The use of phosphate supplementation as an ergogenic aid has been used by athletes for more than 60 years. German physicians during World War I noted increases in “physical fitness” from supplements that consisted of primarily sodium phosphate (16). Phosphates, which often contain sodium, potassium, or calcium, are the mineral salt form of the essential nutrient phosphorus. Phosphorus is an important physiological mineral, functioning as a backbone component for DNA, RNA, and phospholipids. More importantly, and from an athletic perspective, it is essential for the formation of adenosine triphosphate (ATP), our primary energy substrate. Similar to other essential vitamins and minerals, phosphorous must be supplied endogenously through supplementation or the diet in a variety of foods, including beef, chicken, cod, bread, milk, and yogurt.
Phosphate loading, consisting of approximately 1 gram 4 times daily for 3-5 days, is based on the assumption that increased inorganic phosphate levels in humans are associated with an increase in red blood cell 2,3-diphosphoglycerate (2, 3-DPG) concentration (2,8,22). An increased erythrocyte 2,3-DPG concentration has been shown to shift the oxyhemoglobin dissociation curve to the right, allowing a greater amount of oxygen to be extracted from blood, for use by the body (2,3,8). With an increase in 2,3-DPG and a decrease in oxygen affinity, an increase in maximal oxygen consumption (O2max) during intense exercise has been indicated (3). The efficient delivery of oxygenated blood to muscle is an important factor when investigating performance measures. Oxygen is crucial to energy metabolism because of its role as the final electron receptor in oxidative phosphorylation and the synthesis of ATP. ATP is used as energy by skeletal muscle to perform sustained bouts of exercise (23). Inorganic phosphate supplementation has been shown to increase 2,3-DPG in red blood cells (2,4). Phosphate loading has also been reported to enhance cardiovascular response, aid in buffering capacity, and increase scores on cognitive tests (17,19).
Phosphate supplementation has been postulated to trigger numerous physiological mechanisms, including metabolism and cardiovascular health (19). Research has been conducted on the possible association between phosphates and metabolic regulation in both the phosphocreatine and glycolytic energy systems (24). Phosphate loading has also been reported to enhance cardiovascular function and aid in muscle or physiological buffering capacity (19). Although it is hypothesized that phosphate loading has an impact on multiple physiological energy systems, the most supported mechanism is its role in the synthesis of 2,3-DPG and the associated increase in the availability of oxygen to bodily tissues.
Physiological and environmental mechanisms have been found to naturally increase 2,3-DPG levels. Adaptations to cardiovascular disorders (such as anemia and complications from renal disease), living at and traveling to high altitude levels (greater than 3,500 m above sea level), and intense training have shown to elicit this response (17,24,25,28,29). Gender differences have also been reported with regard to 2,3-DPG levels, with women displaying higher resting levels than men with similar fitness backgrounds. This discrepancy may be compensatory and a response to lower hemoglobin levels in women (26). Future research focusing specifically on phosphate loading and women would be beneficial in further exploring this topic.
Normal physiological ranges of 2,3-DPG (3-5 mmol/L) found in red blood cells may not be enough to elicit a rightward shift in the oxyhemoglobin dissociation curve and a decrease in oxygen affinity (8,30). Bremner et al. (2) confirmed the previous work demonstrating that phosphate supplementation can stimulate an increase in red blood cell 2,3-DPG levels (2,3,22,31). Red blood cell 2,3-DPG combines with hemoglobin resulting in a decreased affinity for oxygen. With more oxygen being delivered to muscle tissue through the blood, there is a subsequent increase in the ability of an individual to perform work (24).
Improvements from phosphate loading have been demonstrated in athletic performance and a number of corresponding physiological measures, including maximal oxygen uptake (O2max), anaerobic threshold, and time to exhaustion, can be expected (20). These measures span the gamut of performance variables, and regardless of the chosen sport, improvements in these measures are bound to be beneficial for training and competition. Additionally, phosphate supplementation has also been purported to aid in physiological adaptations at altitude, including increases in plasma phosphate and 2,3-DPG, as well as perceived wellness according to physiological and clinical tests (17). So, phosphate may be a direct benefit to a strenuous hike planned a few weeks out, or it might even make an impact on performance at high elevation, such as in Colorado or Mexico City.
