How to Know if You’re Dehydrated During a Heatwave

Picture a hot June day and an athletic, handsome, strong, and youthful-looking 36-year-old cycling a couple of miles to the bouldering gym. He stretches, starts to climb and then things go downhill. A numb sensation in the right little finger spreads to the lips. A headache appears, and the ability to think in full sentences vanishes.This is exactly what happened to me. At first, I thought I was dying, or at least having a stroke, but I eventually realized I simply hadn’t drank enough water. All of us lose water through sweating, but even simple things like breathing and daily movement dip into our H2O reserve. Often, we’re losing more than we realize.How much fluid we actually need to remain hydrated depends on loads of factors, including your body size, activity level, the climate, and your overall health, but experts recommend about 124 ounces of water for men a day. In my case, I sat down for 20 minutes, sipped some water, and largely felt better. Here’s everything you need to know so you don’t wind up like me, dry as a desert in a heatwave.What happens to my body when I’m dehydrated?Being dehydrated is more than having a dry mouth after half an hour of pickleball. It happens when we lose more fluid than we’re taking in, either through sweating, urination, diarrhea, vomiting, illness, or simply not guzzling enough water. The problem, as Rachael Ajmera, a dietitian and founder of Nutrimental, explains, is that water drives “almost every physiological process inside your body, which is why even mild dehydration can take such a huge toll on health and performance.”When our body’s water reserves drop, our blood volume falls, which makes it harder for the body to regulate temperature and deliver oxygen and nutrients efficiently. “This is why dehydration can affect everything from energy, focus and mood to headaches, dizziness and physical performance,” adds Emily English, a BSC-qualified nutritionist and the founder of Epetōme.Crucially for the sporty among us, dehydration can also alter electrolyte levels, which regulate everything from nerve signaling to muscle contractions and fluid balance. This increases the risk of symptoms like muscle cramps, weakness, dizziness, headaches, and fatigue. In other words, exactly what happened to me on the climbing wall.Your mental performance is not exempt. According to a review published in the Journal of the American College of Nutrition, being dehydrated, even just by 2%, can impair performance in tasks related to attention and memory.It’s an easy trap to fall into. English warns that we’re at the greatest risk of dehydration when exercising, on long travel days when we might not have frequent access to water, when we’re hungover, or when we’re just too busy to go grab a glass. You can even lose a decent amount of water exercising on a cool, cloudy day, when you might feel less thirsty, while spending time at altitude makes us breathe faster, which speeds up water loss. “You do not need blazing sunshine to become dehydrated,” she says. “If you start the day already slightly under-hydrated, even a workout or a long walk can tip you over quite quickly.”What symptoms do I need to look out for?We’ve all been thirsty before, but recognizing the signs of a more chronic condition can be tricky. Here, Ajmera lists the symptoms to look out for, from least to most concerning.ThirstAn obvious one, but Ajmera says “receptors in your brain monitor changes in the concentration of your blood and trigger thirst signals when you're dehydrated,” making this your first warning light.Low energy levelsDehydration affects circulation and cellular energy production, both of which can contribute to low energy and fatigue.Headaches“Dehydration causes the tissues in your body and brain to temporarily shrink, which can cause headaches,” says Ajmera. Reduced blood flow and electrolyte imbalances can also make things worse.Brain fogEver find yourself a bit spaced out at the end of a long run on a sunny day? Reduced blood flow to the brain alongside disruptions in nerve signaling could be to blame.Darker urine“Dehydration triggers the release of a hormone called ADH, which tells your kidneys to conserve water by reabsorbing more fluid back into the bloodstream. This leads to darker, more concentrated urine,” explains Ajmera.IrritabilityHeadaches, brain fog, and fatigue can all be irritating, but dehydration also boosts the production of stress hormones like cortisol, which can lead to changes in mood and irritability. There’s a tipping point too; a paper published in Nutrition Reviews found that when dehydration reduces body mass by more than 2%, it can have a significant impact on mood.What’s the best way to get rehydrated fast?Guzzling water is not advised when you’re dehydrated, the reason being that this can actually lead to hyponatremia, when the sodium in your bloodstream becomes dangerously, or sometimes fatally, diluted. Toral Shah, a nutritional scientist and advisor to the supplements brand Wiley's Finest, recommends small and frequent sips, especially if you’re suffering from dehydration-related nausea. Drinking in this way not only helps the body tolerate water, but it also makes for more efficient absorption over time.It should go without saying that water is usually the best option. Tea, coffee, and Diet Coke are all great, but if you need fluid in a hurry, it’s best to keep it simple. “Water is usually enough,” says English, “but if you have lost a lot of fluid through sweating, vomiting or diarrhea, an electrolyte drink or oral rehydration solution can help replace both fluid and salts.”If water isn’t cutting it, or you’ve been sweating heavily or otherwise losing fluid from either end, Ajmera recommends an oral rehydration solution (like Pedialyte or Liquid IV), too. “They contain water, sodium, and glucose to replenish fluid levels,” she says. “Sodium triggers your body to retain fluid instead of flushing it out, and glucose enhances its absorption in the gut.” Opting for salty foods like bone broth for lunch can also help restore fluid levels and top up those lost electrolytes.In an emergency, Shah recommends getting out of the heat. And, if the situation isn’t improving— and/or you’re experiencing symptoms like confusion, fainting, or an inability to keep fluids down— IV fluids and professional care may be necessary. If you’re at all worried you might be leaning into dangerous territory, call 911 to speak to a medical professional.What’s the best way to prevent dehydration?A proactive approach is best. Around 16 cups of fluid a day is often used as a general guide, but English warns this is “only a starting point.” Sick? Exercising a lot? Traveling? Chugging Honey Deuces at the US Open? You might need to take on more water than you realize.Ajmera warns that getting to know our body’s warning signs–dry skin, low energy—can be a smart move, while keeping a water bottle on hand (and reminders on your phone to actually use it) can make hydration a habit.Finally, Shah reminds us that water can come from foods. According to research published in Antioxidants, a whopping 20 to 30% of our fluid intake comes from foods. Nutrient-dense options like fruits and veggies are, once again, the real winners of the day.A version of this story originally appeared in British GQ.