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Posted: September 29, 2005

Sport Science: Maintaining Hydration And Peak Run Performance

A Relevant Issue As The Thermometer Drops?

By Kim Mueller, MS, RD, Sports Nutritionist

During the fall and winter months, athletes can be tricked into performance declines associated with dehydration. As the cooling effect of air and rain sometimes mask our sense of fluid loss, many athletes fail to take into account the significant amount of water loss that is still occurring. The risk is compounded when runners layer their body with excessive clothing, thereby increasing fluid loss. In fact, fluid deficits of 3-8% of total body mass have been reported in individuals working in cold environments. So, despite many athletes perception that dehydration is a problem that exists only during the summer months, it is quite possible to run yourself dry as the thermometer drops. Join me as we explore water’s role in human performance and address how you can maintain peak performance through proper hydration.

First, let’s start with water’s role in the human body:

Water is the medium for metabolic activity. In order to properly metabolize the calories ingested during activity, it is essential to maintain euhydration. Even slight dehydration, let’s say 1% (1.5 lbs for a 150 pound athlete), can contribute up to a 5% decline in your metabolic efficiency. What does this mean? Well, for one, the calories ingested, especially solid calories, will be left in your stomach, leading to premature muscle fatigue (rather than being distributed to the working muscles). Any fluids you ingest will collect in your stomach until a proper concentration is achieved for optimal digestion. This leads to a shortage of fluid being directed to your working muscles and consequent muscle cramping. When completing such jarring activities as running, all the food stuff and fluids left in your stomach will be jiggling around leading to uncomfortable side stitches, nausea, vomiting, and diarrhea. Worst of all, relieving some of your stomach issues may present a challenge when a bathroom is void from your beloved trail run!

Water is a lubricant for our muscles and joints. Noticeable muscle aches/pains/cramps can occur from the slightest bit of dehydration and become debilitating as dehydration becomes more severe. For every percent drop in hydration, expect a 3-4% decline in your muscle performance; in runner’s terms, this could potentially add an extra 2:48 onto a 40 minute 10k runner’s overall time. Furthermore, a decline in muscle function often diminishes your running form, which can elevate the already compounded risk for muscle injury when training/racing in cold environments.

Water helps cool the body. Water can be compared to the coolant used in our cars. When the coolant runs low, our cars overheat – this can happen in the summer or the winter. During exercise, blood flow to your skin increases as a means to evaporate the internal heat generated (i.e. “sweating”). An athlete may have a flushed appearance, which can be masked as a “wind-burn” in colder conditions. As dehydration becomes more severe, the process of sweating becomes increasingly compromised, causing core body temperature to elevate. The athlete may get the “chills” when the heat generated is not efficiently being released from the body. If ignored, dehydration can lead to frostbite, heat exhaustion or heat stroke, and potentially death.

Table 1 explores the common symptoms experienced by athletes as well as the estimated performance drops at various levels of dehydration.

Table 1: Dehydration & Heat Stress
% Body Weight Loss Estimated % Performance Drop Normal Temperatures High Temperatures and/or Strenuous Exercise
1-2% 5-10% Impaired Judgment, Irritability, Headache, Muscular Aches Sweating, Flushed Face
3% 15% Thirst Reflex Initiated, Sense of Fatigue, Tight Sore Muscles, Increased effects of 1-2% Symptoms Profuse Sweating, Noticeably (to others) Impaired Judgment and Confusion
4-6% 25-35% Profound Thirst, Dizziness, Muscle Cramps, Weakness, Fatigue Very Irritable, May Act Irrational, Pale, Severe Headache Especially at Base of Skull
7-8% 40-45% Nausea, Vomiting, Severe Vertigo or Dizziness, Somewhat Irrational, Severe Muscle Cramps, Staggering Cold, Clammy Skin even though Core Body Temperature may be 104 o F or higher, May Have Stopped Sweating
9-10% 50-55% Collapse, Very Irrational, Unconscious Skin Pale, Cold and Clammy, Stopped Sweating, Muscles Tense & Contracted, Pupils Normal or Dilated, Pulse Weak and Rapid, Low Blood Pressure, Respiration Shallow
8-10%+ 45-55% Heat Stroke *Skin temperature may be 102-104 o F and the core temperature may be over 108 o F which, if prolonged, will lead to kidney failure and death. Heat Stroke Skin Red, Dry, & Hot, Sweating has Stopped, Severe Headache, Extremely Weak, Numbness and Tingling in Extremities, Muscles Tense and Convulsive, Confusion, Dark Urine (if any), Pupils Contracted, Pulse Strong and Bounding, Rapid/Shallow/Labored Respiration, Delirious, Unconscious, Comatose

