It’s unusual, in our commercial sporting culture, that any athlete should be unaware of a nutritional supplement capable of improving his/her performance. Such is the scale of the sports nutrition industry that any potentially ergogenic substance supported by even an iota of evidence is promptly pedalled out to consumers seeking an easy performance edge.
It might come as a surprise to learn that there is one nutritional strategy, backed by quite promising research and theoretical rationale, that most athletes are completely unaware of. That strategy is phosphate loading.
What is it?
Phosphates are an essential part of our body chemistry: they’re a major component of bones and teeth, they’re an integral part of our cell membranes, and even comprise the structural backbone of our DNA. Of particular interest to athletes, however, are their multifarious roles in metabolism. Anyone who has ever dabbled in bioenergetics (the study of how we generate energy) will appreciate the ubiquity of phosphates in this area; the terms “phospho-” and “kinase” abound in the lexicon. Ultimately, our very existence is dependent on a relatively small pool of high-energy phosphate compounds (“adenosine triphosphates”, or “ATP”).
We obtain dietary phosphates from protein rich foods, and as with any major mineral there is a recommended daily intake to avoid deficiency. In fact, much of the interest in phosphate loading has stemmed from observations of deficient individuals (a condition known as hypophosphatemia).
How do they work?
Before looking at any evidence of performance benefit, let’s first look at how increasing phosphate levels (particularly blood phosphates) might benefit an athlete:
- Heart function: hypophosphatemia is known to impair myocardial function20,32. Specifically, low blood phosphate levels are associated with reduced heart contractility (i.e. reduced stroke volume). When phosphate levels are restored, myocardial contractile force recovers in parallel32. Elevating serum phosphates above normal levels, therefore, might further improve cardiac function. Given that stroke volume is typically thought to limit VO2max28, this should be of great interest to athletes.
- Lung function: hypophosphatemia is also associated with impaired diaphragm function and, again, this is can be corrected via restoration of blood phosphate levels3.
- Acid buffering: phosphate compounds are one of the cell’s many defences against changes in pH. However, the relevance of acidosis per se to muscle fatigue is questionable2, so this cited mechanism is unlikely to have a direct effect on performance33.
- ATP synthesis: many of our metabolic enzymes rely on, and are stimulated by, inorganic phosphates5,14; ultimately, the resynthesis of ATP itself is phosphate dependent15. It is at least plausible that these processes might be enhanced by increased phosphate availability6.
- Oxygen delivery: there exists a sigmoidal relationship between the partial pressure of oxygen (PO2) and the % saturation of hemoglobin. Where PO2 is high, such as at the lungs, hemoglobin will be almost 100% saturated with oxygen. When the blood reaches the oxygen-consuming tissues, where the surrounding PO2 is lower, oxygen dissociates from hemoglobin, thus facilitating oxygen uptake. One effect of phosphate loading, and perhaps most salient, is its effect on this relationship. By increasing blood phosphate levels, and thus phosphate availability within the red blood cells8, hemoglobin will more readily offload oxygen at the tissues*4,8,30. Given that aerobic capacity (VO2max) is generally thought to be limited by oxygen delivery29, this effect of phosphate loading is very appealing.
* This largely occurs via an increase in red blood cell “2,3-DPG” concentration. 2,3-DPG is produced via a diversion from normal glucose metabolism, and acts as a regulator of hemoglobin-oxygen affinity. The significance of this mechanism can be seen in that 2,3-DPG concentrations increase in response to altitude27, exercise13 and training35.
Interest in phosphate supplementation dates back to the First World War and the work of German biochemist, G. Embden. The seminal study, however, came in 1984 from Cade et al.13. Using a sample 10 trained distance runners, these authors found that 3 days of sodium phosphate supplementation (4 g/day) increased VO2max by 6-12%, which strongly correlated with the observed increase in red blood cell 2,3-DPG. Research on phosphate loading has since found:
- Further evidence of an increased VO2max 9,16,17,24,25,33,34 (~10% on average).
