How many of you have heard that by the time you are thirsty, it's too late, you're probably dehydrated? Well, let’s see what's really going on when your body signals it's thirsty.

Plasma osmolality is a measure of the concentration of substances such as sodium, chloride, potassium, urea, glucose, and other ions in blood. When you get an increase of plasma osmolality, this means substances in the blood such as sodium have become more concentrated, decreasing the amount of fluid or water in the blood.

Less than a 2 percent rise in plasma osmolality—that's not very much—will elicit thirst. However, it takes a 5 percent rise in your plasma osmolality to indicate true dehydration. Therefore, you could dine out on a high-sodium meal and make yourself extremely thirsty, but this doesn't mean you’re dehydrated. You can get significant increases in thirst without extreme concentration changes in your body fluid.

On the other hand, instances where “thirsty is too late” include water immersion and dehydration in the elderly. Water immersion is where you are plunged in cold water intermittently or for long periods at a time at 60°F or below. In some cultures such as in Russia, they perform such plunging to increase blood flow. Water immersion suppresses thirst response even though you may be dehydrated. And as for the elderly, unfortunately, as we age, our thirst response to dehydration becomes less sensitive, resulting in many elderly not meeting their fluid needs.

So don’t worry; this is just one more of those myths. Just because you're thirsty it doesn't mean you're severely dehydrated or that you've waited too long to hydrate yourself. You may have just had a high-sodium meal or increased your potassium intake by eating bananas, creating a greater concentration of ions in your blood, triggering your thirst.

Who knew? Definitely aim to drink consistently throughout the day. Water is the best hydrating agent and truly is the fountain of youth.