Not true, Canada!
In very hot and humid climates like ours (can be 90% humidity and 40 degrees C or 104 degrees F here in summer), you sweat like a pig made just standing there in the shade doing nothing.
You lose water and electrolytes at an incredible rate. If you take plain water as your fluid, or if your fluid and food have insufficient electrolytes, you can definitely fall into hyponatremia.
In fact, weekend warriors are more at risk than elite athletes; they are slower, so they are out on a Century course far longer than the elites, get pounded on by the sun and humidity.
I am the average recreation / commuter cyclist, and I had it once, not seriously, but I had it. Pounding headaches, gained 4 kilos of weight (purely water), hands feet face swelled up like a balloon. Look in the Internet, caught a clue, ate some salt, and spent the entire night peeing out 4 kilos of water.
Originally Posted by Machka
Well, you have to keep in mind that there is much more danger of dehydration than there is of hyponatremia, especially among the new riders. Hyponatremia isn't really much of a factor until exercisers (cyclists, runners, etc.) are into the advanced levels of their sport (marathons, ultracycling, etc.) and when we get to that level, we should know enough to make ourselves aware of the articles and information out there about those things (like the articles I posted).
The average recreational cyclist or commuter isn't likely to encounter a problem with hyponatremia.