During the non-competition phase of training, one of the major goals of body builders is to increase muscle mass. Weight gain with a positive energy balance promotes an increase in muscle mass when combined with high-intensity resistance training . Adequate protein intake is also required to provide the substrates for muscle accretion. Resistance exercise simultaneously increases both muscle protein synthesis and breakdown, but muscle protein synthesis overwhelms breakdown so that net muscle protein increases . Therefore, in individuals engaging in an intense resistance training regimen, energy requirements and possibly protein requirements are increased. For these reasons, bodybuilders typically consume a high-protein diet in the non-competition phase of training.
There is as yet no definitive protein requirement for bodybuilders, however values in a wide range of 0.8 - 1.8 g/kg/day have been suggested [7, 8, 21]. The participants' average dietary protein intake in this study was 4.3 g/kg of BW/day, which was about 30% of their total caloric intake. The amount of protein was nearly five times higher than that recommended for the general healthy population (0.8 g/kg BW/day) . It was also notably higher than any other recommendations of protein intake for bodybuilders, which have been suggested previously.
It is well known that a high-protein diet induces metabolic acidosis due to acidic residues of proteins. Metabolic acidosis induced by high dietary protein increases urinary acid excretion and also increases urinary calcium and phosphate levels, which may negatively influence bone and muscle protein metabolism. It is presumed that the participants who consumed excessive dietary protein (4.3 g/kg BW/day) in this study may have the risk of metabolic disturbance of acid-base homeostasis, based on the evidences from the previous study, which investigated the effect of high protein diet on metabolic acidosis.
Thus, this study suggested that it is important to determine the protein requirement for bodybuilders, because both over-intake of protein may induce unfavorable health outcomes.
Urinary excretion of nitrogen in response to high protein diet
Protein-rich diets are acidogenic due to the release of excessive non-carbonic acids (e.g., sulfuric anions), which are produced by the metabolism of protein [11, 13]. It is known that the activity of branched-chain ketoacid dehydrogenase is increased in response to a high protein intake . This enzyme facilitates the oxidation and subsequent excretion of the increased amino group. Protein nitrogens are mainly excreted as urea nitrogen via the kidneys . Urinary urea excretion has been shown to increase in response to an elevated dietary protein intake in resistance exercisers, suggesting that amino acid oxidation was increased . On the other hand, the concentrations of urea in plasma and urine also increases during exercise and remains high for some time later, also in proportion to exercise intensity and duration .
In this study, the level of urea in plasma was within the normal range but elevated in 25% of the participants. The levels of UUN were twice as high as the recommended reference range. This result can provide an evidence to assume that elevated excretion of UUN might be due to the high rates of protein catabolism that follow high protein intake. Based on these results from increased UUN and creatinine, it is ascertained that dietary protein consumed by the high-intensity resistance exerciser might be mainly used as the substrates which is needed to release energy and/or to repair muscle mass during exercise.
Urinary excretion of calcium in response to high protein diet
Urinary calcium excretion is ultimately affected by dietary calcium intake. However, high protein intake could not be completely excluded from influence on urinary calcium excretion. The amount of dietary protein as well as the amount of dietary calcium affects urinary calcium excretion . It has been reported that the increases in urinary calcium excretion followed by high protein intake are similar to increases in urinary calcium excretion followed by high dietary calcium intake and independent of the level of dietary calcium . A high-protein diet promotes renal calcium excretion by directly inhibiting renal tubular calcium re-absorption to maintain acid-base homeostasis [28–30]. In the previous interventional study, high protein diet significantly increased urinary calcium excretion in both human and animal model [14, 31]. In the study of Wagner et al. , the urinary calcium excretion of the group received a high protein diet (2.0 g/kg BW/day) was almost two times higher than that of low protein diet group (0.5 g/kg BW/day).
However, although protein intakes (4.3 g/kg BW/day) in this study subjects were twice higher than the amount in Wagner et al.'s study, calcium excretion into urine was only on the border of upper limit of the reference range and the urine pH, which indicates the major evidence of metabolic acidosis, was still in normal range. It has been well-established that high protein intakes increase urinary calcium excretion in general population. However, there is limitation to fully explain the relationship between protein catabolism followed by high protein intake and urinary calcium excretion in the subjects with intensive exercise. It can be presumed that some factors, such as intensive exercise and other dietary factors, would play a role as buffer against increasing urinary calcium excretion in this subjects.
The role of resistance exercise and dietary potassium on the preservation of nitrogen and calcium
Increased protein catabolism, accompanied by high-intensity exercise, may indicate bodybuilder have a higher rate of whole body protein turnover . The participants in this study had high contents of muscle mass simultaneously with high UUN excretion. The plausible reason for increased UUN excretion might be the result from high rate of protein catabolism, using dietary protein as the substrate for muscle accretion. A high amount of dietary potassium also provides an anabolic stimulus for muscle synthesis and buffer against nitrogen excretion in urine . Dietary potassium consumes H+ and reduces both acid production and acid excretion . Ceglia et al. , who studied the effects of a high-protein diet with supplementation of potassium bicarbonate on nitrogen excretion in healthy women, reported that UUN excretion reduced in the participants taking potassium supplements.
Nemoseck & Kern  recently investigated the effects of exercise on urinary calcium excretion, and they reported that urinary calcium excretion in participants who got intensive exercise was lower than those in the group that did not exercise. Dietary potassium also affects calcium metabolism and causes a positive calcium balance by directly or indirectly promoting renal calcium retention and inhibiting bone resorption [36–38].
In this study, participants were in the middle of intensive resistance training with multivitamins and mineral supplements. Multivitamins and mineral supplementation attributed to the high consumption of potassium along with other vitamins and minerals in all participants. The resistance exercise combined with the high dietary potassium intake might be possible to counterbalance the urinary nitrogen and calcium excretion induced by high intake of protein.