I'm not sure how many people commenting here actually read the article which got me thinking about this, but here are a few points I found interesting. I see no reason why the underlying ideas behind them couldn't also apply to backpacking, though perhaps on a different time scale.
"Similarly, there is no link between fluid loss and heatstroke. Human beings can safely lose big volumes of fluid without their body temperature shooting through the roof. Typically, in a marathon on a reasonably warm day, we lose about 2 to 3 L of fluid over many hours. Faster runners lose more - Haile Gebrselassie is reported to have finished his Berlin World Record 5kg lighter than at the start."
"Heatstroke is a viable candidate for the tragic deaths that sometimes happen, but it's a grossly overstated risk and those who diagnose any athlete's collapse or medical condition on a hot day as 'heatstroke' are also taking a lazy and possibly very wrong option. The reality is that heatstroke is a pretty complex phenomenon, and is likely to involve some kind of pathology. Once again, I'd draw attention to the difference between the perception of being hot and actually getting to the kind of dangerous temperatures that characterize heatstroke. We're not talking about feeling hot, uncomfortable and slowing down or stopping here."
"...But recreational athletes don't produce enough heat to develop heatstroke through normal muscle activity. Therefore, we look at alternative theories - either these individuals are failing to lose heat, or they produced excessive heat from unnatural means." (I personally wonder how much obesity and/or poor cardio are at work here creating "unnatural means").
"... So the key points from those case studies - there are 18 documented cases, I've only discussed three - is that the athletes who suffer REAL heatstroke most often are not exercising very hard, they're not in impossibly hot conditions, and they show 'abnormal' heat gain even after they've finished exercise, sitting out of the heat (in a bucket of ice, in one case). Clearly, there's something else going on, and heatstroke does not happen just because we run hard on a hot day."