Plasma amino acids as substrates and nutrition-dependent markers

Amino acid determinations in plasma have been used for diagnostic purposes and as markers of malnutrition since the 1960s. Biochemical research has given an insight into biochemical pathways and their organ specificity, allowing better understanding of the role of amino acids in the organism. In nutrition, amino acid concentrations are used as markers of excess, deficiency or imbalance for optimising the amount and composition of proteins in prepared nutrients. For useful results to be obtained in such investigations correct timing of sampling relative to the end of food intake is essential. Using data from a study in which a reduction of protein to 1.9 g/100 kcal in starter formulas was tested, it is shown that not only does this allow amino acid concentrations quite close to those of breast-fed babies to be achieved; in addition, the levels reached with the optimised composition lead to a balance with less competition from large neutral amino acids with tryptophan transport at the blood-brain barrier (LAT1 transporter), which probably enhances the supply to the brain even when less protein is fed than in conventional formula and when there is no increase of tryptophan in the blood.