Calcium fractional excretion

Ca X P calcium phosphorus product serum calcium multiplied by serum phosphorus CKD chronic kidney disease CPK creatine phosphokinase DEO deferoxamine EPO erythropoietin ESKD end-stage kidney disease ESRD end-stage renal disease FEk fractional excretion of potassium FEn fractional excretion of sodium GFR glomerular filtration rate Hct hematocrit HDL high-density fipoprotein Hgb hemoglobin... 

While alcohol abuse may be associated with a variety of electrolyte and acid-base disorders, the role of the kidneys in this process has only recently been fully defined [164]. Renal functional abnormalities have now been related to chronic alcoholism in patients without liver disease and these defects have reverted to normal with abstinence from alcohol abuse. These abnor-mahties include decreases in the maximal reabsorptive abihty and threshold for glucose, a decrease in the threshold for phosphate excretion, and increases in the fractional excretion of P2-microglobulin, uric acid, calcium, magnesium, and amino acids. Defective tubular acidification and impaired renal concentrating ability... 

It was observed, however, that when the compound was excreted into the urine it carried with it an equivalent of calcium derived from its combination with serum calcium. The net result consequently was an increase in the total urine calcium excretion with no decrease in the ionic calcium fraction. 

But even in osteoporosis due to calcium deficiency we have no means of judging whether the condition has resulted from a lack of absorption of calcium, or indeed whether this factor has played any role whatsoever in its causation. For as is well known calcium is not only absorbed from the intestinal canal, but is excreted into it. It is evident therefore that the calcium in the feces represents not only calcium which has failed to be absorbed but likewise that which has been absorbed and not utilized. We have no means of separating the two fractions and do not know even approximately the percentage of calcium normally excreted through the intestinal wall. In view of this situation it is clear, especially as the alimentary canal constitutes the main path both of intake and output of calcium, that we are in no position to study the question of absorption of calcium. ... 

Table 2. Uricemia, uricosuria, fractional excretion of uric acid and rate of tubular phases that govern the renal excretion of uric acid in patients with calcium urolithiasis and hyperuricosuria and control subjects, under purine-free diet.

Excess dietary protein also leads to an increase in urinary calcium in adult rodents, but there is no attendant loss of calcium from the skeleton or negative calcium balance. This difference between species appears to be due to two factors a smaller fractional excretion of endogenous calcium in the urine of rodents (less than 5%) and a greater capacity to buffer metabolic acid. Only when acid loads are sufficient to depress... 

Many animal species excrete more calcium if fed an acid or acidforming compounds. In the calf, Steenbock and coworkers (13) observed hypercalciuria and acidic urine after feeding hydrochloric acid to the calf. Stehle (14) pointed out that calcium represented the main long-term fixed base to be lost in the urine of the dog loaded with excessive amounts of hydrochloric acid. Walzer and Browder (15) demonstrated that when infused with a sulfate containing solution, the dog excreted several fold more acid and calcium than saline-infused controls the increased calcium loss returned to normal upon removal of the sulfate. Marone, et al. (16) demonstrated increased excretion of calcium in the acidotic dog. Correction of the acidosis reduced the excessive fractional calcium excretion rate, but did not alter sodium excretion. 

Total calcium turnover Is five to ten times greater in the newborn than in the adult This is reflected in the fact that normalized calcium intake is 5-10 times higher in the newborn than in the adult although the fraction absorbed (Va/Vi) is similar A second observation is that in all five newborns more calcium is lost through the endogenous fecal route than by urinary excretion The reverse is true for typical adults Finally, the is in positive balance for both bone and total organism calcium in contrast to the typical adult ... 

Stone formation may occur in those with elevated levels of urinary calcium, Mormally adults excrctc less than 200 mg of calcium in the urine per day, even with relatively high intakes of calcium, A fraction of the population absorbs more calcium than normal and excretes more calcium in the urine, resulting in hyper-calciuria. Hypercalciuria is defined as urinary excretion of calcium of more than 300 mg/day. About half of patients with calcium stones have hypercalciuria and may be calcium hyper absorbers. Persons with hypercalciuria are advised to limit their calcium intake to one serving of milk or cheese per day. They are also advised to limit their protein intake to the RDA. Their protein intake should be limited to minimize the caiciuric effect of protein. They are also advised to fnerense their water intake to produce 2 liters of urine per day and to avoid oxalate-containing foods. Persons with hypercalciuria and with a familial history of stones should not lake calcium supplements to raise their intake above the RDA. 

Calcium absorption can be measured by the double-isotope technique. In this technique a meal containing caldum-45 is consumed and the radioactivity in the urine measured. The measurement of urinary Ca alone cannot provide the fractional absorption, because some of the Ca absorbed is taken up by cells, deposited in bone, or excreted in the bile. A second isotope of calcium, Ca, is used to correct for the fates of absorbed calcium, other than excretion in the urine. The use of the Ca is intended to eliminate cell uptake, bone deposit, and biliary losses as variables in the study of the absorption of the dose of cdcium-45. 

