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We’ve all heard various theories related to hydration. Drink eight glasses of water a day. Drink every 20 minutes while exercising. If your muscles cramp or your pee is dark yellow, you’re dehydrated. Don’t drink too much or you could get . The problem is that the rules of hydration seem to change regularly. Here’s what you need to know: the amount of water you need to stay hydrated depends on many factors, and there’s no prescribed amount of fluids or frequency of consumption that’s right for everyone across all conditions. We asked Dr. Steve Halvorson, an emergency doctor who serves as medical director of the , for his take on how to spot dehydration (whether or not you're exercising) and what to do about it.

Start Hydrated

Ideally, you want to prevent dehydration from happening in the first place. If you’re competing in a big event, like a marathon or a bike race, drink up ahead of time. If it’s an early-morning race start, go to bed well hydrated. “A striking amount of people toe the line at Hardrock 100 already dehydrated,” Halvorson says.

Make Drinking Easy

Halvorson notes that drinking to your thirst is the simplest way to stay well hydrated—although some conditions, like colder temperatures, can impact thirst level. “Your thirst will change as a function of how hot it is, how humid it is, how hard you’re working, and how you’re dressed,” he says. “So your thirst is your best indicator of how much you need to drink.” The key is to pick a hydration system—a backpack with a bladder and a tube or a water bottle—that’s easy to reach and makes taking in fluids hassle-free.

Pay Attention

It’s not that tough to spot dehydration when it happens. Here are the symptoms Halvorson says you should watch out for, in order of increasing concern: thirst, light-headedness, confusion, and a resting heart rate that’s higher than usual and doesn’t come down with rest. Infrequency of urination is another telltale sign—if you haven’t peed in a while, that’s a good indicator that you need to stop and drink. “Symptoms of dehydration do not spell certain doom,” Halvorson says. “First you’ll feel thirsty. If you’ve reached feeling light-headed, make the effort to drink more.”

Sip Whatever Tastes Good

If you’re dehydrated and can stomach fluids, Halvorson recommends drinking whatever’s palatable. But don’t chug—you just need a steady, standard flow. At aid stations in the Hardrock 100, competitors are offered a range of drinks, like ice-cold water, lukewarm water, flavored sports drinks, chicken broth, milk, and soda. “If the person can drink fluids, give them fluids,” Halvorson says. “It matters much less what the fluid is.” He doesn’t recommend sodium tablets or any other supplements, though some other experts do. “They don’t prevent cramps—training does—and they can increase your thirst to the point you drink too much,” he warns. When he’s in the field, having someone drink fluids themselves is more efficient and introduces less risk than starting an IV drip, he says.

Know When to Go to the Hospital

If you start to experience the more serious signs of confusion or nausea, then it’s time to seek medical attention in a hospital. “I call it the BFD: the binary field decision,” says Halvorson. “I think: Can I fix this in the field or not? Does the patient stay or go?” If you are able to keep fluids down, Halvorson says it is OK to stay put. But if the nausea and vomiting persist, or if you’re too confused to follow directions even as you try to drink, it’s time to go to the hospital.

Learn from Your Missteps

Learn how much water you need during specific activities, because everyone is different. Once you’ve dialed in your system, take note of changing temperatures, higher altitudes, excessive exertion, and other variables that might cause you to need more water. Trust that your body knows what it needs. “Your kidneys are far smarter than you are. Our bodies are stunningly resilient at preventing and responding to dehydration,” Halvorson says. “Typically, when we don’t drink enough, we get uncomfortable, and this is easily addressed by drinking fluids and learning for the next effort.”