Originally Posted by MoAlpha

Don't try this again without replacing sodium. Seriously. You can't hold onto intravascular fluid volume without it and it could explain much of what you experienced. Drink enough water without Na+ on a hot ride and you'll find yourself in brain swelling and seizures, potentially fatal-type trouble.

Sugar is also needed to facilitate Na+ and K+ uptake from the gut and K+ uptake into cells. Use any electrolyte replacement drink.