In order to protect against these performance declines, I offer the following hydration tips:

1. Don’t wait until you’re thirsty to start drinking. Perhaps the biggest mistake endurance athletes make is waiting until they are thirsty to start drinking, which generally signals a 3% level of dehydration and up to a 15% decline in maximal performance capacity. To make matters worse, the winter chill can actually further alter thirst sensation by as much as 40% due to a physiological response that occurs when the body does not feel hot. Unfortunately, failure to drink outside of a “thirst window” will not only lead to declines in your physical performance but can also affect your thermoregulatory mechanisms; this ultimately elevates the risk of frostbite, making fluids, especially water, equally important in cold weather as in the heat.

To prevent dehydration, it is best to sip on fluids throughout the day until urine flows clear to pale yellow. Daily fluid needs vary depending on your body composition and total weight. The following equation can be used to estimate daily fluid needs in ounces:

Body weight (in pounds) / 2 = Daily Fluid Needs in Ounces

Fluids will be absorbed more efficiently if taken in incremental fashions during the day. You should aim at consuming ½-1 cup of fluid per waking, non-exercising hour during the day. Note that these fluids should be non-caffeinated, as caffeine levels beyond 300 mg (the equivalent of ~2 cups of coffee) tends to increase gastric emptying of fluids.

2. Top off your fluid tank before you start. The American College of Sports Medicine (ACSM) recommends that athletes fortify themselves with 16 ounces of non-carbonated and non-caffeinated fluids for every hour prior to exercise. If you have trouble holding fluids, try sipping on a sports drink containing sodium for the last hour prior to the event. The sodium in the sports drink will help increase absorption of the water into your cells, protect against hyponatremia, and also prevent muscle cramps. When preparing for a race in the cold, you may consider alternative “warm-up” fluids like hot chocolate as means to help warm your core before taking the frigid plunge.

3. Know your sweat rate as means to keep up with fluid needs during training/racing. Determination of sweat rate is actually quite easy, but it entails the maintenance of a log monitoring the following items:

Case Study

Mary is a competitive ultra-marathon runner from Canada who constantly battles cramping and nausea during her long runs. She admits to not being a fan of carrying anything while running and therefore often fails to take any fluids in during her long runs. She insists that this is not a problem as Canadian air is often cool. To find out, we measured her sweat rate during a recent marathon training run.

Type of Workout: 4 hour run at 75% max heart rate

Pre Workout Naked Weight: 125 pounds

Post Workout Naked Weight: 116 pounds

Total Fluids Ingested During Workout: 32 ounces (2 water bottles)

To Determine Mary’s Hourly Sweat Rate:

1. Pre workout weight (125) – Post workout weight (116) = 9 lbs
2. 9 lbs x 16 ounces fluid per lb lost = 144 ounces
3. 144 ounces + fluids ingested (32 ounces) = 176 ounces
4. 176 ounces / total duration of workout (4 hours) = 44 ounces

Mary’s 9-lb weight loss represented a 7% level of dehydration, which is why such symptoms as nausea and cramping have occurred during the later stages of her runs. To counteract these profound losses, Mary’s goal is to consume 1.3 liters of fluid per hour. Note that many cannot physically absorb this amount so Mary should gradually increase her fluids to a liter of fluid per hour, breaking her fluid intake into 5-7 ounce increments every 10 minutes to allow for optimal absorption. Mary was encouraged to plan her run routes around known water stops, practice carrying fluids in either a water bottle or Fuel Belt, and carry money in her shoes to allow for pit stops at convenience stores. After all, the performance declines associated with dehydration will be much larger than that seen because of the extra weight of any fluids being carried.

o	Pre-Workout Naked Weight
o	Post-Workout Naked Weight
	*Make sure to take naked weight 
	as those sweater winter clothes can add a hefty load!
o	Total Duration of Workout
o	Workout Intensity
o	Weather Conditions
o	Volume of Fluid Ingested during Workout

Each pound lost during your workout is equivalent to 16 additional fluid ounces (~1 water bottle) you should have added to your fluid regimen. To determine hourly fluid needs, subtract your post workout weight from pre-workout weight and multiply by 16. This gives you an idea of how many additional fluids you should consume during the duration of your workout. Add this number to volume of fluid ingested during your workout to determine your total fluid needs. Divide total fluid needs by total duration of workout to determine the athletes hourly fluid replacement needs.