Calcium absorption can also be measured by determining the fraction of the mineral in a test dose that is not absorbed. Here, a test meal containing calcium-47 is consumed and the feces collected for 12 days to measure the isotope excreted. The calcium lost in biliary and intestinal secretions may be corrected for by an intravenous injection of radioactive calcium (Spencer et al, 1978). 

Hypocalcemia can result from inadequate dietary intake, decreased fractional calcium absorption (as seen with increasing age), or enhanced calcium excretion. To restore calcium homeostasis after hypocalcemia, PTH concentrations rise, and vitamin D metabolism increases to enhance intestinal calcium absorption (see Fig. 88-3), renal calcium reabsorption, and bone resorption. Fracture risk is greatest with low calcium intake and low fractional calcium absorption." ... 

Now and again doubt arose Key s (1895) and other experiments did not go very deeply into the problem, and new studies were performed. Nicolaysen was the first to explore the problem fully (1934) and to reach a conclusive result. As the poorly founded postulate of active re-excretion and secretory function of the colon is again being given consideration, a brief summary of Nicolaysen s (1934) experiments will not be out of place. The argument was comparatively simple, namely that the endogenous fraction of the fecal calcium could not be studied except on a diet free of calcium. 

All the hyperuricosuric patients in this investigation had a urate clearance of the same order as the controls. Only one hyperuricosuric patient had a clearance urate/clearance creatinine of 14.5%, thus exceeding the 95% tolerance limit in the controls. In this patient, however, 95% of the excretion was PZA suppressible. The PZA-suppressible fraction of urate excretion and the tubular reabsorption of filtered urate in hyperuricosuric stone formers did not differ from the controls. Thus it is concluded that the renal tubular handling of urate in hyperuricosuric calcium stone patients does not differ from that of healthy subjects. 

These components represent diverse functions including the transport of nutrients to the tissues and of waste products from the tissues to be reused or excreted, the maintenance of blood pH within physiological limits and the movement of effectors such as hormones from their sites of synthesis to their respective target tissues. Some of these components are actually dissolved in the plasma, and some, such as lipids that are insoluble in an aqueous environment, are transported in the form of lipid-protein complexes, the lipoproteins. In other cases, for example, calcium and hormones, some may exist free in the plasma, and some may be protein bound. In these cases, only the free fraction is biologically active. 

In all, 40% of calcium in serum is boimd to protein and 10% to anions like citrate, hydrogen carbonate, etc. 50% is ionized (free) and only this fraction influences nervous excitability. Parathyroid hormone and calcitriol increase calcium concentration (Table 1) their antagonist is calcitonin. Input of calcium is by food, and output by incomplete tubular reabsorption of the glomerular filtrate and intestinal excretion (Table 6). 

The average daily intake of calcium is about 1250 mg, although there is a considerable variation between different parts of the world. Fifteen to forty percent is absorbed from the intestine. The intestine also secrets calcium, and the net calcium absorbed each day approximates 200 mg. At calcium balance, the kidneys therefore excrete slightly less than this amount. In case of low calcium intake a greater fraction is absorbed than at a higher calcium intake. This adaptation to the calcium content of the food is a slow process, which is connected to complex changes in the concentrations of the calcium-transporting mechanisms in the mucosa cells [2]. 

The non-protein-bound fraction of Ca is filtered fteely (8 g/24 hr) in the glomeruli and then most of it (7.8 g/24 hr) is reabsorbed in the tubuli yielding a net daily excretion of about 200 mg (5 mmol). The bulk calcium reabsorption is accomplished in the proximal nephron and the thick ascending limb in association with sodium reabsorption and with no additional energy requirements. The calcium reabsorption along the nephron is seen in Fig. 2. 

Human adults fed experimental diets high in purified proteins exhibit an increased loss of calcium in the urine and an increase in the amount of dietary calcium required to maintain balance. Calciuria is due mainly to increased acid production arising from the oxidation of excess sulfur amino acids. Acid excretion is associated with a decreased fractional reabsorption of calcium from the renal tubules. 

The kidneys are two fist-sized organs whose primary function is to generate urine for excretion of water and metabolic waste products. The kidneys not only remove accumulated nitrogen products (urea, creatinine, uric acid, and others) but also maintain homeostasis of water and electrolytes (sodium, potassium, chloride, calcium, phosphate, magnesium) and regulate acid-base balance. In addition, human kidneys perform a few endocrine and metabolic functions, such as production of the hormone erythropoietin (a hormone that stimulates blood cell production) and conversion of vitamin D to its active form. Because of the tremendous overcapacity of normal kidney function, a person can live with only a fraction of normal kidney capacity, and the 0.1% of the population who are bom with a single kidney often are not even aware of the missing kidney. 

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