High-protein diets differentially modulate protein content and protein synthesis in visceral and peripheral tissues in rats

Abstract

Objective

High-protein diets give rise to increased amplitude in the diurnal cycling of protein gains and losses at the whole-body level, but the tissue localization and mechanisms underlying these metabolic adaptations remain unclear. We investigated tissue-specific responses to increasing protein intakes in rats.

Methods

Protein synthesis rates (flooding dose with ¹³C-valine) and accretion were assessed in individual tissues of fasted or fed rats (n = 32) after a 2-wk adaptation to a normal- or high-protein (HP) diet.

Results

In livers of HP rats, a strong inhibition of protein synthesis rates (−34%) occurred in the fasted and fed states, whereas a higher protein content (+10%) was observed. In the kidneys, a slight inhibition of synthesis rates after the HP diet was also observed but remained without effect on kidney protein pool size. Stomach and skin protein synthesis rates were significantly increased under HP conditions, whereas protein anabolism in skeletal muscle remained insensitive to the dietary protein level. This was also true for specific muscle protein fractions: myosin, mitochondrial, or sarcoplasmic protein synthesis rates were influenced by neither the dietary protein level nor the nutritional status.

Conclusion

Modulation of protein kinetics and accretion by the HP diet is tissue-specific and the liver plays a critical role in such adaptations in a unique situation associating an inhibition of protein synthesis and protein pool expansion. The mechanisms underlying these changes and their physiologic incidence remain to be elucidated.

Introduction

Protein consumption exceeds recommended levels in most developed countries [1], [2], [3] and may be extreme in specific populations using high-protein (HP) diets to improve their muscle mass and physical performance or to lose weight. However, the metabolic adaptations resulting from prolonged exposure to an HP diet are not fully elucidated [4], [5]. Unlike the effects of low-protein diets, which have been extensively investigated [6], little is known about the adaptation of protein turnover to increased protein intake, particularly in individual tissues. Even if some pioneering works have generated valuable data regarding enzyme induction and tissue responses to HP diets [7], the tissue localization, physiologic implications, and mechanisms underlying such metabolic adaptations to HP diets have not been established. Hence, it is difficult to draw a clear picture of the changes to protein turnover induced by an HP diet at the tissue level during fasting and feeding periods that could be extrapolated to humans consuming high levels of dietary protein.

In humans, whole-body protein turnover studies have revealed that an increase in protein intake is associated with marked increases in amino acid oxidation and stimulation in the fasted state and strong inhibition in the fed state of the protein breakdown rate [8], [9], [10]. Conversely, whole-body protein synthesis rates are little affected by HP levels, with only moderate stimulation in the fed state [8], [9], [10]. These changes result in a greater amplitude of body protein gains and losses during fed and fasted periods of the day [11]. In specific tissues, in humans and rats, most studies have focused on mixed muscle protein synthesis and the findings are conflicting, showing protein synthesis rates that are inhibited [12], [13], [14], [15], enhanced [9], [16], or unchanged [17] with increasing protein intakes above the requirement. There are, however, some evidences for a differential regulation of protein synthesis rates between mitochondrial and cytoplasmic proteins by hormones and amino acids [18], [19] that question the possibility of different individual responses of these fractions to increased protein levels.

In visceral organs, the few results available show contrasting tissue fractional synthesis rate (FSR) responses to increasing protein intakes, with unchanged or enhanced rates in the liver, unchanged rates in the intestinal mucosa, and increased rates in the kidneys [13], [15], [16], [20]. Only one of these studies has examined this tissue response to an HP diet in the fasted and fed states [15], which is necessary to have a true reflection of diurnal metabolic fluxes. However, the results obtained in neonatal piglets [15], [16] are difficult to extrapolate to growing or mature animals with slower rates of growth.

In this context, the purpose of this study was to assess the responses of protein synthesis rates at multiple tissue and protein fraction levels to elevated protein intakes in fasted and fed growing rats. We hypothesized that increasing protein intake would differently affect protein synthesis rates of individual tissues and of individual proteins in tissues such as muscle.