Note that as the intensity of your workouts increase, sweat rate will go up to keep the core body temperature in check. For reference, the average athlete will lose approximately 1 liter of fluid per hour (~2 water bottles), which also is the approximate max we can absorb per hour. When racing or training for long periods of time in cold environments, you may consider carrying warm fluids like hot chocolate in a thermo as means to help hydrate and warm your core.

4. Don’t neglect electrolytes when training/racing longer than an hour. In addition to fluid replacement, replacement of electrolytes becomes instrumental in your endurance bouts lasting longer than 1 hour. The five most important electrolytes to consider include sodium, chloride, potassium, magnesium, and calcium:

Sodium: Sodium enables ATP (energy) generation and also allows cells to retain more water, thereby helping to prevent dehydration. For events lasting longer than five hours, dangerously low sodium (aka hyponatremia) is a concern. Hyponatremia, or low blood sodium, generally happens after drinking too much plain water (over a quart and a half per hour) over several hours. Slightly low levels of sodium are fairly common in distance runners, and usually go unnoticed, but become more pronounced during marathon and ultra-marathon training where the athlete is training/racing for 3+ hours. A prospective study was performed on 36 athletes during a three- to four-hour endurance event, and another on 64 athletes during an endurance event lasting between 8 and 17 hours (Ironman). No athlete was hyponatremic (low sodium levels) after the shorter race, but 27 percent were hyponatremic following the Ironman. In fact, an average of 17 percent of the Ironman™ participants required medical attention, most for hyponatremia. Because fluid balance is intimately tied to sodium, hyponatremia can lead to damage of certain kinds of tissues in your body. Changes are most noticeable in the nervous system, where seizures, coma, and even death can result.

Recognizing over-hydration is challenging because it may appear much like an exercise-induced heat illnesses. Early symptoms are vague, and include confusion, nausea, fatigue, muscle cramps, and weakness. Worse cases can include vomiting, muscle twitching, delirium, seizures, and coma. Differentiation from heat stroke or heat exhaustion can be difficult, even for trained medics. The main difference is that heat stroke, and frequently heat exhaustion, includes a high temperature, whereas over-hydration does not. The final diagnosis must be made at a medical facility where appropriate treatment can occur.

To prevent hyponatremia you should avoid excessive consumption of water during prolonged training, opting for a salt-containing sports drink instead. It is also recommended that you consume 100-250 mg of sodium for every 8 ounces of water ingested during endurance training and racing. Beyond encouraging fluid balance, the sodium will actually improve the palatability of the sports drink as it offsets the sweetness of added sugars. Consumption of saltier foods (pretzels, broth, sauces, salt bagels, V-8 juice) leading up to race day and on race morning is also appropriate for prevention.

Chloride: Chloride is an anion generally consumed as sodium chloride (NaCl) or table salt. There is a high correlation between the sodium and chloride contents of the diet, and only under unusual circumstances do levels of sodium and chloride vary in your diet independently. Sports drinks use table salt to fulfill both sodium and chloride requirements during training. Adequate intake of sodium chloride is required for maintenance of extracellular fluid volume. Chloride is both actively and passively absorbed. Urine excretion reflects chloride intake, with low or no chloride found in deficiency states.

Potassium: Potassium is a mineral and is responsible for regulating total body water and stabilizing contractions (both controlled and automatic). A deficiency of potassium (hypokalemia) can manifest as weakness, fatigue, confusion, heart irregularities, and sometimes cause problems in muscular coordination. Insufficient potassium can also exaggerate the effects of a lack of available sodium. The first sign of a potassium deficiency is usually a generalized weakness - not something you wish to experience during an Ironman or marathon. Endurance athletes have higher potassium needs due to heavier sweat loss; in a study of athletes running 40 minutes at 70 degrees Fahrenheit, potassium loss was estimated at 435 mg/hour. Based on this rate of potassium loss, supplementation both during and after short and long events is warranted and necessary. You should aim at consuming 50-80 mg of potassium for every 8 ounces of fluid ingested during training as well as ~100 mg for every pound of weight loss post training.

