The first rule of preventing overheating during triathlon training is to stay hydrated. That means you should consume plenty of liquids. It also means that you shouldn't wait until you become thirsty before you consume them. You can be in the early stages of dehydration long before you start to feel thirsty, so start drinking a couple of hours before you begin strenuous training.
Don't overdo it, though. It's also possible to overhydrate. How, then, can you tell how much fluid you should consume? A method that some athletes use is to weigh themselves right before working out and weigh themselves again immediately after. The drop in weight is roughly equivalent to the amount of fluid that the body has lost during training. A pound of body weight is equal to 16 ounces of fluid, so multiply the number of pounds you drop by 16 ounces and that's how much fluid you'll need to drink in order to not become dehydrated. Incidentally, an obvious sign that you aren't staying hydrated is the color of your urine. If it becomes unusually dark, then you should be consuming more fluids.
However, you shouldn't just consume any old fluid. Avoid drinks with caffeine, including non-decaf coffee, caffeinated sodas and energy beverages, because caffeine is a diuretic. It can actually make you less hydrated. Your best bet is to drink a combination of good-old-fashioned water and electrolyte-balanced sports drinks. These will not only keep you cool but will maintain your muscles and nervous system in proper working order. You can even carry them with you so that you can rehydrate while cycling or running. Most bicycles have a special rack for water bottles; you'll quickly learn to drink with one hand while steering with the other.
The second rule of preventing overheating is to rest if you feel any of the symptoms of dehydration coming on. These include weakness, dizziness, nausea, confusion and lethargy. Remember that you're only training, not running an actual race; there's no shame in sitting down in the shade on a warm day while your racing buddies are still out there risking heat stroke. Take advantage of your resting time to rehydrate and make plans for the rest of the day. As likely as not, your training buddies will probably join you for a moment of rest and some conversation.
And that part about having training buddies is important: Should you experience heat stroke, you may no longer be able to help yourself and you'll need a companion to lead you to a shady spot and to call for emergency assistance. And should you lapse into unconsciousness, make sure your companion knows in advance not to try to force liquids into your mouth. You could choke on them or vomit them back up. It's more important that they cool you with ice packs, if any are available, and elevate your feet above the level of your head. To avoid heat stroke in the first place, you might even consider carrying some ice with you on hot days, so you can place it on the back of your neck to help you stay cool.
Finally, should you find yourself needing to train in warmer weather than you normally do, take time to acclimate yourself. Your body will get used to the increased temperatures and adapt, but this can take a while -- up to ten days before you can return to your full training routine. Until then, take it a little easier than you normally would. One of the main reasons for becoming a triathlete is to stay in good health. You want to be sure that your training routine makes you healthier and stronger. If you're not careful, training in the heat can do just the opposite.
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