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The metabolic mechanisms of accommodation to a low-protein diet in elderly women were studied. Diets containing 1.47 (low) or 2.94 (adequate) g protein . kg body cell [mass.sup.-1].[d.sup.-1] (0.45 or 0.92 g . kg body [weight.sup.-1].[d.sup.-1], respectively) were fed for 10 wk. Fed state leucine kinetics, fasted and fed metabolic rate, and acute-phase responses to a bout of exercise were measured. Leucine uptake into protein, breakdown, and acute-phase responses were not significantly different within or between diet groups. In the low-protein group, leucine flux and oxidation were lower after 3 wk and oxidation continued to decrease throughout the 10-wk study. When expressed per body cell mass, metabolic rate increased over time in the low-protein group only. Leucine oxidation was a more sensitive index of the adequacy of protein intake than synthesis, flux, metabolic rate, or acute-phase response.

A 9-wk study of adaptation to marginal protein intakes was conducted in 12 elderly women. Subjects were randomly assigned to two groups fed a weight-maintenance diet containing either 1.47 (low) or 2.94 (adequate) g protein . kg body cell [mass.sup.-1].[d.sup.-1] (0.45 and 0.92 g.kg body [wt.sup.-1].[d.sup.-1], respectively). Mean nitrogen balance in the low-protein group remained negative throughout the study. These subjects experienced significant losses in lean tissue, immune response, and muscle function. The adequate-protein group was in nitrogen balance throughout the study, without changes in lean tissue, and with improvements in immune response, serum immunoglobulins, albumin, total protein values, and muscle function. Thus, elderly women fed the low-protein diet accommodated to the diet by compromising functional capacity, whereas those fed the adequate diet maintained functional capacity.

Dietary protein requirements of elderly people were determined by short-term nitrogen-balance techniques and using calculations recommended by the 1985 Joint FAO/WHO/UNU Expert Consultation. Twelve men and women aged 56-80 y were randomly assigned to groups that consumed either 0.80 [+ or -] 0.01 or 1.62 [+ or -] 0.02 g protein. [kg.sup.-1].[d.sup.-1] ([chi] [+ or -] SEM. Net nitrogen balance was negative for the lower-protein group [+ or -] 3.4 mg N.[kg.sup.-1].[d.sup.-1]; and positive for the higher-protein group (13.6 [+ or -] 1.0 mg N.[kg.sup.-1].[d.sup.-1]); the intake required for nitrogen equilibrium was estimated to be 1.00 g.[kg.sup.-1].[d.sup.-1]. Nitrogen-balance data from three previous protein requirement studies in elderly people were recalculated by using the same balance formula and combined with the current study data to provide an overall weighted mean protein requirement estimate of 0.91 [+ or -] 0.043 g.[kg.sup.-1].[d.sup.-1]. Together, the current and retrospective nitrogen-balance data suggest that the mean protein requirement in elderly adults is considerably greater than the 0.60 g.[kg.sup.-1].[d.sup.-1] established by the 1985 Joint FAO/WHO/UNU Expert Consultation. A safe protein intake for elderly adults would be 1.0 - 1.25 g.[kg.sup.-1].[d.sup.-1] of high-quality protein. Am J Clin Nutr 1994;60:501-9.

Body composition and the components of energy metabolism were examined in 12 men and women, aged 56-80 y, before and after 12 wk of resistance training. Subjects were randomly assigned to groups that consumed diets that providing either 0.8 or 1.6 g protein [multiplied by] [kg.sup.-1] [multiplied by] [d.sup.-1] and adequate tota energy to maintain baseline body weight. Fat mass decreased 1.8 [+ or -] 0.4 kg (P < 0.001) and fat-free mass (FFM) increased 1.4 [+ or -] 0.4 kg (P < 0.01) in these weight-stable subjects. The increase in FFM was associated with a 1.6 [+ or -] 0.4 kg increase in total body water (P < 0.01) but no significant change in either protein plus mineral mass or body cell mass. With resistance training, the mean energy intake required for body weight maintenance increased by [nearly equal to] 15%. Increased energy expenditure included increased resting metabolic rate (P < 0.02) and the energy cost of resistance exercise. Dietary protein intake did not influence these results. Resistance training is an effective way to increase energy requirements, decrease body-fat mass, and maintain metabolically active tissue mass in healthy older people and may be useful as an adjunct to weight-control programs for older adults. Am J Clin Nutr 1994;60:167-75.

