary total nitrogen excretion decreased 14% for the entire group.
Results obtained therefore offer a mechanism by which phosphorus
directly improves the non-parathyroid hormone portion of tubular
calcium reabsorption. The magnitude of the effects seen however suggest
that phosphorus-stimulated ammoniagenesis only plays a small role in
the mechanism of the hypocalciuric effect of phosphorus in young men. === Graduation date: 1996
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