The Measurement of Muscle Mass in Children Using [15N]Creatine

Abstract

Extractcreatine in normal individuals (13,19,21) and is remarkablyIn eight infants and children who had recovered from proteinenergy malnutrition (PEM), muscle mass was estimated by measuring creatine turnover by an isotope dilution technique using [15N]creatine, creatine concentration, and urinary creatinine output. Creatine turnover varied from 1.5 to 2.6% of the muscle creatine pool per day and muscle creatine concentration ranged from 1.7 to 3.9 rglPg muscle D N A . Muscle mass was between 15% and 37% of total body weight. The results indicate that daily creatinine output is not a reliable indicator of muscle mass in children who have recently recovered from severe PEM. Speculation constant although there is sufficient variation between individuals to affect the creatinine excretion per unit of body creatine. The concentration in muscle also shows individual variation (I). When the daily excretion of urinary creatinine is used to estimate muscle mass, it is important to recognize that both creatine turnover (13,35) and creatine content of muscle (1) vary in the individual in differing clinical states. Other workers have pointed out the unreliability of using a single constant to relate muscle mass to creatinine excretion (21). In the present study we have measured creatine turnover and creatine concentration in muscle in estimating muscle mass in children. THEORETICAL CONSIDERATIONS AND CALCULATIONS Short term fluctuations in the daily creatinine output in young children may be due to variations in muscle creatine concentrationSince more than 90% of body creatine is found in muscle, from a knowledge of the size of the creatine pool and the concentration of since creatine turnover, in the individual subject, remains re-creatine in muscle, muscle mass can be estimated markably constant. The wide variation in the proportion of body weight that is muscle may reflect significant body composition Muscle mass (kg) . -. changes in fat, water, and muscle in children who have recoveredcreatine pool (g) from severe protein-energy malnutrition.(1) creatine concentration in muscle (lg/mg wet weight of muscle) Skeletal muscle is the major component of lean body mass and is also the largest protein reservoir in the body. However, there is no accurate method for measuring muscle mass in man. A limited number of whole body dissection studies have been performed in relatively well nourished and in severely malnourished children (16,18,27). This direct method has limitations because it is difficult to ensure complete dissection of all muscle tissue, variable amounts of water and other fluids are lost during and after dissection, and studies are done a t varying times after death. It is therefore difficult to extrapolate these findings to the dynamic situation in the living individual. Of the alternative and indirect methods of estimating muscle mass, that based on the 24-hr creatinine excretion is most widely used. The strong association between creatinine excretion and body weight led to the suggestion that muscle mass could ...

Muscle Mass and Composition in Malnourished Infants and Children and Changes Seen after Recovery

“…These authors have concluded that creatinine output can change independently of lean body mass. The data presented in this and other studies (9,10,25) support the view that there may be considerable error in predicting muscle mass from the daily excretion of urinary creatinine.…”

“…Recently we reported a more direct method of measuring muscle mass in children (25). This method has been used in the present study to determine quantitative changes in muscle mass, total cell number, and functional cell size in malnourished and recovered children.…”

“…The method for measuring total muscle mass in vivo with [I5N]creatine has been described in detail (25). A tracer dose of ['Nlcreatine was injected intravenously to label total body creatine nearly all of which is jn muscle (22,32).…”

“…Isolation and purification of urinary creatinine and its analysis for I5N abundance by mass spectrometry have been previously described (25). Creatine was measured enzymically (3).…”

A Proposed Mechanism for Reduced Creatinine Excretion in Severe Chronic Renal Failure

“…include diminished creatinine production owing to reduced lean body mass [ 1 ], decreased dietary creatine [2], or reduced body creatine pool [3,4]. Another mechanism for decreased creatinine excretion could be that creatinine produced is accumulated in body fluids.…”