- A higher VO2/power at the second ventilatory threshold 16,17,25.
- Increased maximal ventilation 16,17,24(perhaps due to improved diaphragm function).
- Reduced resting HR 17.
- Improved 5 mile run time 25.
- Improved 40 km cycling time trial time 9,24.
- Improved 10 mile cycling TT performance 19.
It must be acknowledged that not all studies are in agreement with these findings1,7,18,22,36,37. These conflicting results can, however, be reconciled on the basis of several methodological inconsistencies, for example:
- Calcium phosphate may be less effective than sodium phosphate (e.g. refs 7, 21, 31 vs. 13, 16, 19).
- Why? This might be explained a) by the existence of a sodium-dependent phosphate transport mechanism5,15,23 and/or b) by the alkalinizing effects of sodium35.
- The use of single21,31 and/or large phosphorous doses7,21 seem to be less effective than a loading protocol of smaller doses8.
- Why? Blood phosphate levels are carefully regulated by homeostatic mechanisms. As such, a large spike in blood phosphates will invoke hormonal responses that rapidly restore normal levels35. Similarly, the use of absolute doses, rather than relative doses9,10,16, might lead to varied treatment effects - for example, Czuba et al.16 administered sodium phosphate doses relative to fat free mass.
- It has been suggested that the effects of phosphate loading might persist for upwards of two weeks13, hence any study that failed to incorporate a sufficient washout period between treatments may have been confounded by residual treatment effects (e.g. ref. 18).
- Females may be less susceptible to the effects of (sodium) phosphate loading11. Therefore, conflating results from both males and females may obscure any significant effects37.
- *Why? There are a number of physiological differences between sexes (e.g. females typically have higher 2,3-DPG concentrations, smaller hearts, less red blood cell mass, and exhibit greater blood phosphate fluctuations due to the actions of oestrogen) that would render them less responsive to the effects of phosphate loading12.
- Well-trained athletes might be more responsive to phosphate loading35, and this could explain some of the disparities in the literature7,31.
- As with any nutritional ergogenic aid, there will inevitably be individual variation in responses37. This might be related, in part, to baseline phosphate levels; for example, some of the most promising published results16 were obtained from a sample of elite mountain bikers who were borderline hypophosphatemic (i.e. clinically low blood phosphate levels). This latter observation is by no means trivial as athletes may find themselves in a state of hypophosphatemia as a result of high training loads, incomplete recovery, and/or dietary deficiency.
Although there is some disagreement in the literature, it should be borne in mind that no study has found evidence of harm as a result of phosphate supplementation (apart from the occasional report of gastrointestinal distress). Therefore, the potential benefits of phosphate loading should surely make it an attractive nutritional aid to most, if not all, endurance athletes.
Current consensus would recommend that 1 g of sodium phosphate be taken four times a day, for a period of 3–6 days prior to competition12,26. For athletes competing in multi-day events, it has been shown that a dose of ~400 mg (again, taken four times daily) can maintain effects, post-loading, for as long as 3 weeks16. Those that stand to benefit most from phosphate loading are male endurance athletes, cyclists particularly, competing in events > 15mins10. Supplementation may be enhanced by the co-consumption of vitamin D3 and glucose5,7,31,35. If purchased in capsule form, phosphate absorption may also be enhanced by dissolving the capsule contents in water37. The only sports nutrition company I have been able to find that currently offers sodium phosphate supplements is Zipvit
- Ahlberg A, Weatherwax RS, Deady M, Perez HR, Otto RM, Cooperstein D, Smith TK, Wygand JW. Effect of Phosphate Loading on Cycle Ergometer Performance. Med. Sci. Sports Exerc. 18: S11, 1986.
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- Buck CL, Dawson B, Guelfi KJ, McNaughton L, Wallman KE. Sodium Phosphate Supplementation and Time Trial Performance in Female Cyclists. J. Sports Sci. Med. 13: 469–475, 2014.
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