Does increasing sodium concentration impact plasma sodium and volume with ad libitum drinking in healthy, trained males?

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The role of sodium ingestion on the development of exercise associated hyponatremia (EAH) is uncertain (Hew-Butler, Loi, Pani, & Rosner, 2017). There is, however, evidence from controlled, long lasting exercise trials in the heat to indicate that sodium intake, both acutely (Vrijens & Rehrer, 1999) and chronically (Koenders, Franken, Cotter, Thornton, & Rehrer, 2017), can significantly affect plasma sodium concentration during exercise. Sodium balance is important not only with respect to risk of hyponatremia, but also with respect to retaining water and reinstating hydration after significant sweat losses (Merson, Maughan, & Shirreffs, 2008; Shirreffs, Taylor, Leiper, & Maughan, 1996). In most sport drinks, beverage sodium concentration is considerably less (Chatterjee & Abraham, 2019), than the typical sweat sodium concentration (Baker, 2017). In previous research (Rehrer, Wijering, & Cotter, 2017) we have shown that with prescribed drinking volume to replace losses a beverage of 60 mmol/L sodium maintains both plasma sodium and plasma volume better than the same beverage but with 21 mmol/L. But to test the ecological validity we need to see if drinking ad libitum will give similar results or not. Thus, the purpose of this study is to find out if ad libitum consumption of a beverage with ~ 3 times that normally in sports drinks will maintain plasma sodium and volume better than a typical sports drink, during 3 h of exercise in the heat, as is observed when the volume of beverage is prescribed. The hypothesis is that ad libitum ingestion of a beverage with 60 mmol/L of sodium during 3 h of cycling in the heat will result in better maintenance of plasma sodium concentration and plasma volume than a typical sports drink with 21 mmol/L sodium.

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