Summer running and racing are favorite activities. Several hundred research papers show that the more sweat and weight lost during a race, the more drastically our running performance declines. The rate we lose sweat depends on a variety of factors including individual sweat rate, and the temperature, humidity, and length and intensity of your run.
How much sweat can we lose during a race or summer running training effort? Lots! Fluid losses of over 1 liter (34 ounces) of fluid (sweat) per hour have been recorded. On a hot humid day an average sized person (110-165 lbs) can lose 1.6 to 2 liters of fluid, or 2.5% to 3.5% of body weight.
Acclimatization can help you adjust—if you adapt to the hot and humid climate by spending two weeks training in it, your sweat rate increases and starts earlier on each training run. With acclimatization, your plasma volume increases to support these higher sweat rates, and you can maintain your plasma volume and cardiac output getter. Your electrolyte loss is also reduced by 50% with acclimatization, so you retain these important minerals for longer.
Dehydration puts tremendous stress on our body. One study found that for each liter of sweat lost through dehydration, exercise heart rate increases by 8 beats per minute, and decreases cardiac output by 1 liter. Another found that a water loss of 4% to 5% of body weight impairs physical work capacity and function.
In one study, subjects exercised for 2 hours, to lose 4% of their body weight. Their time for walking to exhaustion was reduced by 48%, and VO2 max was reduced by 22% with a fluid loss of 4.3% of body mass. After this, they drank enough to replace 20%, 50%, or 80% of their sweat loss, resulting in dehydration of 3%, 2% and 1% of body weight. The researchers found that the better the subjects maintained their body weight through drinking, the better their performance.
Another study concluded that if the temperature is above 20 degrees C (68 degrees F) and exercise is longer than 90 minutes, performance is impaired. Obviously if it is hotter than this, the earlier we will feel it. In a temperature of 31 degrees Celsius (87.8 degrees F) our sweating rate is much higher, so even 60 minutes of running in this heat is enough to cause your body to lose 2% of body weight, or enough to impair performance.
What causes the decline in our summer running performance when we run in the heat? A formidable combination of detrimental effects on the thermal, cardiovascular, metabolic and central nervous systems conspire to make us suffer: a drop in the blood flow to our muscles, skin and brain; a decrease in blood pressure; a decrease in oxygen delivery to the working muscles; earlier onset of anaerobic metabolism; and hyperthermia-induced fatigue, all of which severely slow us down.
How Effective are Sports Drinks for Summer Running?
Almost all of the studies investigating the effectiveness of sports drinks show that they have enhance our performance. Studies have found that a 5.5% to 7% carbohydrate-electrolyte drink improves summer running performance when compared with an artificially sweetened placebo.
One study showed the importance of preventing dehydration and providing carbohydrate during summer running exercise. Subjects cycled for 50 minutes at 80% of VO2 max, then ended with a sprint finish. Performance was improved by 6% when 80% of their sweat loss was replaced, and by another 6% when they drank 79 grams of carbohydrate, compared with when they drank fluids with no carbohydrate, for a total improvement in performance of 12%. Another study found that carbohydrate feeding improves performance when exercise lasts longer than 90 minutes at 70% of VO2 max.
What Should We Be Drinking?
Hundreds of studies show that carbohydrate and sodium in sports drinks have beneficial effects. Carbohydrate solutions help maintain energy levels for racing by replacing depleted muscle glycogen stores, and sodium helps retain water, stimulates thirst and prevents low plasma sodium.
Preserving our blood plasma volume is a key factor in sustaining optimal summer running pace, especially in events lasting more than 2 hours. Elite athletes who sweat heavily can lose as much as 4800 mg of sodium per hour in extreme heat and humidity, although a sodium loss of 1100-1900 mgs/hour is considered normal. This may not be as much of a problem as you might think, because much of the sodium present in our body is “recycled” from our sweat glands, so high sodium levels are not necessary in sports drinks.
If you are concerned about replacing sodium, or are a heavy sweater, the sodium concentration in electrolyte replacement drinks ranges from 232-845 mg/deciliter (about .4 of 1 pint). The optimal combined concentration of carbohydrates and electrolytes is less than 8% equating to 30-60 grams per hour during exercise.