The effects of red blood cell 2,3-DPG on oxygen affinity are well documented, and the inverse relationship between these 2 physiological measures has been studied for quite some time (8). In addition, phosphate supplementation has been shown to elicit an increase in red blood cell 2,3-DPG levels (2). With an increase in 2,3-DPG and a decrease in oxygen affinity, an increase in maximal oxygen consumption (O2max) during intense exercise has been postulated (3). Cade et al. (3) were the first to consider phosphate loading as an ergogenic aid and found that it leads to an increased O2max of 6-12% in well-trained male runners (3). Further research followed, with a number of studies showing similar impacts on O2max (3,5,20,21,31); notably, Stewart et al. (31) demonstrated a 10% increase in male cyclists (31), Kreider et al. (20,21) reported a 9% increase in various athletes (20,21), and, most recently, Czuba et al. (5) posited a 5% increase in off-road cyclists (5). A variety of phosphate formulations have been used, but when significant increases have been achieved, sodium phosphate has typically been the method of supplementation. Folland et al. (11) examined sodium phosphate and its effect on cycling performance. Although only nonsignificant increases were shown in O2max values, mean power output and 16.1-km time-trial performance were both very tangible variables that did experience improvements (11). Although this data support improvements in oxygen consumption, some studies display contradictory results (1,7,11-13,22). Acute and 4-day loading protocols of calcium phosphate have elicited no changes in O2max (1,12,22).
Differences in results between studies may be attributed to various loading protocols, using acute dosing (up to 6 days) or chronic supplementation (approximately 30 days), with the acute strategy demonstrating the most practical and effective way to improve performance (9,20). Acute dosing has shown increases in O2max of up to 12%, whereas long-term supplementation has shown only minimal changes in aerobic fitness. Additionally, most studies have used sodium phosphate, potassium phosphate, or calcium phosphate as forms of oral supplementation, with sodium phosphate showing the most consistent positive results. Another limitation to the available research is that only male subjects have been studied with regard to phosphate loading and its effects on O2max. The effects on women have yet to be researched.
Reports regarding increases in 2,3-DPG levels without supplementation and high-intensity interval training (HIIT) have been mixed (15,18). Although it is known that HIIT elicits increases in cardiorespiratory fitness in both men and women (6,14,27), the concomitant increases in 2,3-DPG and O2max after phosphate loading and HIIT have not been measured. However, it would be reasonable to believe that coupling the benefits of HIIT and phosphate supplementation could lead to compounded improvements in performance. Specifically, for endurance athletes, more oxygen availability because of phosphate loading at the end of a HIIT cycle could be beneficial to enhance training volume or for competition.
Phosphate salts are commonly included as part of the formulation of nutritional supplements, and phosphorous is found in many foods. Research also shows that individuals tend to consume extra phosphorous during daily dietary consumption (29). Creatine phosphate, for example, has been shown to provide more performance benefits than supplementation with creatine alone (10). Beta-alanine is another widely used supplement available with phosphorous as an additional ingredient. The possibility of weight gain associated with creatine or the tingling sensation sometimes caused by beta-alanine may be the detracting factor with these supplements, although phosphate supplementation on its own may be an attractive alternative to elicit performance and training benefits.
The recommended phosphate loading dosage consists of 1 g of sodium phosphate salt 4 times daily for 3-5 days (19). Loading protocols have been shown to be the most effective for increasing 2,3-DPG and induce little to no adverse effects in well-trained subjects. People with kidney disease are often advised to closely monitor phosphorous levels and should avoid loading protocols (29). It has been proposed that regular long-term use can upset the balance of phosphates and other minerals within the body, irritating the digestive tract and possibly leading to upset stomach, diarrhea, and/or constipation. However, of all the previously published data with a total of 88 participants, only 1 person reported to have suffered from gastrointestinal distress (11). Previous research has suggested strictly aerobic improvements from phosphate loading, but phosphate also plays a role in the phosphocreatine energy system and the provision of anaerobic energy. Further research involving anaerobic performance and phosphate loading is warranted. Improvements from phosphate loading have shown to elicit increases in O2max up to a week after the cessation of dosing (3), so it may not be necessary to supplement immediately before competition.
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