✅ Blood in the upper gastrointestinal (GI) tract is degraded to its constituent parts, including hemoglobin-derived amino acids. These amino acids are absorbed by the intestinal epithelial cells and transported to the liver.
✅ In the liver amino acids may be used to synthesize new protein (anabolism) or they may undergo catabolism/degradation to generate energy and/or to synthesize glucose and fatty acids.
✅ Amino acid catabolism/degradation results in the release of ammonia (NH3), which is then converted to urea by the urea cycle (the latter being confined to the liver).
✅ The urea is released into the circulation and filtered by the kidney.
✅ Blood urea concentration is dependent on renal function and on non-renal factors including the amount of protein ingested. Upper GI bleeding is akin to a blood meal, presenting a bolus of amino acids to the liver.
✅ By contrast, creatinine concentration, another marker of renal function, reflects muscle mass and is not influenced by dietary protein intake (or, by extension, blood in the gastrointestinal tract).
✅ As a result, the BUN to creatinine ratio is increased in upper GI bleeding.
✅ Blood has low biological value (poor nutritional availability for use by the body).
✅ Proteins of low biological value give rise to higher ammonia and urea production compared with balanced proteins. As a result, blood in the stomach (e.g., from a bleeding peptic ulcer) will cause a greater increase in the BUN compared with an isonitrogenous protein meal.