Leucine metabolism and body cell mass in cystic fibrosis.

To further investigate the nature of malnutrition in cystic fibrosis (CF), specifically to evaluate the influence of body composition on leucine metabolism and determine whether measures used as reference standards alter comparative differences in protein metabolism, we measured [13C]leucine kinetics in nine well-nourished and eight malnourished, height-, FEV1-, and sex-matched CF subjects. Body cell mass was measured by total-body potassium (K40) analysis. Leucine turnover did not significantly differ between groups in absolute values or when corrected for body weight, body height, or surface area. However, malnourished CF patients had accelerated leucine turnover based on body cell mass measurements. Mean leucine synthesis per unit body cell mass in the malnourished group was 147% higher than that of the well-nourished group (p < 0.001). These data indicate that altered body composition is an important factor influencing protein metabolism in CF and should be considered in future metabolic studies. The importance of potentially serious but possibly correctable deficits in body cell mass in CF is emphasized.