Someone once told me that once you are thirsty you are already dehydrated. Is this true? Another thing I am curious about: I typically wait until I am hungry before I eat dinner, rather than eating at a prescribed time. Should I be sticking to a schedule for my eating and drinking?
Nah, thirst is a pretty sensitive mechanism as long as you’re not involved in strenuous activity or in high temperatures. Same for hunger.
If we only felt thirsty when we were in danger of dropping dead, we would have gone extinct long ago. That would be incredibly maladaptive in any environment. (In addition, I would have died at a young age.)
As an aside, I bet the person who told you this picked it up (directly or indirectly) from one of the websites/emails that claims we’re all chronically dehydrated but for some reason don’t know it. Strictly woo-woo stuff.
I read an article by an MD who had studied the problem. His conclusion was that you don’t tend to feel much thirst until you’re about a pint to a quart low. That isn’t serious dehydration, but it is enough for effects (mostly on mental performance) to be measurable.
I have no similar information re hunger. We often hear of the value of regular meals, but skipping one never seems to lead to much in the way of problems. I generally fast two days a month, and I’ve never noticed any grim consequences from being hungry.
This is commensurate with the advice given to runners for many years: drink before you get thirsty. When you are thirsty, you are already dehydrated. Not serious dehydration, but enough to affect your running.
I also do not eat if I’m not hungry. Most people adhere to a regular regimen regarding eating meals, but few do so regarding drinking.
Yeah, Lord knows, the entire country is experiencing hunger and underconsuming calories… Listen to your body. Actually, if you do reading on “calorie restrition” and life extension, it looks like it may be beneficial to let yourself get hungry (and even really hungry) on a regular occasion.
This last year there was shown to be a specific benefit to fasting for a day, even if you made up most of the calories the next. The benefit was supposedly to do with insulin production, basically exercising your pancreas. There seemed to be a benefit from eating widely varying amounts of carbohydrates. Note: this is not advice for diabetics. This was toward preventing type II diabetes. I don’t have a current cite. It made the news rounds in past year and a half IIRC.
I googled and found this
I googled and found this
So if you are fasting, the pancreas gets a rest from producing insulin and produces glucagon, which use up stored glucose in the liver, the muscles, the kidney and fat cells. 
[sup]I hope not necessarily in that order.[/sup]
For the “already dehydrated when you feel thirsty” adage, I got it from a RN who trains people for the Wilderness Medicine Institute. That doesn’t mean it’s true; but it seems to be coming from reliable sources.
The most recent dope coming from the running gurus is that you shouldn’t drink until you feel thirsty. However, this is because of a spate of athletes who have suffered hyponatremia. So, instead of warning runners to take some sports drinks or other methods of getting some salt while running long in hot weather, they are advising not to drink so much water. (Then you won’t have to worry about hyponatremia, only heat strokes.)
While dangerous, hyponatremia is pretty much a red herring. You can’t OD on water, but you can screw up your electrolytes if you don’t consume enough salt. Just throw some Gu or Gatoraide in the mix and drink all you want. It’s much more likely that you’ll get heat stroke (an extremely dangerous and often deadly situation) than the much rarer hyponatremia.
Most people engaging in strenuous activity are dehydrated to some degree. When you feel thirsty, yes, you are probably down a quart so to speak. Drink copious amounts, with some salt intake to keep your electrolytes in balance, for best performance.
Always pee clear.
Truly.
Recent observations about the way certain clothes fit, in spite of nearly daily workouts and a reasonably health diet have forced me to conclude that I’ve had enough to eat as soon as I no longer feel grindingly hungry, as opposed to feeling really satisfied.