Magnesium: As the fourth most abundant element (behind sodium, potassium and calcium) found in your body, with a total of 50 to 60 percent being stored in your skeletal system and the remainder being stored in muscles and soft tissues, magnesium plays an instrumental role in maintaining both structural (bone) and biochemical (muscle contraction, nerve transmission, enzyme production) homeostasis within your body. In fact, magnesium is responsible for 80 percent of all enzymatic reactions in the body, regulating virtually every body activity. Because of these established roles, attainment and maintenance of magnesium balance is crucial for optimal health and peak physical performance.

A magnesium imbalance is triggered by changes that occur in one or more of the stages of metabolism, such as reduced dietary intake, reduced absorption, redistribution and increased excretion. Endurance athletes, whose sweat rate ranges from just under 1 liter per hour to over 1.5 liters per hour, seem to be at a greater risk for magnesium imbalance. To prevent a magnesium imbalance secondary to sweat loss, athletes training or racing greater than 3 hours should aim at replacing 20-30 mg of magnesium per 8 ounces of fluid ingested.

Calcium: Calcium is one of the most important nutrients for athletes, aiding bone health, nerve transmission, and muscle function. One study discovered that athletes lost on average 422 mg of calcium over 3 days of training which lead to a 3.8% loss of bone mineral density over a period of 3 months. Furthermore, consistent loss of calcium in this magnitude can enhance risk for muscle cramping during intense training and racing. To offset the calcium losses experienced during endurance training, athletes should aim at consuming 10-15 mg of calcium per 8 ounces of fluid ingested.

5. If traveling for a “warm-weather” race, be sure to heat acclimate. During the winter months, many add a warm-weather marathon or half marathon (e.g., Honolulu, Disney) to the racing schedule as means to escape the cold. If you are planning a winter marathon in a warmer environment, it will be essential to acclimate to this environment. Heat acclimatization is a process by which your body makes adjustments to promote better cooling in hot environments. As you acclimatize to heat by training in warm conditions (e.g., hatha yoga, spin class without fans, indoor tracks), your sweat will become more dilute, the threshold at which sweating begins is lowered, and your sweat rate is increased. These and other changes take time to fully complete, with the most fit athletes achieving acclimatization at a more rapid rate. Significant acclimatization to heat can be attained in 4-5 days, although full heat acclimatization takes 7-14 days with 2-3 hours per day of carefully supervised exercise in the heat.

Another important tip when planning a race in the heat during the winter months is to cut warm-up time in half. Many endurance athletes who competed in the 2004 Summer Olympics in Athens also wore a cool vest as means to keep their core body temperature down prior to starting their race. A cool vest can be mimicked by placing ice packs around your core as you warm-up/stretch prior to a warm race/training bout.

6. Be weary of medications that interfere with cooling. Certain drugs may cause dehydration or interfere with sweating. Antihistamines and some blood pressure medications decrease sweating. Caffeine and alcohol are diuretics and thus cause your body to lose water. You should avoid their use for several days prior to the race. If you are concerned about overheating, discuss your racing/training schedule, as well as any medications taken, with your personal physician.

7. Rehydrate. For every pound of body weight lost during exercise, consume 16-24 ounces of fluid as means to re-hydrate and enhance recovery from exercise. Failure to replenish will contribute to feelings of sluggishness/fatigue the remainder of the day and may prevent you from completing a planned second run or forthcoming workout. Electrolyte containing beverages such as a sports drink are desirable for post-workout re-hydration due to the sodium (which increases fluid absorption) and carbohydrate (replenishes lost glycogen) content. If returning from a rather frigid run, consider a nice cup of hot chocolate prepared with warm milk (see recipe below), which has an excellent electrolyte profile, to aid re-hydration and help you thaw out!

Happy winter hydrating trails….

Kimberly J. Mueller*, MS, RD (*formerly Brown) is a Registered Sports Dietitian and competitive endurance athlete who provides nutritional counseling and meal planning to athletes all around the world. Kim can be reached at . More info on Kim’s services can be found at

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© Copyright 2004 Peak Running Performance. All Rights Reserved. Reprinted with permission

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