Bobby Boucher (Adam Sandler) was right. Water is better. Sports drinks are full of bad stuff, but I won't get into that now. I wanted to use this post to talk about how to properly acclimate and hydrate when exercising in the heat.
The human body regulates internal temperature through radiation, conduction, convection and evaporation (yay, engineering terms!!). The one that is most critical to athletes is evaporation, aka sweating. When sweat is secreted through your pores onto the surface of your skin, it evaporates and changes from a liquid to a gas. This cools your skin. So in order to actually cool your body, the sweat must evaporate. That's why when it's humid outside, it's harder to keep cool. In dry conditions, this sweat evaporates more quickly - cooling the body more efficiently. But in order to sweat, your body needs to by hydrated.
Dehydration is a term that is thrown around a lot. The medical definition is that it's a "reduction in the total body water content." When fluid is lost from the body through sweat, the osmolality of the blood increases. This rise stimulates the brain to release a hormone, whose function is to increase water reabsorption by the kidney. The kidney then reduces the amount of fluid it secretes - reducing urine flow into the bladder. If you do not do something to reverse this process, you become dehydrated. The good news is that our body knows when this is happening. At the same time, another part of the brain is stimulated, increasing your thirst. In the ideal situation, you drink more water - the kidney starts to secret more fluid - your bladder fills - and you pee. So the first symptom of dehydration is thirst. If you do not quench your thirst, the body activates a series of emergency adaptations that prolong life...but trying to continue to exercise while the the body is going into "emergency mode" is what causes serious problems.
I want first address how much or how often you should be drinking water when exercising. There are lots of formulas out there to calculate your required fluid intake. Some are based on distance covered in training (7-10 ounces of water for every mile covered). Some are based on the amount of time you are exercising (12-24 ounces per hour). Some are based on your body weight (0.5-1 ounce per pound). Some people even suggest that you set a schedule (drink three sips of water every 10 minutes).
Personally, I don't think that any of these are good methods. Everyone has a different sweat rate. The best way to determine how much water you need is to weigh yourself naked before and then after your workout. If you take in adequate water, these weights should be close to the same. If they are not, you need to take in 24 ounces of water for every pound of weight lost. Then revise your fluid intake next time you workout and try to dial it in. Note that you must do a workout at race-pace intensity in order to know how much to drink during a race.
Personally, I know that if I have to pee about once an hour while I'm exercising, then I'm taking in adequate fluids. If I don't get that urge after about 45-60 minutes, I will immediately start to increase my water intake. I've used this method on all of my long rides and runs while training for Ironman and my post-workout weight is always within a pound or so of my pre-workout weight.
There's also the "Tim Noakes" school of thought. He proposes that your body possesses mechanisms to self-regulate and will tell you (through the thirst mechanism) that you need water. He says that you should only drink when you are thirsty and never on a set schedule or based on an amount per hour or body weight - no matter how extreme the physical activity. He sites several studies where runners have finished races with a body weight 10-12% less than when the race started and showed no signs of dehydration. Furthermore, he suggests that most people drink too much water during an organized race - leading to a whole other list of problems. All that being said, when pushed, Noakes does recommend fluid intake in the range of 13-26 ounces of water per hour across all endurance events (based on his studies)...which is not far from what others suggest is necessary.
You can drink an adequate amount of water and still not have the race you wanted. Heat and humidity play a huge role in what your body is capable of. Gradually exposing yourself to heat stress improves your ability to tolerate heat. These changes improve heat transfer from the body core to the skin and improve cardiovascular function.
There are several ways to acclimate to heat. In order to do so, you have to put yourself in a hot environment fairly often. The most obvious way is to do your workouts outside, in the heat of the day. Moving 2-3 workouts per week to the hottest parts of the day will help your body learn how to exercise in the heat and humidity.
If you don't have the option to exercise in the afternoon, you can expose your body to heat other ways. One option is to exercise while wearing more clothing than you normally would. You might get some weird looks running in Under Armour in July, but who cares.
Another strategy is to use saunas or steam rooms. This is what I did leading up to Ironman Louisville in 2011. I would sit in the steam room at the gym for 30 minutes twice a week. I did this for six weeks leading up to the race. Boy, do I have some interesting stories from those sessions - people do weird stuff in a steam room!
This summer, I haven't trained much in the afternoon heat, and I no longer have time to go to gym and sit in a steam room; so I'm trying another method this time around. I'm turning my truck into a "sweat locker" for my drive home from work every day. It's about a 25-30 minute drive and I wear some sweats and crank up the heat as high as it will go. It's not particularly fun, but neither is being forced to walk the entire marathon of an Ironman!
No matter what your method of heat acclimation, be sure to drink plenty of water while you are sweating! The human body is amazing, it will teach itself how to cool down properly if given the chance.
The other thing to consider when exercising in the heat is the fact that the more lean you are, the easier it is for your body to cool itself. If your muscles are heating up during exercise and the distance between your muscle and your skin is very close, the heat will come off of your core much faster. Just image trying to exercise in the heat with a coat on. Fat acts as insulation - and you don't want to trap the heat inside!
So the take-away from all this is that you need to be proactive about training or racing in the heat. Know how much water you need to drink based on the conditions and try to acclimate your body to the heat prior to race day.
You might be wondering about sodium intake. From studies I've seen, your body has more than enough sodium on board to make it through any workout or race (even Ironman) without the need for supplementing. That salt you see on your skin as you sweat is just your body getting rid of excess sodium, likely from food you have consumed in the previous 24 hours. Knowing this, there is no harm in taking sodium supplements or salt tablets during a race if it makes you feel better (placebo effect is real). Your body will just sweat out the excess anyway.
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