Nitrogen excretion products, rates

The fact that relatively large amounts of urea can be produced by subjects in whom an enzyme defect of the urea cycle has been proved has perplexed many investigators. Despite the fact that the activity of the rate-limiting enzyme has been reduced even to zero as measured by a sophisticated method, urea production still continues. It must therefore be concluded that, in all these cases, urea production is only impaired, not abolished. In all normal circumstances, all but a small fraction of the nitrogen in excess of tissue protein requirements is still excreted as urea and/or as the amino acid whose further metabolism is blocked. The impairment shows itself in the elevated levels of blood ammonia and consequently of glutamine, which vary according to the stress placed upon the urea cycle by varying the rates of protein intake. 

Chronic kidney disease (CKD), as evidenced by the inability of the kidneys to excrete nitrogenous and other waste products, usually develops over months to years (see Chap. 43). Malnutrition secondary to reduced oral nutrient intake frequently is evident when the glomerular filtration rate (GFR) drops below 20 to 25 mL/min. Patients with CKD are considered to have ESKD when the GER falls below 15 mL/min (see Chap. 44). Malnutrition is also a common occurrence in ESKD, not only because of decreased oral intake, but also due to increased nutrient losses via the various renal replacement therapies. Because of its chronicity, malnutrition in these patients is treated most frequently in the ambulatory setting with EN. 

---