Index

Warning: include_once(../../../includes/reprint_box.php): failed to open stream: No such file or directory in /home/worldult/public_html/sections/MOVEDarticles/nutrition/hyponatremia2.php on line 31

Warning: include_once(): Failed opening '../../../includes/reprint_box.php' for inclusion (include_path='.:/usr/local/php71/pear') in /home/worldult/public_html/sections/MOVEDarticles/nutrition/hyponatremia2.php on line 31

Water and Salt Intake During Exercise

by Lulu Weschler

Lulu Weschler is a physical therapist, ultra cyclist and regular contributor to UltraCycling

Hyponatremia: How To Recognize, Treat And Prevent It

As little as 2% dehydration adversely affects performance[1]. But as little as 2% overhydration can cause life-threatening hyponatremia[2]. Clearly, you need to get your hydration needs just right. Hydration must be coupled with adequate sodium intake; sodium losses can be significant during prolonged exercise[3,4]. Barr et al[3] observed an average loss of 12 g salt in six hours of cycling at 86 degrees F and 30% relative humidity (2 g more than the average US daily intake of 10 g).

I. Water and Sodium Requirements

Since sodium requirements depend on water requirements, you must first figure out hydration needs. This you can do only by experiment, for which you must first have a reasonably good idea of your optimally hydrated weight.

Your optimally hydrated weight will vary. For day-to-day training and for multi-day tours, your pre-breakfast weight gives a reasonable estimate of what your weight should be at the end of the day's ride. But at the beginning of an event for which you have tapered, you may well weigh about 2.5% more than your optimally hydrated weight. This is because you start with a full load of stored glycogen plus the water molecules that "stick" on the glycogen. Burning glycogen both liberates this attached water and produces water as a metabolite[5,6]. Several hours after the start of an event, your weight should level out at roughly 2.5% less than what it was at the start. Keep track of various weights - morning, pre-ride, post-ride and evening, and put them in the context of what kind of riding you're doing, weather, and how you feel.

There is a simple test you can use to check optimal hydration weight. Bend your hand backwards, and pick up a fold of skin (lengthwise, using the thumb and forefinger of your other hand) on the back of your hand. A thin fold that stays raised (doesn't flatten out quickly) signals dehydration. A fold that you can't even pick up indicates bloating. A somewhat thin fold that disappears fairly quickly when you let it go represents optimal hydration. Skin elasticity decreases with age, making this a crude test (except in relative extremes of dehydration or overhydration) on people other than yourself.

Estimating Optimal Hydration

The goal is to find out how much water to ingest per hour, so that at ride's end you are at your optimally hydrated weight. In the examples In Table 1, this is assumed to be your morning weight.

Weigh yourself before and after a ride. Keep track of how much fluid you consume in fluid ounces. (You need to know the volume of your water bottles and CamelBak. Mark your bottle and CamelBak bladder in eight ounce gradations. Use a measuring cup to pour in eight ounces, mark the level with a magic marker, pour in another eight ounces, mark again and so on.) Note the time for the ride. Then calculate fluid ounces per hour (see Table 1). The key equivalence: 1 lb = 15.2 fluid oz. water.

The longer the ride time, the more accurate your results. Bathroom scales are precise to no more than +/- 1 lb. Notice also that the less you weigh, the larger the impact of an error. Women's typically lower body weight is part of the reason they turn up hyponatremic more often than men [2,7].

Repeat the experiment at different temperatures. If you can't do this, then use Lon Haldeman's 75% for 10 degrees F rule as a guide: Haldeman needs 60 oz/hour at 100F, roughly 75% of that, or 45 oz/hour at 90F, and 32 oz/hour at 80F. At 70F and below, he needs about 15 oz/hr[8].

Table 1 Calculating Your Fluid Needs
Weigh yourself before and after a ride. Keep track of how many fluid ounces you ingest. 1 lb = 15.2 fluid ounces.
    A B C D E F
    Time Riding, Hours (direct measure) Fluid Ingested During Ride, Oz (direct measure) Weight Change, Lb Wt after - wt before Fluid Deficit or Excess, Oz
15.2 x C
Total Fluid You Need, Oz B-D Fluid You Need Per Hour, Oz E/A
Example I 10 400 +2 30 excess 370 37
Example II 6 160 0 0 160 27
Example III 8 310 -3 46 deficit 356 45

Estimating your sodium needs

Sodium, sodium chloride and table salt

Sodium chloride is table salt. Table salt dissolves in water to give sodium and chloride ions. Dissolved salt is invisible, but a taste of its solution will confirm its presence!

