Dietary intake data of 15 treated patients with phenylketonuria (PKU) (7-17 years) and six control siblings (6-14 years) were used to evaluate the effectiveness of medical foods to balance energy, protein, vitamin B-6, and individual amino acids from natural foods and to evaluate vitamin B-6 status. Four-day diet records were computer-analyzed and the contribution of medical foods and natural foods to the total diets of the patients was determined. A fasting blood sample and two 24-hour urine collections were obtained from each subject. In eight patients consuming a strict diet, natural foods provided 0.9 g protein and 39 mg phenylalanine (phe)/100 calories, a significant reduction from control intakes of 3.3 g protein and 153 mg phe/100 calories. However, plasma phe levels were above the acceptable treatment range. Medical foods were consumed in less than the recommended quantities to meet approximately 120% of the Recommended Dietary Allowances (RDAs) for protein. These foods contributed 73% of the total protein RDAs but only met 22% of the energy RDAs of patients consuming the strict diet. Taste qualities of certain L-amino acids (L-AAs; Lmethionine, L-glutamic and L-aspartic acids), which constitute the protein in elemental medical foods (EMFs) for school-aged patients, make acceptance of these products difficult. Intakes of each of these L-AAs were above nutritional standards and suggested that their levels could be safely lowered in EMFs. Natural foods and medical foods provided 0.057 and 0.046 mg vitamin B-6/g protein, respectively, above the 0.020 RDA standard and the mean control intake of 0.018 mg vitamin B-6/g protein. The mean plasma pyridoxal 5'- phosphate (PLP) concentration for the patients with PKU was over twice that of the mean control concentration and above literature values. However, more than half of the patients excreted less than 30% of their vitamin B-6 as 4-pyridoxic acid; values below the criterion suggested for inadequate status. This work provides data for a better understanding of vitamin B-6 metabolism and status in PKU and supports the design and testing of a new EMF to balance energy, protein, and amino acids from natural foods and which may improve dietary adherence. / Graduation date: 1991