[Estimation of potentially available protein in infant starting formulas for term and preterm neonates]

Sixteen milk-based starting formulas were analyzed with the aim of calculating their "true protein" content and assessing "in vitro" protein digestibility, in order to estimate levels of potentially available protein. Ten of them were designed for term infants: 7 had a casein:whey protein ratio 40:60 (adapted formulas) and 3 a ratio 80:20 (non-adapted); the 6 remaining formulas (all adapted) were for preterm infants. Nitrogen was determined by the Kjeldahl method. True protein was calculated as (total N--non-protein N) x 6.25. NPN was determined in the soluble fraction, after protein precipitation with 24% trichloroacetic acid and centrifugation. Digestibility was assessed by digestion with pepsin and pancreatin, and defined as the increase in non-protein N after enzymatic digestion. Values for true protein were from 1.3 to 2.3 g/dL and for non protein N, from 4.5 to 13.7%. Digestibility values varied between 59.0 and 92.5%; an inverse trend was observed between protein digestibility and protein content. Considering both the "true protein" levels and their digested proportions, all preterm and 60% of the term formulas would present potentially available amounts below those recommended. These observations constitute an alert, even though this method of assessing "in vitro" protein digestibility represents only an approximation to physiological processes; however, it could be useful in order to evaluate the intensity of the heat treatments to which these formulas were subjected. On the other hand, since NPN allows the estimation of the true protein provided by the formulas, either its percentage or the true protein content could be included on the label.