Combined Effect of Dietary Phosphorus and Structured Exercise on Body Composition, Energy Balance, and Protein Synthesis Initiation in Rats

Abstract

Individually, both phosphorus (P) ingestion and physical exercise (E) have been found to alter body composition measures and energy balance parameters. Additionally, regular E was reported to be associated with energy compensation either through an increase in energy intake (EI) and/or a reduction in total energy expenditure (TEEx). However, it is not clear whether the ability of dietary P to stimulate energy expenditure (EE) would affect energy compensation following structured E. Accordingly, one of the prominent aims of this study is to assess the combined effect of dietary P and moderate running exercise routine on body composition, energy balance and energy compensation.

P has been associated with the availability of cellular energy for protein synthesis, while regular physical E has been reported to activate protein synthesis in skeletal muscle, but not necessarily so in other tissues. Yet, to date, there are no studies investigating the combined effect of P and E on the mTOR pathway and markers of protein synthesis initiation in various tissues. Correspondingly, the second major aim of this research is to evaluate the impact of P and E on liver and skeletal muscle protein synthesis initiation factors and investigate the signaling pathway involved.

After receiving approval from Institutional Animal Care and Use Committee of the American University of Beirut, two experiments were performed, Low Phosphorus (LP) (0.1%P, 0.2%P, and 0.3% P) and High Phosphorus (HP) (0.3%P, 0.6%P, and 1.2% P) diets. In each experiment, male rats were randomly divided into 3 groups (n=8), in which a sedentary or a moderate-intensity exercise routine (30 minutes 5 days a week) was implemented. EI, body weight and composition, TEEx, energy efficiency, and energy stores were monitored for 6 weeks. Following sacrifice, signaling proteins involved in initiation of protein synthesis translation were measured in liver and gastrocnemius muscle.

In the LP experiment, EI and weight gain were the lowest in the 0.1%P and 0.2%P as compared to the 0.3%P. In the HP experiment, EI was highest in the high P (0.6%P and 1.2%P) groups, while weight gain was reduced. In both experiments, E was able to reduce body fat accumulation and to maintain a higher %LBM. In the LP experiment, the similarity in TEEx between the sedentary and E groups suggests the probability of a reduction in normal daily activities, which indicates the presence of compensation for the energy expended during exercise by a subsequent reduction in EE. In contrast, the elevated TEEx in the HP exercising groups (0.6%P and 1.2%P) argue against the presence of energy compensation. Therefore, high dietary P decreases the body’s capability to compensate for the energy deficit induced by E, consequently maintaining an elevated TEEx.

In reference to protein synthesis, the degree of activation of signaling proteins varied between liver and gastrocnemius muscle. Mammalian target of rapamycin (mTOR) activation increased in the liver as dietary P level increased, while no changes were detected in downstream expression of phosphorylated eukaryotic initiation factor 4E-binding protein-1 (4E-BP1) and ribosomal protein p70S6 kinase (S6K1). The eukaryotic initiation factor 4E (eIF4E) was activated in the exercising groups as the level of P increased. Essentially, E did not result in a change in phosphorylated AMP-activated protein kinase (AMPK) levels, which implies that P availability may have prevented the activation of AMPK in liver, thus maintained mTOR activation and resultant protein synthesis initiation stimulation.

In gastrocnemius muscle, AMPK activation in the 1.2%P exercising group may have resulted in lowering the levels of phosphorylated mTOR and eIF4E, yet, may not have affected protein synthesis considerably. Phosphorylated mTOR was highly expressed in the 0.6%P exercising group which may indicate a greater activation in muscle protein synthesis, though translational initiation factors other than 4E-BP1 and S6K1, which remained unchanged. Further, an mTOR- independent pathway may be responsible for the effect of high P intake (1.2%) on eIF4E activation. Hence, both an mTOR-dependent and mTOR-independent pathway may be involved in protein synthesis in response to P and/or E at the level of skeletal muscle.

In conclusion, these collective results demonstrate that P ingestion above standard level of 0.3%P combined with regular structured moderate-intensity E favours improvement in body composition measures and energy balance outcomes, and may lead to enhancement in protein synthesis through activation of various initiation factors in liver and muscle.