I believe it’s best to go by how you’re feeling or expect to feel during a run, in regards to hydration.
The biggest myth I see when it comes to drinking fluids during running is that losing sweat is bad. You can drop water weight during a run or race and be fine! I ran the Austin Half Marathon in conditions of 70 degrees F and 90% humidity without a single water stop, and had zero issues with cramping or fatigue
Below is an excerpt from one of my favorite authors and books on the subject of hydration, Dr. Tim Noakes and the book, Waterlogged.
You shouldn’t relate overheating to dehydration. You overheat when you run too fast. That’s the key. You don’t overheat because you become dehydrated. The brain’s too clever. If you’re not going to drink, the brain will slow you down, and that will lower your body temperature, not raise it. So, we’ve got some great studies where we look at people running half marathons, marathons, short ultramarathons, and long ultramarathons. The longer the race, the lower the temperature, because they are running slower. Their levels of dehydration are pretty much the same whatever distance they run. There’s some sort of regulation, that whatever distance you run, if you drink appropriately, you always get the same level of dehydration, however far you run. But the key is that the faster you run, the hotter you are. But it’s still absolutely safe to expect your body temperature to rise. And the fact is that heat stroke occurs very, very infrequently. It’s the exception, not the rule. And when it does happen, there are exceptional circumstances. Most of those people have some other genetic circumstances that are a problem, or they are taking drugs, or they have an infection. It’s not normal to develop heatstroke during a race. If you do develop heatstroke during a race, something else is going on, and that’s affected your body’s ability to control it’s temperature, but it’s not the normal procedure. Normally it’s perfectly safe to run in the heat, and your body will make sure that your to the finish before your temperature rises too high.
Let’s say that again:
You shouldn’t relate overheating to dehydration. – Click to Tweet
Tim Noakes on the Serious Problem of Overhydration in Endurance Sports | Dr. Timothy Noakes| “The brain, unfortunately, can’t tell you that when you overdrink, you’re going to go slower. So you don’t pick up the messaging. You just go slower without realizing it. It’s very important.”
The Basics of Hyponatremia | Marty Hoffman, MD | “Therefore, NSAIDs not only increase the risk for acute kidney injury, but also increase the risk for the development of EAH. It should be apparent that the use of NSAIDs during endurance events is risky business.”
5 Scientific Ways to Stop Muscle Cramps | Armi Legge | “There are four reasons why losing electrolytes and water probably doesn’t cause — or isn’t the primary cause — of your muscle cramps.2-5”