Multiple organ failure (MOF) is accompanied by muscle wasting, but changes in body composition are frequently obscured by fluid retention (edema), mainly in superficial and visceral tissue. There is a need to assess body composition and changes in body composition in these circumstances independently of edema. A relation was sought between fat-free (lean tissue) mass [calculated from body weight and skinfold thicknesses and measured by using dual-energy X-ray absorptiometry (DXA)] and muscle thickness (measured using ultrasound at a variety of sites accessible in an unconscious supine subject) to determine which sites correlated best with lean body mass. The three best sites were midbiceps, midforearm anteriorly, and midthigh anteriorly: R2 for the simple sum of the three sites correlated with fat-free mass from skinfold thicknesses was 71.1%, and with lean tissue mass from DXA was 76.1%. Serial measurements of both muscle thickness and midupper-arm circumference in nine patients with MOF showed a complete dissociation; in all nine there was a significant negative correlation of muscle thickness with time (P < 0.05) but changes in arm circumference were random. Only one patient showed a significant negative correlation with time, seven showed no change, and one other showed a significant increase. The muscle thicknesses that correlate best with lean body mass are measured over the biceps, anterior forearm, and anterior thigh. Monitoring muscle thicknesses at these three sites identifies wasting in edematous patients as it is happening.

We studied the effects of preexercise meal composition on metabolic and performance-related variables during endurance exercise. Eight well-trained cyclists (maximal oxygen uptake 65.0 to 83.5 ml . kg-1 . min-1) were studied on three occasions after an overnight fast. They were given isoenergetic meals containing carbohydrate (CHO), protein (P), and fat (F) in the following amounts (g/70 kg body wt): high-carbohydrate meal, 215 CHO, 26 P, 3 F; high-fat meal, 50 CHO, 14 P, 80 F. On the third occasion subjects were studied after an overnight fast. Four hours after consumption of the meal, subjects started exercise for 90 min at 70% of their maximal oxygen uptake, followed by a 10-km time trial. The high-carbohydrate meal compared with the high-fat meal resulted in significant decreases (P < 0.05) in blood glucose, plasma nonesterified fatty acids, plasma glycerol, plasma chylomicron-triacylglycerol, and plasma 3-hydroxybutyrate concentrations during exercise. This was accompanied by an increase in plasma insulin (P < 0.01 vs. no meal), plasma epinephrine, and plasma growth hormone concentrations (each P < 0.05 vs. either of the other conditions) during exercise. Despite these large differences in substrate and hormone concentrations in plasma, substrate oxidation during the 90-min exercise period was similar in the three trials, and there were no differences in performance on the time trial. These results suggest that, although the availability of fatty acids and other substrates in plasma can be markedly altered by dietary means, the pattern of substrate oxidation during endurance exercise is remarkably resistant to alteration.

These results highlight that far from being slow in the uptake of amino acids, the erythrocyte in fact sequesters amino acids at an appreciable rate during exercise without a corresponding elevation in the plasma amino acids. For a greater understanding of amino acid changes during exercise, the analysis of both plasma and erythrocytes is recommended.