The Anabolic Response to Dietary Protein Is Not Limited by the Maximal Stimulation of Protein Synthesis in Healthy Older Adults: A Randomized Crossover Trial

Sanghee Park,Myung Dong Choi,Jiwoong Jang,Yun-A Shin,Arny A Ferrando,Scott Schutzler,Il-Young Kim,Robert R Wolfe,Gohar Azhar

doi:10.3390/nu12113276

Abstract

We have recently demonstrated in young adults that an anabolic response with mixed meal protein intake above ~35 g/meal, previously recognized as an “optimal” protein dose, was further stimulated. However, it is unknown if this applies to older adults. We therefore examined anabolic response to a mixed meal containing either 35 g (MOD, moderate amount of protein) or 70 g (HIGH, high amount of protein) in a randomized cross-over metabolic study in older adults (n = 8). Primed continuous infusions of L-[2H5] phenylalanine and L-[2H2]tyrosine were performed to determine whole-body protein kinetics and muscle protein fractional synthesis rate (MPS) in basal fasted and fed states. Whole-body protein kinetics (NB, net protein balance; PS, protein synthesis; PB, protein breakdown) and MPS was expressed as changes from the baseline post-absorptive state. Consistent with our previous findings in young adults, both feedings resulted in a positive NB, with HIGH being more positive than MOD. Furthermore, NB (expressed as g protein∙240 min) increased linearly with an increasing amount of protein intake, expressed relative to lean body mass. The positive NB was achieved due mainly to the suppression of PB in both MOD and to a greater extent HIGH, while PS was only increased in HIGH. Consistent with the whole-body data, MPS was significantly higher in HIGH than MOD. Plasma concentrations of essential amino acids and insulin were greater in HIGH vs. MOD. We conclude that in the context of mixed meals, whole-body anabolic response linearly increases with increasing protein intake primarily through the suppression of PB, and MPS was further stimulated with protein intake above the previously considered “optimal” protein dose in older adults.

Highlights

Sarcopenia, a major factor of the fragility syndrome, is defined as progressive decrease of muscle mass, strength, and function
It was based entirely on muscle protein fractional synthesis rate (MPS) data despite the fact that the whole-body anabolic response is determined by the balance between protein synthesis and breakdown [9,10]
Last and importantly, when quantifying an anabolic response it is necessary to take into account the entire body protein pool, because more than half of whole-body anabolic response occurs at organs such as gut [9]. With these points in mind, we previously found in healthy young adults that increasing amounts of protein intake induced greater anabolic responses in the context of a mixed meal [13]

Summary

Introduction

Sarcopenia, a major factor of the fragility syndrome, is defined as progressive decrease of muscle mass, strength, and function. It is considered a strong predictor of disability and mortality in older adults [1,2]. The anabolic response in older individuals has been reported to be maximized with consumption of 0.40 g protein/kg BW/meal or ~ 32 g protein/meal for an 80-kg person [8]. This conclusion seems to be incomplete for several reasons. It was based entirely on muscle protein fractional synthesis rate (MPS) data despite the fact that the whole-body anabolic response is determined by the balance between protein synthesis and breakdown [9,10]

Methods

Results

Discussion

Conclusion