Figure 3 Predicted percentage body mass loss due to water deficit (%BML) and plasma sodium concentration (mEq/l ) under three levels of sweat sodium loss for 90 kg subjects of varying body composition drinking 400–800 ml water/hour while running at 10 km/h in cool (18°C; upper panels) and warm (28°C; lower panels) weather. The solid shaded areas demark when water loss would be sufficient to modestly (yellow) and substantially (red) diminish performance. The hatched shaded area demarks the presence of hyponatraemia. The two lines sharing the same line style in the sodium figures are the predicted outcomes for each fluid replacement rate (400 ml/h  =  solid line, 600 ml/h  =  dashed line, 800 ml/h  =  dash‐dot line) when total body water accounts for 50% and 63% of body mass. M indicates the finishing time for the marathon run. IT indicates the approximate finishing time for the Ironman distance triathlon based on literature values for participants running a marathon portion at 10 km/h.