Commentary on Myths and Methodologies: Making sense of exercise mass and water balance.

Abstract

The article by Drs Cheuvront & Montain (2017) kicks off a Mini Review Series on Myths and Methodologies, which explores the underlying principles of a selected methodology, considers appropriate and inappropriate uses and outlines best practice in physiology for particular methods or equipment. Their review concisely summarizes water and body mass balances during exercise, with explanation as to why these differ and therefore when such differences might become problematic. The sources of water and mass exchanges are well contextualized. As water generated from oxidative metabolism offsets that lost from ventilation, body water loss approximates that lost from the skin (i.e. eqn 4; or sweat loss) if ingestion and excretion are accounted for. In turn, body water loss and sweat loss approximate body mass loss minus 48 g (mJ energy expended)−1 (∼eqn 9). This equation is applied to quantify dehydration (eqn 11), before other applications are addressed, including worked examples for a marathon run by two individuals of different fitness. The matter of chemiosmotically bound water being released from macronutrients during their metabolism is then addressed because of its relevance to practical applications of these equations and its potential to affect the validity of (some) consensus fluid replacement guidelines. The net effect of liberating this existing water is concluded to be small, e.g. <200 ml water exchanged between compartments for energy expenditures up to 2000 kcal (approximately a marathon). Thus, the efficacy of drinking during exercise is considered to be synonymous with percentage dehydration. This conclusion is appealing but belies physiological and methodological complexities that remain unresolved and are beyond the scope of this short review that otherwise does an excellent job of summarizing issues of water and mass balances. Physiologically or functionally, efficacious hydration behaviour in exercise concerns not only body water balance but also its composition and distribution. Whether these are impacted meaningfully by glycogen metabolism will depend on the mass of water liberated and its potassium content, both of which remain to be clarified and are likely to vary (e.g. Maughan et al. 2007; King et al. 2008). Cheuvront & Montain (2017) also point out that glycogen is not the only substrate oxidized or releasing water during exercise (Candlish, 1981). That is true, but glycogen would usually be most important not only because it binds the most water (one to four times its own mass) but also because it is the major substrate mass oxidized in most endurance and team sports, while also fuelling the glycolysis that predominates in team sports. Glycolysis permits the release of more water relative to energy expenditure and is coincident with high heat strain and thus rate of dehydration. Of course, if the liberated water is low in volume or high in potassium it will have little effect (systemically at least), as concluded by Cheuvront & Montain (2017), but more research is needed to elucidate its role and importance.