Sodium is 0.39 the weight of sodium chloride, so table salt is approximately 2/5 sodium. A gram of salt (or salt tablets) contains 400 mg of sodium. USDA regulations require that "mg sodium" be printed on food labels (but not supplements). Since table salt is the stuff you can see and measure, it is useful to know that one teaspoon of table salt weighs about 6 g, or 6,000 mg. There are about 2.4 g sodium in one teaspoon salt. The average American daily intake of salt is estimated to be 10 g, or about one and two-thirds teaspoons salt.

Once you know your fluid intake under various conditions, then you can estimate your sodium requirements. Sodium intake is estimated proportionally to fluid intake, not to riding time. How much sodium you need depends on how much fluid (sweat and urine) you're losing and also on the sodium concentration in the fluid you've lost. During exercise, urine has approximately the same sodium concentration as sweat. A liter of sweat contains, on average, 50 meq sodium[3,9,10,11,12] which equals 1000 mg sodium/quart of sweat (see Table 2).

Example: You consume a 20 oz. water bottle of a sports drink. Since one quart equals 32 oz., you need 20/32 x 1000 = 625 mg sodium. The sports drink gives you 440 mg/quart, or 275 mg. To complement the fluid intake you still need about 350 mg sodium, which you can get from food or a salt capsule.

The volume of blood plasma increases by about 10% on the first day of hot weather riding[13], even if you are already fit. You need more sodium when you are acclimating to heat. For a 154 lb athlete, that extra plasma requires about 2.5 g salt (~ 1000 mg sodium) to be brought up to normal sodium concentration. Do this by increasing the ratio of salt capsules to fluid intake, by eating more salty snacks and/or adding more table salt to food.

Withholding sodium won't "teach" your body to conserve sodium. The trigger for sodium conservation appears to be core temperature, not sodium concentration[14]. You produce as much of the sodium-conserving hormones on your first day of heat acclimatization as on the last day; what changes is theorized to be the sensitivity of the kidneys and sweat glands to those hormones.

Competing in hot weather after you've trained in cool weather should be treated like the first few days of heat acclimatizing: increase your sodium intake.

Acclimatization decreases the sodium-in-sweat concentration. Studies of unfit subjects have shown as much as a 60% reduction of sodium-in-sweat as a consequence of acclimatization. So, you need somewhat less sodium when you are acclimatized to heat.

Table 2 Sodium Concentration Of Various Fluids
Since one teaspoon of table salt has about 2400 mg sodium, normal blood plasma has about one and one-third teaspoons of salt per quart. 1 g = 1000 mg.
To convert from meq/liter to mg/quart, multiply by 21.8
To convert from mg/quart to mg/fluid ounce, divide by 32
Fluid Sodium Concentration (Meq/Liter) Sodium Concentration (Mg/Quart) Sodium Concentration
(Mg/Fluid Ounce)
Blood plasma, normal 135-145 2940-3160 91.9-98.8
Blood plasma, Hypo-natremic <135 <2940 <91.9
Blood plasma, Hyper-natremic >145 >3160 >98.8
Fluid outside cells 135-145 2940-3160 91.9-98.8
Fluid inside cells 14 305 9.53
Sweat Average 50; (range, 20-80) 1090
(440 - 1740)
34.1
(13.8-54.4)
Urine 1-500 22 - 10,900 1.5-341
Normal (0.9% sodium chloride) IV 154 3360 105
3% sodium chloride IV 513 11,200 350
Campbell's chicken broth 263 5730 179
V-8 juice 114 2490 77.8
Sports drinks Average 20;
(range, ~18-23)
440
(390 - 500)
13.8
(12.2-15.6)

II. Hyponatremia

"Hypo-" means "too little." "Natremia" comes from the Latin word for sodium, and means "sodium status." Be careful to keep "hypo-" distinguished from "hyper-." Hyper means "too much."

Concentration is the key to understanding hyponatremia: concentration is the amount of sodium divided by the volume of blood plasma (typically expressed in milliequivalents, or meq, per liter). The body doesn't care about the total amount of sodium. It cares how much sodium you have for a given volume of blood plasma. Increase the amount of blood plasma by drinking too much water too fast, and you may not have enough sodium for that increased amount. You're dividing sodium by a larger number, and hence the concentration becomes smaller. If the volume of blood plasma is large enough, you've got hyponatremia, even if the total amount of sodium hasn't changed.

