The design and validation of a diet for studies of amino acid metabolism in adult humans

A dietary protocol developed for use in studies of amino acid and protein metabolism is reported in detail. The diet consisted of a mixture of free amino acids, an amino nitrogen-free liquid formula and protein-free cookies. Energy requirement for a group of adult men (n=10) was determined by multiplying each individual's resting metabolic rate, measured by indirect calorimetry, by a factor of 1.7. Subjects maintained a constant weight while consuming the experimental diet and had few complaints regarding palatability during a nine day study period. No significant variation (P=0.69) was evident over a six hour period in measurements of ¹³CO₂ enrichment in the breath of subjects (n=4) consuming small hourly meals of the diet. Dietary adaptation to a constant level of ¹³C enrichment in breath CO₂ with this dietary protocol was accomplished in a short period of time as shown by the lack of change in the mean (±SD) ¹³C enrichments of 0.02764±0.00084, 0.02818±0.00067 and 0.02812 ±0.00062 atoms percent enrichment in subjects (n=7) on days 3, 6 and 9 of the dietary period, respectively. The small within subject variation in repeated ¹³CO₂ enrichment measurements indicates the reliability of the dietary protocol in studies where a steady isotope background is essential. This dietary protocol provides a constant and stable metabolic milieu for studies of amino acid metabolism, especially, for those involving the use of stable isotopes.