Tumors--effective nitrogen traps independent of protein intake

Despite abnormalities in host nitrogen metabolism, animal studies have demonstrated that tumors successfully compete with host tissues for substrates during periods of nutrition support. The present study was performed to determine the effect of protein depletion on protein content of both tumor and host tissues in tumor-bearing animals. Following subcutaneous mammary tumor (AC-33) implantation, 56 female Lewis rats were randomly assigned to one of two nutrient regimens: (1) protein-depleted chow (0.05% protein) or (2) standard rat chow (22.1% protein) ad libitum per os. Animals were sacrificed after 7 or 14 days on each diet and protein content of tumor and three metabolically active host tissues (liver, gastrointestinal mucosa, and bone marrow) was determined. As tumor growth progressed, tumor protein content increased while liver and gastrointestinal mucosa protein content was depleted; no significant change occurred in protein content of bone marrow. These metabolic alterations occurred regardless of protein intake. Thus, tumors are effective nitrogen traps independent of protein intake and despite wasting of normal host tissues (liver and gastrointestinal mucosa).