The Recommended Dietary Allowance for Protein May Not Be Adequate for Older People to Maintain Skeletal Muscle

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Abstract

Inadequate dietary protein intake results in loss of skeletal muscle mass. Some shorter-term nitrogen balance studies suggest that the Recommended Dietary Allowance (RDA) of protein may not be adequate for older people. The aim of this study was to assess the adequacy of the RDA of protein for older people by examining longer-term responses in urinary nitrogen excretion, whole-body protein metabolism, whole-body composition, and mid-thigh muscle area. This was a 14-week precisely controlled diet study. Ten healthy, ambulatory men and women, aged 55 to 77 years, were provided eucaloric diets that contained 0.8 g protein.kg(-1).day(-1). The study was conducted at a General Clinical Research Center using an outpatient setting for 11 weeks and an inpatient setting for 3 weeks. The main outcome measures included urinary nitrogen excretion, postabsorptive and postprandial whole-body leucine kinetics via infusion of L-[1-(13)C]-leucine, whole-body density via hydrostatic weighing, total body water via deuterium oxide dilution, and mid-thigh muscle area via computed tomography scans. Mean urinary nitrogen excretion decreased over time from Weeks 2 to 8 to 14 (p =.025). At Week 14, compared with Week 2, there were no changes in postabsorptive or postprandial leucine kinetics (turnover, oxidation, incorporation into protein via synthesis, release via breakdown, or balance). Whole-body composition (% body fat, fat-free mass, and protein + mineral mass) did not change over time in these weight-stable subjects. Mid-thigh muscle area was decreased by -1.7 +/- 0.6 cm(2) (p =.019) at Week 14 compared with Week 2. The loss of mid-thigh muscle area was associated with the decrease in urinary nitrogen excretion (Spearman r =.83, p =.010). The maintenance of whole-body leucine metabolism and whole-body composition is generally consistent with a successful adaptation to the RDA for protein. However, the decrease in mid-thigh muscle area and the association with decreased urinary nitrogen excretion are consistent with a metabolic accommodation. These results suggest that the RDA for protein may not be adequate to completely meet the metabolic and physiological needs of virtually all older people.

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Elderly women accommodate to a low-protein diet with losses of body cell mass, muscle function, and immune response.

Stacy Crim, Greg W. Evans, C Castaneda … (1995)

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-1.d-1 (0.45 and 0.92 g.kg body wt-1.d-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.

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Increased energy requirements and changes in body composition with resistance training in older adults.

V Leroy Young, David J. Evans, C Crim … (1994)

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.kg-1.d-1 and adequate total energy to maintain baseline body weight. Fat mass decreased 1.8 +/- 0.4 kg (P < 0.001) and fat-free mass (FFM) increased 1.4 +/- 0.4 kg (P < 0.01) in these weight-stable subjects. The increase in FFM was associated with a 1.6 +/- 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 approximately 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.

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Serum albumin is associated with skeletal muscle in elderly men and women.

Niklas K. U. Koehler, Markus Baumgartner, P Garry … (1996)

Serum albumin concentrations decrease with age and values < 38 g/L are associated with increased morbidity, mortality, and disability in the elderly. It is not clear to what extent the decreases are associated independently with changes in metabolism, dietary intake, physical activity, morbidity, or body composition. We examined associations of serum albumin with age, protein and energy intakes, physical activity, morbidity, and muscle mass in 275 men and women aged 60-95 y. Serum albumin was measured with the bromcresol green procedure. Usual dietary intake and physical activity were quantified through questionnaires. Morbidity was ascertained from medical history, questionnaire, and examination. Muscle mass was estimated from dual-energy X-ray absorptiometry. In multivariate analyses, serum albumin was associated significantly with muscle mass after age, protein intake, physical activity, and comorbidity were controlled for in men and women. This study suggests that decreases with age in serum albumin concentrations are associated with muscle loss (sarcopenia) in the elderly. This association is independent of other factors that may affect muscle mass and albumin concentration. We suggest that the increased risk of disability with low serum albumin concentrations observed in the elderly may actually reflect an association with sarcopenia.