Effect of sodium phosphate loading on endurance and sprint performance in trained females

Nutritional ergogenic supplements have long been used by athletes in an attempt to improve exercise performance. One nutritional supplement that has shown some positive benefits for sporting performance is sodium phosphate (SP). Specifically, SP is supplemented orally in capsule form, commonly at a dose of 3-5 g or 50 mg·kg-1 of fat free mass (FFM) a day for between 3-6 days to benefit aerobic and endurance performance. Numerous mechanisms have been proposed to explain the performance improvements seen following SP ingestion. These include: enhanced 2,3-diphosphoglycerate (2,3-DPG) concentration in red blood cells, which allows for a greater unloading of oxygen to the peripheral tissues; improved buffering capacity of hydrogen ions due to increased hydrogen phosphate concentrations; improved myocardial efficiency, which results in more efficient oxygenation of the exercising muscle; potentially greater adenosine triphosphate/phosphocreatine (ATP/PCr) resynthesis due to increased availability of extracellular and intracellular phosphate; and an enhanced rate of glycolysis, due to the positive effects of phosphate on the glycolytic energy pathway.
Notably, no studies to date have assessed the effects of SP supplementation on repeated-sprint ability (RSA) as performed in a team-sport game and only one study has investigated the effect of SP loading in females (using an aerobic capacity test). Further research is needed here, as it is possible that females may respond differently to SP loading than males, due to several gender related physiological differences that may affect the aforementioned mechanisms associated with SP supplementation. Specifically, compared with males, females have a decreased affinity of oxygen with haemoglobin, which may dampen the potential positive effect of any increases in 2,3-DPG concentration. Furthermore, females have higher natural levels of (and fluctuations in) oestrogen, which plays a role in renal phosphate reabsorption within the body.
In addition, it is common for two (or more) nutritional supplements to be ingested together to further enhance exercise performance, however no studies have assessed the effect of SP loading in combination with other common ergogenic aids. Notably, caffeine and nitrate represent two legal, beneficial supplements that if ingested in combination with SP may further improve exercise performance than if taken alone, as each of these supplements is proposed to enhance exercise via completely separate mechanisms.
Similar to SP, nitrate in the form of beetroot juice (BJ) has been found to improve both exercise tolerance (by 15-25%) during high-intensity, incremental cycling tests and cycling time-trial (TT) performance (by 2.7% for 4 km, 1.3% for 10 km and 2.8% for 16.1 km) following 5–11.2 mmol of nitrate (BJ or sodium nitrate) ingestion, as well as exercise efficiency, based on reductions in submaximal oxygen uptake (VO2). Mechanisms proposed to account for these effects include more efficient mitochondrial respiration, due to reduced proton leakage through the inner mitochondrial membrane and more efficient ATP metabolism during muscle force production, due to lower levels of calcium ATPase and actomyosin ATPase activity. However, similar to SP, no studies have assessed the effects of BJ supplementation on RSA as it relates to team-sports performance.
Similarly, numerous studies have reported that consuming 3-6 mg·kg-1 of body mass of caffeine 60 min before exercise resulted in significant improvements in endurance performance. More recently, the effects of caffeine on RSA, as performed during team-sports, have been investigated, with some studies finding benefit while others have not. Improvement in exercise performance following caffeine ingestion has primarily been attributed to adenosine receptor antagonism, which is proposed to result in reduced sensations of effort and pain, increased alertness, improved neural firing rates and enhanced motor unit recruitment and frequency of activation. Notably, all studies that have assessed the effects of caffeine on RSA have used male participants.
Consequently, the purpose of this thesis was to investigate the effect of SP, BJ and caffeine supplementation, either alone or in combination, on endurance and RSA in female athletes with sprints performed prior to, midway and at the end of a simulated team-sport game.
Specifically, study one investigated the effect of six days of SP supplementation on 500 kJ (simulated 20-km) cycling TT performance. A secondary aim of this study was to determine the optimal SP loading dose for females, as it may differ to that used in males. Consequently, three different dosing protocols were compared (25, 50 and 75 mg·kg-1 of FFM doses). Overall, results indicated that none of these SP doses improved 500 kJ TT performance in female cyclists when compared with placebo.
The second study investigated the effect of SP loading (50 mg·kg-1 of FFM per day for 6 days) on RSA (3 sets of 6 x 20 m sprints) performed at the start (set 1), during the half-time break (set 2) and at the end (set 3) of a 60 min simulated team game circuit. This study also aimed to determine if RSA performance could be further improved by combining SP with caffeine (SP+C) supplementation. It was observed that both SP and SP+C resulted in some improvement to RSA for various sets compared with placebo and caffeine, but caffeine alone had minimal effect on RSA.