We have studied in a prospective manner the evolution of three urinary indices which measure protein catabolism: 3-methylhistidine, the creatinine level index, and the excretion of urinary nitrogen, the latter being calculated based on the excretion of urea in urine. These studies have been performed using healthy patients only being given parenteral nutrition. Our data shows a very significant decrease in 3-methylhistidine (p = 0.001) and in the excretion of nitrogen calculated (p = 0.026), there being a correlation among the differences between these two parameters.