1. The classical literature shows that wasting of body protein takes place mainly from the cellular compartment leaving high proportions of extracellular collagen. Whole body nitrogen (WBN) is proportional to whole body protein (cellular + extracellular) while whole body potassium (WBK) is almost entirely intracellular. WBK might be an adequate index of cellular wasting.
2. WBN was measured by neutron activation and WBK by counting 40K in 29 healthy males and 131 male and female patients with wide ranges of body composition.
3. Our wasted patients with Crohn's disease or ulcerative colitis, had higher WBN/WBK ratios than matched healthy controls and the difference between the two groups was in the cellular ratio (23.6 mol of N/mol of K, 0.33 g of N/mmol of K, 3 mmol of K/g of N). A multiple regression model for all the subjects represented cellular nitrogen by a term in WBK and extracellular nitrogen by simple anthropometric measurements. The partial regression coefficient of WBN on WBK was 22.6 ± 1.1 (se) mol of N/mol of K; this was also similar to the cellular ratio.
4. Our results were compatible with extracellular protein (mainly collagen, which is 35% of normal whole body protein) remaining resistant to wasting even when severe loss of cellular protein occurs. The high ratios of WBN/WBK in wasted patients can be explained by this disproportionate wasting of cellular substances and they do not imply alterations in the cellular N/K ratio.
5. We suggested that the stable ratio of WBK to cellular protein makes it an effective index of cellular wasting. The resistance of collagen to wasting and the preponderance of extracellular mass in the fat-free mass of wasted subjects, make WBN and fat-free mass unreliable guides to the extent of wasting.