The urine of sexually mature male rats contains a protein of hepatic origin, α2u-globulin, not found in the urine of immature or female rats; output of this protein is greatly reduced by fasting. We have examined the effects of feeding and of fasting for various lengths of time on urinary output and hepatic synthesis of α2u-globulin. Rats eating ad libitum showed diurnal rhythms of urinary α2u-globulin excretion reaching maxima between 2000 and 0800 hours, thus coinciding with the daily feeding period of the rat. Fasting for 12 hours extinguished this diurnal rhythm. When fasting was prolonged up to 36 hours, urinary excretion of α2u-globulin was reduced to very low levels. Hepatic synthesis of α2u-globulin under these nutritional conditions was investigated by incubating liver polyribosomes with [3H]-leucine and a preparation of soluble enzymes for protein synthesis and separating the labeled α2u-globulin peptides by immunoprecipitation followed by electrophoresis on sodium dodecyl sulfate-polyacrylamide gels. By this technique, it was shown that only membrane-bound ribosomes in the livers of mature male rats make this protein. Semi-quantitative measurement suggested that the proportion of liver polyribosomes synthesizing α2u-globulin was unchanged after 12 hours of fasting, but was reduced after 24 and 36 hours of fasting. It is proposed that the diurnal rhythm in α2u-globulin output in the urine represents translational control of its synthesis in the liver, whereas the more extensive reduction with prolonged fasting is partly due to a selective reduction in transcription of the messenger RNA for this protein.

Creatine metabolism was studied in relation to creatine intake and creatinine excretion. Young men were fed 0.23 g creatine/day for 9 days and then 10 g/day for 10 days consecutively. Thereafter, the diet fed was creatine-free. From day 81 through 90, isonitrogenous amounts (4 g N/day) of either an equimolar mixture of the creatine precursors arginine and glycine or of alanine were added to the diet. As reported in a previous paper, creatinine excretion increased during creatine feeding, continued to remain elevated immediately afterwards and then decreased gradually during the period of feeding the creatine-free diet, whereas two subjects not fed creatine showed no significant changes in creatinine output throughout the experiment. The present paper describes studies in which di-15N-creatine was injected into the same subjects on two occasions during the creatine-free period, near the beginning of this period and 43 days later. By isotope dilution, the creatine pool sizes were calculated and the rate of conversion of this pool to creatinine was computed. The pool of body creatine diminished during the creatine-free period in parallel with the daily output of creatinine, that is, the fractional rate of conversion of creatine to creatinine was very similar for all subjects (0.0169 ± 0.0006 day-1, n = 13). In contrast to the marked constancy of the rate of conversion of creatine to creatinine, apparent fractional creatine synthesis rates were much more variable between subjects (0.011 to 0.016 day-1). Administration of the creatine precursors arginine and glycine significantly increased apparent creatine synthesis, whereas administration of alanine depressed synthesis. From these data on di-15N-creatine metabolism, it can be concluded that (a) the size of the body pool of creatine can be influenced by dietary creatine, (b) administration of precursor amino acids can increase the rate of synthesis of creatine, (c) creatinine output is a constant fraction of the body creatine pool and can change independently of lean body mass.

Changes in dietary creatine intake often result in changes in the daily urinary creatinine excretion of humans. In the present study, the metabolic effects of the presence or absence of creatine in the diet were studied. Young men were fed creatine, 0.23 g/day for 9 days and 10 g/day for 10 days, consecutively. The subjects were physically trained for vigorous exercise during the 9-day low creatine dietary period. The diet was creatine free for 71 days after creatine feeding. During the last 10 days of the creatine-free period, isonitrogenous amounts (4 g nitrogen/day) of either an equimolar mixture of arginine and glycine (precursors to creatine synthesis) or alanine were added to the diet. Creatinine excretion increased during creatine feeding and continued to be elevated afterwards (+10% to 30%). Creatinine excretion decreased gradually during the period of feeding the creatine-free diet. The creatinine excretions of two subjects not fed creatine were unchanged during the same period. Creatinine excretion increased in two of four subjects fed precursor amino acids and was unchanged in the others and in the alanine-fed subjects. Balances of nitrogen, potassium, and phosphorus were all positive during creatine feeding, and nitrogen balance remained positive throughout the study. Potassium accumulation (1.3 g/day) during the creatine feeding period was in excess of that predicted from nitrogen balances. Positive potassium balance did not continue during the creatine-free period and did not occur in the subjects not fed creatine. Creatine deposition itself and/or posttraining glycogen accumulation are possible explanations for the potassium retention. Maintenance of nitrogen and potassium balances coincident with declining creatinine excretion indicated a degree of independence between lean body mass and urinary creatinine excretion.