Summer Training and Competition: The Lowdown on Hyponatremia

Summer Training and Competition: The Lowdown on Hyponatremia
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Introduction
Since most of the country is in the full swing of summer it seems appropriate to discuss a situation potentially dangerous to athletes competing and training in hot and humid conditions. No, this time it's not dehydration taking the center stage, in fact, it's quite the opposite…a condition called hyponatremia.

Hyponatremia literally means ‘too little sodium in the blood'. Sodium is an important electrolyte responsible for numerous actions and metabolic functions in the body. Normal blood sodium concentrations lie in the range of 136-142 mmol/L (311-325 mg/dL). An individual is technically deemed hyponatremic when sodium levels fall below 135 mmol/L (309 mg/dL). This fluid-electrolyte disruption can have serious consequences, especially when blood sodium concentrations remain low for an extended period of time and/or when they continue to decrease. Severe hyponatremia (sodium concentrations below 120 mmol/L) can result in a rapid influx of water into the brain, causing the brain to swell and to then set off a chain of serious neurological responses that can ultimately end in death via a rupture of the brainstem.

Keep in mind that fatal hyponatremia is rare in athletes; however, because it can be a very serious and dangerous condition it's important to be aware of the causes, symptoms and precautions of water intoxication. This knowledge is especially significant for athletes competing in long events (> 4 hours) while in conjunction with hot temperatures and high humidity.

Causes and Risk Factors
Exercise associated hyponatremia originates from some combination of unusual sodium loss and/or water retention. Although numerous sources can be responsible, the main suggested etiologies are listed below.

High sodium losses in sweat
Not surprising to anyone, exercising in hot and humid conditions typically results in high sweat rates. Typical sweat rates vary from 1.0-2.5 L/hour, depending on the athlete, and on average sweat contains between .9 – 1.4 grams of sodium per liter. Take an endurance event (adventure race, Ironman-distance triathlon, ultramarathon, tennis tournament, etc) and use the conservative end of the above figures as an example. An athlete with a low sweat rate of 1.0 L/hr and a sodium concentration of .9 g/L would lose over 9 grams of sodium in 10 hours of competition. Athletes with higher sweat rates and sweat sodium concentrations will lose significantly more. Salt losses from sweat can add up quickly and therefore be a key contributor to the development of hyponatremia if not replaced.


Excessive fluid intake
To some athletes, it may seem very surprising that drinking excessive amounts of water during training or competition could be fatal Who would have thought that too much water was a bad thing? But it's true---overdrinking can (and will) dilute sodium levels in the blood. At rest, excessive water is eliminated without difficulty in urine, but during exercise urine production decreases by as much as 20-60% from resting values. Although it is very important to stay hydrated during exercise, replace lost fluids wisely. Athletes that stop and take a drink of water at every aid station and drink as much as possible are setting themselves up for water intoxication later in exercise.


SIADH
SIADH is short for the ‘syndrome of inappropriate ADH release'. ADH (anti-diuretic hormone) regulates sodium and water balance in the body. In response to certain conditions (e.g. exercise, stress, drugs) the pituitary gland can release an abnormal amount of ADH into the blood. The kidneys respond to the released ADH by retaining water and reducing urine production. Typical sodium losses from sweat coupled with a situation of SIADH can also therefore lead to hyponatremia.


Other causes/risk factors


Medications – NSAIDs, or non-steroidal anti-inflammatory drugs (e.g. ibuprofen, ketoprofen, acetaminophen, etc) can disrupt kidney function and thus contribute to hyponatremia

Body size – smaller individuals are more susceptible to hyponatremia for the simple reason that it takes less water to dilute the blood plasma.

Poor fitness and acclimatization to heat – Unfit and/or poorly acclimated athletes can exceed 1.4 grams of sodium/L of sweat and also tend to have higher than normal sweat rates as compared to fit and well acclimated athletes. As blood sodium levels fall faster and lower, the risk of dangerous and life-threatening consequences increase.
Symptoms
The symptoms associated with hyponatremia are very similar to dehydration and can thus further exacerbate the problem. Be familiar with the warning signs. In order of increasing severity, symptoms can be as follows:

Mild hyponatremia:
Bloating, nausea, muscle cramps


Moderate/Serious hyponatremia:
Throbbing headache, vomiting, wheezing, swollen hands and feet, restlessness, confusion, disorientation, slurred speech


Severe hyponatremia:
Seizure, respiratory arrest, coma, permanent brain damage, death.
Precautions and Practical Avoidance
To recap, the most likely scenario for the majority of exercise associated hyponatremia cases results from a combination of excessive water consumption and high sodium losses from sweat. Know your body well! Get familiar with your own sweat rate and fluid/salt needs during training and competition in the heat. Several guidelines are listed below as an additional aid.

Drink during exercise! Don't be scared by everything you've read here– staying hydrated is vitally important. Protecting water loss and blood volume is a basic survival mechanism. In fact, when faced with a state of dehydration and hyponatremia the body will ALWAYS try to hold on to water and combat dehydration first. So be smart about rehydration—remember to replace the sodium as well as the water.
Quick tip: Most sport drinks don't contain enough sodium to replace the amounts lost in sweat. ‘Spiking' your drink or water with a mere ¼ teaspoon of table salt will significantly boost the sodium content.


Don't overload on water prior to the event! A lthough it is good practice to stay well hydrated the week leading up to competition, don't overdo it and guzzle 5 liters of water the night before your event. Excessive drinking before exercise dramatically increases the risk of hyponatremia.


Weigh yourself. Know how much water you're losing. Any weight loss from pre to post exercise should be replaced with fluids. This is good practice to follow during training AND competition. (Note: 1 lb of water is equal to 16 fluid oz)


Replace electrolytes through salty foods and drinks. The best way to ensure that sodium losses from sweat are replaced during and post-exercise is to consume foods and drinks that are high in sodium (see Chart 1 for a list of examples). Salt tablets should be avoided when possible as it can be easy to ingest too much sodium via this route. Use your taste buds instead—your palate will tell you when enough is enough!


Keep salt in your diet. Don't restrict yourself from the salt shaker when in heavy training or leading up to a big competition. Athletes have a much higher need for sodium intake than non-athletes.



Salty foods/drinks
Food/drink Serving Size Sodium (mg) Carbohydrates (g)
Table salt ¼ tsp 590 -
Pretzels 1 oz 480 22
Tortilla chips 1 oz 140 24
Saltines 1 oz 130 11
Dry salted peanuts 1 oz 230 6
Tomato juice 1 oz 410 5
Chicken broth 1 oz 360 <1
Energy gels 1 oz 133 19-25
Sports drinks 8 oz 188 6-19