Influence of purine intake on uric acid excretion in infants fed soy infant formulas

Objective: These studies tested the hypothesis that increasing intake of purines. delivered as RNA from soy protein-based infant formula, would increase urinary uric acid excretion in infants. Methods: Study One examined the influence of feeding on serum uric acid in a total of 178 infants from four separate trials with infants fed commercial and experimental soy-based and milk-based infant fi,formulas or human milk. Studies Two and Three compared the effect of a standard purine soy formula (STD Purine; 180 mg purines/L from RNA) and a reduced purine soy formula (Reduced Purine; 65 mg purines/L: 26 mg/L from RNA and 39 mg/L from ribonucleotides) on urinary uric acid excretion in infants. In Study Two. 11 infants ranging in age from 16 to 128 days of age were fed both formulas in a random crossover design. Complete 72-hour urine collections were done at the end of each 11-day feeding period. Urinary uric acid excretion was expressed as mmol/day. In Study Three, 33 infants were enrolled before eight days of age and randomized to one of the formulas one week later. Spot urine samples were collected at 28 and/or 56 days of age and urinary uric acid concentration was expressed as mmol/mmol creatinine. Results: In Study One, each of the feedings resulted in mean serum uric acid levels within normal reference ranges. Soy formula led to higher serum uric acid levels than human milk, and human milk to levels indistinguishable from cow milk-based formulas. In Study Two, infants excreted significantly more uric acid in the urine when fed the STD Purine formula compared to the Reduced Purine formula(0.8 +/- .04 vs. 0.57 +/- .04 mmol/d) (p=0.006). In Study Three, infants fed the STD Purine formula had a significantly higher concentration of uric acid in their urine compared to those fed the Reduced Purine formula (2.1 +/- 0.2 vs. 1.4 +/- 0.1 mmol uric acid/mmol creatinine) (p=0.0001). Conclusion: These data indicate that healthy infants can digest RNA and subsequently absorb the liberated purine ribonucleotides as determined by urinary uric acid concentration.