There are a few HOT and HUMID long distance races in the next few months and a very important race for myself (and many of my athletes who I have helped with nutrition) in 9 days (OH MY!).
In addition to fluids and electrolytes during training/racing, acclimating to the hot and humid temperatures is another way to assure yourself of reaching the finish line at your next long distance event.
If you've been training in the heat and humidity, it’s likely that you will lose a lot of sweat during your training sessions. However, seeing that it is August, if you've trained in the heat, you are probably used to the heat. If you are from an area of dry or cool temperatures in the summer or prefer to train before the sun comes up, it is likely that you are not adjusted to the rising temps which easily reach 85% humidity and 84-degrees by 8am here in Florida (this was the temp and humidity on Thurs when I was working out).
Extreme heat may cause athletes to lose up to 2-3 L/hr of sweat, whereas the majority of athletes lose around one liter (34 ounces) per hour of exercise. Because the body only requires around 20-25 ounces of fluid (lighter athletes around 18-24 ounces and heavier athletes around 24-28 ounces) per hour of exercise to fulfill hydration requirements under most conditions, it's vital that you focus on your fluid intake during each hour of long distance exercise.
The problem I find with many athletes is that they feel acclimated to the heat when really, their body has not physiologically adjusted to the heat. Because athletes are nervous about racing in abnormally hot and humid temps, there is an effort to do anything and everything to prepare for race day. Wearing heavy clothes, taking walks at 12pm in heavy clothes, exercising in sweat pants and a jacket and training indoors with no fans and no AC...I've heard it all.
I did a Pubmed search of research articles of training indoors in hot temperatures and wearing heavy clothing to prepare for the heat and I didn't find anything that I could relate to endurance athletes and preparing for the heat. Having said that, there may be research articles out there but I can't locate them.
Although exercising indoors in hot temps and wearing thick clothing will absolutely make you sweat, this is not the best way to prepare for the heat. Your blood is working overtime to support working muscles and cool the body and this will make you feel super hot, as if you are preparing yourself for the heat.
Exercising in thick or bulky clothing may alter your running gait or cycling form, thus causing you to possibly injure yourself. In addition to exercising in a hot room, wearing thick clothing while exercising may increase core body temperature too quickly and compromise your body by not being able to cool yourself adequately.
Relative humidity is the most importance factor that affects sweating and cooling. Therefore, even if you feel hot exercising (indoors, middle of the day, with clothing) you may be getting use to the heat but you are not teaching your body to cool itself.
In terms of humid races, humidity causes sweat to evaporate more slowly than on less-humid days. Therefore, it is important to train your body to efficiently evaporate sweat for cooling purposes.
Through heat acclimatization, the body learns to remove heat more efficiently by repeatedly teaching the body to adapt to hot conditions. In a non-acclimatized athlete, sweating will occur later, thus the acclimatized athlete will demonstrate a higher quantity and rate of sweating, resulting in a cooler body temperature and a more comfortable racing experience. Considering that blood must cool the body as it facilitates working muscles, an acclimatized athlete will have more blood for working muscles and less blood needed to cool the skin. Due to the efficiency of sweating during exercise, acclimatization causes the heart rate to rise more slowly due to an increase in stroke volume. Lastly, sweat becomes more diluted and there is less sodium loss from the kidneys.
An acclimatized athlete can work harder in extreme conditions, with less heat-related consequences, compared to a non-acclimatized athlete. Having said this, do not think that if you trained in hot and humid temperatures that you can race at a higher intensity than normal during your long distance race. If you haven't trained your body to sustain 85% max HR for a 3+ hour race, acclimated or not, your body will not be able to clear lactate as quick as it is produced. Therefore, you must always be aware of how you are going to train your body to prepare for a race. As you acclimate and train for a race, find an acceptable HR that you can sustain to metabolize a mix of carbs and fats for a long period of time. If your lactate threshold is low, it doesn't matter how comfortable you are in the heat. High intensity HR's to attempt to sustain high intensity efforts will quickly deplete stored glycogen thus causing you to rely on a heavy intake of calories to sustain that high intensity. This is a worse scenario than just feeling hot at your race because a glycogen depleted, bloated athlete generally drinks too much fluid, takes in way too many calories, pops too many pills during the race (electrolytes) which may = DNF during an IM distance race.
If possible, give yourself around 5-14 continuous days to acclimate to heat, immediately prior to race day, if you are expected to race in hotter conditions than normal. Give yourself a mix of interval, tempo, long distance and recovery workouts so that you learn your RPE (Ratings of perceived exertion), HR or pace depending on the temperatures. The acclimatization generally requires 2-4 hours of daily heat exposure but some research studies have shown less volume. Even if you can only give yourself 4 days to prepare for the heat when you arrive to the race venue, plan your training accordingly (ex. workout at 2-3pm instead of 6am) and give yourself a proper warm-up so that you don't exhaust yourself during your taper period.
Acclimatization to the heat does not reduce your risk for dehydration. Pre-exercise dehydration as well as dehydration during exercise impairs physiologic functions and decreases sweating rates and blood flow in both the acclimated and non-acclimated athlete. Though the body can become more efficient at cooling the body, acclimatization does not train your body to need fewer fluids than recommended. Furthermore, drinking a lot of fluids (more than recommended) does not ensure that your body will stay cool during your training session or race. Pay attention to your fluid intake when training in the heat. Fluid requirements: 20-28 ounces of fluid per hour of exercise, drinking 5-6 ounces every 10-15 minutes. Also, use water to cool the body during training/racing.
If you can't acclimate to heat/humidity:
1) I read a research study showing that 4 x 30-45 min. high intensity sessions during a 10 day period (with 1-2 days rest in between) at 30 degrees C, 27% relative humidity did induce acclimatization (Sunderland, 2008). If you can't exercise for long periods of time (especially when you are tapering before a race) on the weeks leading up to a race, when it is hot or humid during the day, exercising for a short period of time in the heat may help prepare your body for the heat.
2) To prepare yourself for a humid and hot-weather race, dress in appropriate clothing that promotes evaporation from the skin. This is not limited to racing: dress appropriately during training. Lightweight, loose-fitting clothing will allow more air to pass over the body for cooling. Also, avoid dark clothing which absorbs light rays and promotes radiant. By wearing clothes well-suited for hot and humid temperatures, you will encourage evaporation from the skin more easily, in order to prevent extreme fluid loss.
3) As sweat soaks your clothes and drenches your body, avoid drying off your skin with a towel or changing clothes (ex. IM race). Because your body’s primary method of cooling is through evaporation, dry clothing will hinder heat exchange compared to wet clothing.
4) Sweat loss is not calorie loss. Do not feel proud of yourself if you lost weight after a hot and humid run. Additionally, do not feel like water loss provides you with an opportunity to eat more than normal because you technically "lost weight" during a 90 minute workout and you want to refuel for upcoming workouts. I am sure everyone has heard to drink 16 ounces for every 1 lb. lost during exercise in hot temperatures. However, this doesn't start and stop right after the workout. You should be drinking water regularly during the day since it may feel uncomfortable to drink an excessive amount of water immediately after exercise. More so, recognize if you are thirsty when a food craving hits. Generally, most athletes eat when they are in need of fluids so always drink before you eat (8-12 ounces before and with meals) and carry around a water bottle (20-24 ounces) of cold water to sip on in between meals.