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Tryptophan Intake Influences Infants' Sleep Latency^1,2,

Division of Nutritional Sciences, University of Illinois, Urbana, IL 61801 ^* Carle Foundation Hospital, Urbana, IL 61801

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 plasma 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 µmol/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 < 0.0005). Infants fed commercial formula or the formula without added tryptophan had lower (P < 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 < 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 development. This result was obtained with variations in the level of tryptophan within the range found in commercially available formulas.

KEY WORDS: • infant feeding • tryptophan • behavioral tests • infant growth • humans

¹ Presented in part at the Annual Meeting of the Federation of American Societies for Experimental Biology, March 1989, New Orleans, LA [Goff, N. C., Picciano, M. F., Steinberg, L. A., Birch, L. L. & Hatch, T. F. (1989) Plasma tryptophan concentration: is it related to quality or quantity of protein in infant formulas? FASEB J. 1: A764 (abs.), and Steinberg, L. A., Birch, L. L., Goff, N. C. & Picciano, M. F. (1989) Infant nutrition and behavior: Neonatal Behavioral Assessment Scale differences among infants fed formula varying in amount of tryptophan. FASEB J. 1: A765] and also presented in part at the Biennial Meeting of the Society for Research in Child Development, April 1989, Kansas City, KS [Steinberg, L. A., Goff, N. C., Birch, L. L., Picciano, M. F. & Hatch, T. F. (1989) Infant behavior and nutrition: behavioral differences among infants fed formula varying in tryptophan. Soc. Res. Child. Dev. J. 6: 165].

² Supported in part by Ross Laboratories, Columbus, OH and NIH Grant R01HD19752-07 to Leann L. Birch.

³ Current address: Department of Nutrition, 5126 Henderson Building South, Pennsylvania State University, University Park, PA 16802.

⁴ To whom correspondence should be addressed.

Manuscript received 12 December 1991. Revision accepted 12 May 1992.

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