Heat, Hydration & Sports Drinks
Staying hydrated on a hot day is important, as adequate hydration helps maintain our body temperature within an acceptable range and keep our mineral (electrolyte) balance in check. Even on days when we're not breaking a sweat, we constantly loose moisture with every breath. Now combine a solid workout with a hot day, where you're sweating and breathing more than usual, and you might find yourself less-than-optimally hydrated. The recommended water intake is half your body weight in ounces every day, plus more during activity. Yes, that's a lot of water!
Cramps and fatigue are two of the most common symptoms of dehydration, but getting too hot can lead to conditions as severe as heat stroke, so it's an important topic. If it's going to be a hot day and your workout time is flexible (such as a bike ride or a run), move your workout to an earlier time to avoid the hottest parts of the day. If your time is set (such as a soccer game or other organized activity), make sure to bring plenty of water, ice, and have a spot where you can get out of the sun. A sun-shade or other pop up tent can come in handy for shade, and keeping a couple of towels on hand to make an ice towel (soak the towel in ice water and drape around the back of your neck and over your shoulders) can help to cool you down.
Sports drinks can be particularly helpful if you're active for longer than an hour (regardless of the temperature), as they also replace electrolytes lost in your sweat. Not all sports drinks are created equal, so look for one that contains salt (preferably sodium chloride, as you also loose chloride when you sweat), potassium, and has 30-60 g. of carbs. When working out for an hour, 30-60 g. of carbohydrates has been shown to sustain blood glucose levels and increase performance. If you're working at a high intensity for 45 minutes or so, this same carbohydrate recommendation applies.
It's hard to make a global recommendation for hydration since sweat-rates vary widely from person to person, so pay attention to how you're feeling (especially when it's hot) and know what's worked well for you in the past. Keeping an eye on the temperature and and making an effort to stay hydrated can help keep you active without any ill effects on days when mother nature turns up the heat!