Amino Acid Medical Foods Provide a High Dietary Acid Load and Increase Urinary Excretion of Renal Net Acid, Calcium, and Magnesium Compared with Glycomacropeptide Medical Foods in Phenylketonuria

Bridget M Stroup, Emily A Sawin, Sangita G Murali, Neil Binkley, Karen E Hansen, Denise M Ney, Bridget M Stroup, Emily A Sawin, Sangita G Murali, Neil Binkley, Karen E Hansen, Denise M Ney

Abstract

Background. Skeletal fragility is a complication of phenylketonuria (PKU). A diet containing amino acids compared with glycomacropeptide reduces bone size and strength in mice. Objective. We tested the hypothesis that amino acid medical foods (AA-MF) provide a high dietary acid load, subsequently increasing urinary excretion of renal net acid, calcium, and magnesium, compared to glycomacropeptide medical foods (GMP-MF). Design. In a crossover design, 8 participants with PKU (16-35 y) provided food records and 24-hr urine samples after consuming a low-Phe diet in combination with AA-MF and GMP-MF for 1-3 wks. We calculated potential renal acid load (PRAL) of AA-MF and GMP-MF and determined bone mineral density (BMD) measurements using dual X-ray absorptiometry. Results. AA-MF provided 1.5-2.5-fold higher PRAL and resulted in 3-fold greater renal net acid excretion compared to GMP-MF (p = 0.002). Dietary protein, calcium, and magnesium intake were similar. GMP-MF significantly reduced urinary excretion of calcium by 40% (p = 0.012) and magnesium by 30% (p = 0.029). Two participants had low BMD-for-age and trabecular bone scores, indicating microarchitectural degradation. Urinary calcium with AA-MF negatively correlated with L1-L4 BMD. Conclusion. Compared to GMP-MF, AA-MF increase dietary acid load, subsequently increasing urinary calcium and magnesium excretion, and likely contributing to skeletal fragility in PKU. The trial was registered at clinicaltrials.gov as NCT01428258.

Figures

Figure 1
Figure 1
Potential renal acid load was calculated for 10 different AA-MF and 3 different GMP-MF, based on mineral and amino acid analysis, n = 2-3 per medical food. AA-MF, amino acid medical foods; GMP-MF, glycomacropeptide medical foods; PRAL, potential renal acid load; PE, protein equivalent.
Figure 2
Figure 2
Renal net acid excretion (a), urinary calcium excretion (b), urinary magnesium excretion (c), and urinary sulfate excretion (d), based on 24-hr urine collections in participants with PKU who consumed AA-MF or GMP-MF, n = 8. Values are means ± SE. The dashed lines (b, c, d) represent the reference range for urinary calcium excretion (100–300 mg/d), magnesium excretion (12–293 mg/d), and urinary sulfate excretion (0–30 mEq/d). Results indicate significant increases in renal net acid excretion (p = 0.002), urinary calcium (p = 0.012), magnesium (p = 0.029), and sulfate (p = 0.0008) excretion with AA-MF compared to GMP-MF. AA-MF, amino acid medical foods; GMP-MF, glycomacropeptide medical foods.
Figure 3
Figure 3
Renal net acid excretion (a), urinary calcium excretion (b), urinary magnesium excretion (c), and urinary sulfate excretion (d), based on 24-hr urine collections in participants with PKU who consumed AA-MF or GMP-MF, n = 8. Statistical significance reflects significant treatment effects, as shown in Figure 2. Results show increases among all 8 participants in urinary excretion of renal net acid, calcium, and sulfate and in 7 of 8 participants in urinary excretion of magnesium with AA-MF compared to GMP-MF. AA-MF, amino acid medical foods; GMP-MF, glycomacropeptide medical foods.
Figure 4
Figure 4
Urinary calcium and renal net acid excretion were positively correlated (r = 0.57, p = 0.03) with AA-MF and GMP-MF, n = 14 from 7 participants. One participant was omitted, due to use of a therapeutic dose of calcium to treat low BMD-for-age. AA-MF, amino acid medical foods; BMD, bone mineral density; GMP-MF, glycomacropeptide medical foods; RNAE, renal net acid excretion.

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