Four adult men received a 48-h constant intravenous infusion of [2H4]lysine, [2H3]leucine, L-[ring-13C6]phenylalanine, and L-[1,2,3,-13C3]alanine. Subjects ingested hourly meals for two 12-h periods, separated to two 12-h fasting periods. The isotopic enrichments of free amino acids in venous plasma and in VLDL apolipoprotein B-100 (apoB)-bound amino acids, plasma alpha-keto isocaproic acid (alpha-KIC) and plasma pyruvic acid (PYR) were measured by negative chemical ionization gas chromatography-mass spectrometry. By 7 h of infusion, all four amino acids achieved an equilibrium isotopic enrichment (EIE) in plasma and in apoB. In the fed state, the EIE of the amino acids in apoB was lower than that in plasma free amino acids. The ratio EIE-apoB:EIE-plasma differed significantly among amino acids in the fed state (alanine 0.30; lysine 0.64; leucine 0.70; phenylalanine 0.81). In the postabsorptive state, the EIE-apoB:EIE-plasma ratio rose significantly compared with the fed state (alanine 0.38; lysine 0.73; leucine 0.94; phenylalanine 1.05). Plasma PYR and apoB-alanine were in isotopic equilibrium irrespective of nutritional state. The EIE-apoB-leucine:EIE-plasma-alpha-KIC ratio rose from 0.75 in the fed state to near 1 in the postabsorptive state. We conclude that the contribution of systemic amino acids to apoB-100 synthesis is sensitive to nutritional state, and that systemic essential amino acids seem to be preferentially incorporated into apoB.