Tryptophan intake influences infants' sleep latency.

Formula-fed infants have depressed plasma tryptophan concentration relative to human milk-fed infants. Because tryptophan alters sleep-waking patterns in adults, a study was designed to determine whether additional dietary tryptophan could elevate plasma tryptophan concentrations of formula-fed infants to concentrations characteristic of human milk feeding and whether differences in plasma tryptophan or the ratio of plama tryptophan to the sum of the other plasma large neutral amino acids (tryptophan:LNAA) were of behavioral significance. Infants were fed a formula (13 g protein/L; whey:casein, 34:66) containing either 0, 294, 588 or 882 mumol/L of added tryptophan. Infants fed human milk or commercial formula (15 g protein/L; whey:casein, 18:82) were included for comparison. In formula-fed groups, plasma tryptophan was directly related to tryptophan intake (r = 0.46, P less than 0.0005). Infants fed commercial formula or the formula without added tryptophan had lower (P less than 0.001) plasma tryptophan compared with infants fed human milk. Only the infants fed the highest tryptophan formula had significantly higher plasma tryptophan:LNAA ratios than the other experimental groups, and these ratios were similar to those of infants fed human milk. The plasma tryptophan:LNAA ratios, not plasma tryptophan concentrations, were predictive of differences in the infants' sleep latency; infants fed formula containing the highest tryptophan had sleep latencies of 18.7 min, significantly shorter (P less than 0.05) than those of infants fed formulas containing less added tryptophan (27.7 min). Feeding infants formulas differing in tryptophan concentration produced differences in sleep latency, which could influence neurobehavioral developments.(ABSTRACT TRUNCATED AT 250 WORDS)