SummaryBackground and objectives Patients with advanced chronic kidney disease (CKD) are in positive phosphorus balance, but phosphorus levels are maintained in the normal range through phosphaturia induced by increases in fibroblast growth factor-23 (FGF23) and parathyroid hormone (PTH). This provides the rationale for recommendations to restrict dietary phosphate intake to 800 mg/d. However, the protein source of the phosphate may also be important. Design, setting, participants, & measurementsWe conducted a crossover trial in nine patients with a mean estimated GFR of 32 ml/min to directly compare vegetarian and meat diets with equivalent nutrients prepared by clinical research staff. During the last 24 hours of each 7-day diet period, subjects were hospitalized in a research center and urine and blood were frequently monitored. ResultsThe results indicated that 1 week of a vegetarian diet led to lower serum phosphorus levels and decreased FGF23 levels. The inpatient stay demonstrated similar diurnal variation for blood phosphorus, calcium, PTH, and urine fractional excretion of phosphorus but significant differences between the vegetarian and meat diets. Finally, the 24-hour fractional excretion of phosphorus was highly correlated to a 2-hour fasting urine collection for the vegetarian diet but not the meat diet.Conclusions In summary, this study demonstrates that the source of protein has a significant effect on phosphorus homeostasis in patients with CKD. Therefore, dietary counseling of patients with CKD must include information on not only the amount of phosphate but also the source of protein from which the phosphate derives.

Achievement of maximal calcium retention during adolescence may influence the magnitude of peak bone mass and subsequently lower the risk of osteoporosis. Calcium retention is generally considered to reach a plateau at a certain calcium intake. To test this hypothesis, calcium balance was measured in 35 females with a mean (+/-SD) age of 12.7 +/- 1.2 y (range: 12-15 y) who consumed from 841 +/- 153 to 2173 +/- 149 mg Ca/d. Subjects ate a basal diet that included a fortified beverage containing different amounts of calcium citrate malate. Twenty-one subjects were studied at two dietary calcium intakes with use of a crossover design. Results from a previous study in 14 subjects who were studied at only one calcium intake were included in the data analysis. Calcium retention was modeled as a nonlinear function of calcium intake that included a parameter representing mean maximal retention. Mean maximal calcium retention was 473 mg/d (95% CI: 245, 701 mg Ca/d). At higher postmenarcheal ages, maximal calcium retention was lower but the intake required to achieve this was not affected. Calcium intake explained 79% and 6%, respectively, of the variation in fecal and urinary calcium excretion. Intake of 1200 mg Ca/d, the recommended dietary allowance for calcium published in 1989, resulted in a mean calcium retention that was 57% of the maximal value (95% CI: 25%, 89%). Intake of 1300 mg Ca/d was the smallest intake that allowed some adolescent females to achieve 100% of maximal calcium retention (95% CI: 26%, 100%). These data support the idea that calcium retention plateaus at a certain calcium intake although it continues to increase at intakes > 2 g/d.

Increased sodium (Na(+)) retention in blacks could be related to the high prevalence of hypertension in adult blacks. Na(+) retention in response to controlled dietary Na(+) has not been rigorously compared in the different race groups. The present study assessed Na(+) retention in 22 black and 14 white girls, 11-15 yr old, during 3 wk on a low (1.3 g, 57 mmol)- and during 3 wk on a high (4 g, 172 mmol)-Na(+) diet in a randomized order, crossover design. Subjects were matched by postmenarcheal age and weight. After a 1-wk equilibration period, the mean daily Na(+) retention was 357 +/- 69 mg (15.5 +/- 3.0 mmol) in blacks and 239 +/- 37 mg (10.4 +/- 1.6 mmol) in whites on the low-Na(+) diet and 991 +/- 138 mg (43.1 +/- 6.0 mmol) in blacks vs. 334 +/- 90 mg (14.5 +/- 3.9 mmol) in whites (P < 0.001) on the high-Na(+) diet. The greater Na(+) retention in blacks was not accompanied by an increase in fecal or sweat Na(+) excretion. Blood pressure and weight did not increase despite the Na(+) retention, and thus, the retained Na(+) appeared to reside in a nonextracellular compartment that we speculate to be bone. In summary, black girls showed greater Na(+) retention compared with white girls. The difference in Na(+) handling may contribute to underlying racial differences in susceptibility to hypertension.

Dietary calcium requirements did not differ with race. Higher calcium retention at all calcium intakes during adolescence may underlie the higher bone mineral content of adult blacks than of adult whites.