In previous blogs we’ve discussed what sodium is and how it might be helpful during exercise. In this article, Dr Alan McCubbin will take a look at the scientific studies that have explored consuming sodium during exercise, and whether it improves performance.
How many studies on sodium intake and performance are there?
As part of my PhD on sodium for endurance athletes, I conducted a systematic review, a comprehensive sweep of the academic literature to find any and all studies that have investigated sodium replacement during exercise and subsequent endurance performance. Whilst I was aware that not much had been done in this space, even I was surprised to find only five studies ever published on the topic.
What did they find?
Of these five studies, none of them were conducted in hot conditions (temperatures varied from 2.6-22.5°C). None of them personalized the athlete’s sodium replacement either, instead giving the same amount to each participant, usually estimated as the average expected loss. It is likely then that some participants received too little sodium to replace their losses, whilst others replaced more than what they lost during exercise. Whilst some studies allowed the participants to drink as much or as little fluid as they liked (replacing sodium did make them drink more in the studies that measured this), one study provided a prescribed amount that was so great that a significant proportion of the participants developed mild hyponatraemia by the end. This same study also had participants exercising outdoors on two separate days, with temperatures so different (as low as 2°C on one day and 20°C on the other) that performance was more likely to be influenced by the weather than the sodium.
Of all the studies (four outdoor, one in the lab), only one showed any benefit from sodium supplementation, and it involved 26 male competitors in an actual Ironman 70.3 triathlon taking salt or placebo capsules. However, other factors on the day like competitors and transitions may have influenced this result, and the reported benefit in the sodium supplemented group (26 minutes, or an 8% improvement) is well beyond what would be expected from any nutritional intervention, such as optimised hydration or carbohydrate intakes.
Of all the studies (four outdoor, one in the lab), only one showed any benefit from sodium supplementation and there are questions marks about the reliability of that study.
What other evidence exists?
The other type of study that might point to a potential performance benefit from replacing sodium during exercise is those done at a fixed exercise intensity in the lab, where perceived effort is measured throughout. A reduced perception of effort when exercising at the same intensity or pace would suggest that the athlete could work harder, and therefore improve overall performance. Studies of sodium intake during exercise with fixed intensity models however, also fail to show differences that could imply a potential performance benefit.
Another study, not included in the systematic review above, measured running time-to-exhaustion on a treadmill, after 2 hours of interval running. In this study participants drank beverages with different concentrations of sodium, and the amount consumed was calculated to also create differences in fluid losses and replacement. When the performance trial was conducted, differences in hydration (water consumed to offset losses) had a significant impact on performance times, but the sodium concentration of the drinks made no difference at all.
Despite the often claimed benefits of sodium for exercise performance, there is surprisingly little research in this area.
Summary
Despite the often claimed benefits of sodium for exercise performance, there is surprisingly little research in this area, none of it has been done in hot weather, and none meets the often promoted personalized sodium replacement strategies for athletes. From the limited evidence however, it does not appear that sodium replacement during exercise is likely to positively impact on exercise performance, unless it directly contributes to the athletes drinking more and therefore maintaining more optimal hydration throughout exercise.
References
McCubbin AJ & Costa RJS. Impact of Sodium Ingestion During Exercise On Endurance Performance: a Systematic Review. Int J Sports Sci. 2018; 8(3):97-107.
Sanders B et al. Sodium replacement and fluid shifts during prolonged exercise in humans. Eur J Appl Physiol. 2001; 84(5):419-425.
Baker LB et al. Quantitative analysis of serum sodium concentration after prolonged running in the heat. J Appl Physiol. 2008; 105(1):91-99.
