Perspective: Protein Requirements and Optimal Intakes in Aging: Are We Ready to Recommend More Than the Recommended Daily Allowance?

Abstract

The Dietary Reference Intakes set the protein RDA for persons >19 y of age at 0.8 g protein ⋅ kg body weight-1 ⋅ d-1. A growing body of evidence suggests, however, that the protein RDA may be inadequate for older individuals. The evidence for recommending a protein intake greater than the RDA comes from a variety of metabolic approaches. Methodologies centered on skeletal muscle are of paramount importance given the age-related decline in skeletal muscle mass and function (sarcopenia) and the degree to which dietary protein could mitigate these declines. In addition to evidence from short-term experimental trials, observational data show that higher protein intakes are associated with greater muscle mass and, more importantly, better muscle function with aging. We are in dire need of more evidence from longer-term intervention trials showing the efficacy of protein intakes that are higher than the RDA in older persons to support skeletal muscle health. We propose that it should be recommended that older individuals consume ≥1.2 g protein · kg-1 · d-1 and that there should be an emphasis on the intake of the amino acid leucine, which plays a central role in stimulating skeletal muscle anabolism. Critically, the often-cited potential negative effects of consuming higher protein intakes on renal and bone health are without a scientific foundation in humans.

FIGURE 1

Mean amount of dietary protein expressed according to ideal body weight among men (n = 324) and women (n = 281) aged >70 y. Data were derived from NHANES 2005–2006, as published by Berner et al. (7). Horizontal dotted lines indicate the current EAR and RDA, and dashed lines indicate the proposed EAR (0.9 g · kg⁻¹ · d⁻¹) and RDA (1.2 g · kg⁻¹ · d⁻¹) based on values from Table 1. The data indicated that 6% of men aged >70 y and 12% of women aged >70 y consumed less protein than the current EAR (0.66 g · kg⁻¹ · d⁻¹). EAR, Estimated Average Requirement.

FIGURE 2

Mean amount of dietary protein during breakfast, lunch, and dinner at total protein intakes of 0.8 and 1.2 g · kg⁻¹ · d⁻¹ in a skewed and even protein intake distribution. The distribution of protein over the main meals is derived from Farsijani et al. (96), with the most skewed and most even distribution of protein intake that is typically seen in older men. The horizontal dashed line refers to the proposed optimal per-meal amount of dietary protein for maximal stimulation of MPS of 0.4 g · kg⁻¹ · meal⁻¹ (95% CI: 0.21, 0.59 g · kg⁻¹ · meal⁻¹) derived from Moore et al. (28). With the use of a skewed pattern at the RDA, only dinner reaches the optimal amount of dietary protein. We recommend protein intakes of 1.2 g · kg⁻¹ · d⁻¹. With the use of a skewed pattern at this higher amount of dietary protein, both lunch and dinner may provide ample protein for increasing MPS rates. Alternatively, more evenly distributing protein intake throughout the day at a higher amount of dietary protein may allow for a stimulation of MPS after all of the main meals, thereby supporting muscle mass maintenance with aging. MPS, muscle protein synthesis.