If sports drinks upset your stomach, water may be your best option. It has the fastest absorption rate of all fluids. However, before you ditch sports drinks completely, try watering down your favorite sports drinks to a concentration that works for you. This may be all it takes to make it tolerable to your system. Some research indicates that the fluid should be cooled for the best absorption.
How Much Should We Drink?
Exercise scientists agree that we need to rehydrate at a rate not exceeding a loss of 2% of body weight loss. This is difficult because we simply can’t ingest water quickly enough to replace our sweat losses. Our stomach can only hold 900-1300 ml, or 30-44 ounces and we can only empty about 800 mls per hour while exercising. Larger volumes of excess water pool in our gastrointestinal tract, sloshing around causing gastric discomfort, nausea, even vomiting. Yet research also tells us that it’s crucial for us to drink large volumes of water early in our marathons and ultras, because there’s a 40-60 minute lag time for it to clear the gastrointestinal tract and get to where it is needed in the body’s cells and plasma.
This practice is not necessary for shorter events like the 10K because it only takes an hour for the majority of runners to finish this event. The message here for 10K runners is to hydrate 40-60 minutes before your race. However, if you feel thirsty during a 10K, by all means drink. Heat exhaustion and heat stroke have happened in races as short as 10K if climatic conditions are wicked enough, so make sure you are adequately hydrated at the start line of your 10K.
How Frequently Should We Drink?
It’s important to keep drinking at regular intervals during your race at a rate that replaces fluid loss, about 400-800 ml of fluid per hour. This averages out at roughly 5-7 ounces, or 250 ml of fluid every 15-20 minutes.
Post-Race and Post-Training Rehydration
Studies also show that carbohydrates consumed immediately after and two hours after exercise enhance muscle glycogen restoration. This is most effective if ingested from fluid, as fluid is absorbed faster. Many studies also show that electrolyte balance is restored almost to pre exercise levels when an electrolyte beverage is drunk immediately after exercise.
Tips for Surviving High Heat and Humidity and Maintaining Performance Intensity
Summer Running Training Advice:
Drink lots of cold water before, during and after your training efforts. Select running routes that have water fountains along the way. Drink 200-500 mls 15 - 20 minutes before starting and drink at least one cup of water every 20 minutes during your activity. Carry a water bottle.
There is nothing macho or intelligent about the archaic practice of shunning water on your summer running training efforts thinking it will toughen you up—it could kill you.
Post training or post race rehydration: weigh yourself before and after your race or training effort. Make sure you drink that weight back on within an hour or two of finishing. Choose carbohydrate rich fluids such as juices that replace both water losses and muscle glycogen. Juices contain more carbohydrates than sports drinks, so drink your fill of your favorite fruit juices.
Summer Racing Advice:
On hot, humid days don’t push your pace beyond your current level of fitness. Above all, do not be tempted to go out fast from the start. Early dehydration invites disaster. Realize that your performance in heat and humidity will be substandard, and accept this.
Never drink sports drinks in a race without having experimented with them previously on your longer training runs.
A brief break combined with drinking cold fluids, and taking a sponge bath, provide great relief in the middle of the race. The cooling effect more than compensates for the few seconds you’ve taken to do this in terms of recovery time.
Summer Running General Advice:
Food digestion interferes with the blood flow to the working muscles, so avoid large meals before running in the heat.
Electrolyte sports drinks with sodium have been found to help endurance athletes retain water in their system, so you’re well advised to try them. But avoid the imposters that are loaded with sugar and no better for you than soft drinks. If electrolyte drinks make you feel nauseated, they’re too concentrated, so dilute them 100% or more. Avoid alcohol—its diuretic effect causes you to dehydrate quicker.
You’ll be able to tell whether you are hydrating adequately by the color of your urine. Dark yellow indicates low hydration, and pale to light yellow is good. The old adage of 8 glasses a day of fluid is not quite right. You should ingest ½ ounce of fluid per pound of body weight if you are running in hot conditions. 80-100 ounces of fluid covers almost every runner.
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