Popular press accounts of exercise-associated hyponatremia have emphasized hyponatremia associated with too much ingested water leading to water retention or bloating. All deaths from hyponatremia, and all serious cases reported in the literature have involved bloating[2,15,16,17,18]. The reason is brain swelling[19]. Extra fluid goes first from the gut into the bloodstream. Then it seeps out through the capillary membranes (the smallest blood vessels) into the extracellular space[5], mostly between muscle and skin, where because of the skin's stretchiness, there is plenty of room for expansion. But the brain is encased in a bony vault (the skull) that allows no room for expansion, and the result is an intolerable pressure, symptoms of which can be any or all of the following: change in mental status, confusion, incoordination, seizures (convulsions), coma, death.

A dehydrated athlete (dehydration is the exact opposite of bloating) can be hyponatremic. In a study of Ironman triathletes (9-15 hour events), there were more dehydrated, hyponatremic athletes at the finish than bloated, hyponatremic athletes[20]. But the dehydrated, hyponatremic athlete does not have enough fluid with which to develop brain swelling, and is not in imminent danger. However, a dehydrated, hyponatremic athlete who re-hydrates too quickly and without adequate sodium, could develop brain swelling.

Compounding the danger of bloating hyponatremia is that it can be misdiagnosed and treated as dehydration[17]. The reason for this is that bloating hyponatremia has two counter-intuitive features: the bloating itself and urination shutdown.

The Bloating Paradox

We tend to associate water retention with excess sodium, but studies point to bloating as a red flag warning of hyponatremia in exercising athletes. In these studies, athletes are weighed pre- and post event, and their post-event plasma sodium levels are measured. The universal finding is that the more an athlete weighs at the finish compared to the start, the lower the plasma sodium is likely to be. Athletes whose weight has increased (the "bloaters") have a high probability of being hyponatremic. Not one bloated athlete, nor even any athlete who has maintained weight, has been found to have plasma sodium concentrations that are too high[20,21,22,23,24,25,26,27,28].

Nor is it necessarily excess sodium that causes a bloating athlete to have retained water in the first place. In a heat acclimation experiment[29] one subject (of 10) bloated and became hyponatremic within four hours of beginning the experiment. During this time, the subject was consuming water at twice the rate of his fellow subjects. He was also urinating more as well, but not enough, and he developed bloating and hyponatremia. His bloating was not due to excess sodium: he started the experiment with a blood plasma sodium concentration of 134 meq/liter compared to the other subjects' 140+/-1 meq/liter (normal is 135-145 meq/liter).

Nor is the generalization that excess sodium always causes water retention true: water retention was not observed in an experiment30 with 20-24 year old healthy males whose sodium chloride intake was ramped up from their daily average of ~12 g to 31 g.

The Urination Shutdown Mystery

It is possible to grossly overdrink[17,31] and overwhelm even the best of kidneys. Maximal urination rates are in the neighborhood of one quart/hour [29,32]. If you're taking in fluid at a rate that exceeds what you're putting out via sweat plus urine, then you will bloat. Overdrinking has been the cause of many cases of hyponatremia[2,18,31].

It is also possible for urination to shut down in the presence of a moderate fluid overload, one that the kidneys at rest will excrete[33]. Why the urinary overflow valve jams shut for some people during exercise is not understood[5], but the result is bloating.

Urination shutdown can be dangerously misleading. If an athlete is not urinating, we think dehydration - but here is the exact opposite, an overhydrated athlete who has stopped urinating! To avoid making a mistake, consider the context: is the non-urinating athlete bloated? Has his/her weight increased? What has been his/her fluid intake over the last few hours?

Recognizing bloating hyponatremia

The only way to definitively diagnose hyponatremia is to take a blood sample and analyze it for plasma sodium. Fortunately bloating hyponatremia has its own set of symptoms, first of which is the bloating itself:

A bloated, hyponatremic athlete will typically not have an elevated core temperature[16]. The only definitive measure of core temperature is rectal temperature[35], but you can make an educated guess by checking skin temperatures at several different locations. Core temperature reaching ~104 degrees F is what signals the fatigue or malaise that forces an exercising athlete to stop[36]. Higher core temperatures are more likely in short events (for example, a 40 km time trial) than endurance events[37], but also represent a medical emergency for which the proper treatment is cooling.

Treating bloating hyponatremia

First, stop drinking (except as a means for salt ingestion)! Second, ingest salt! How much? Follow the emergency room treatment[2,38,39,40]: aim for a rapid but incomplete correction of hyponatremia. If you have salt tablets/capsules, take 300-500 mg sodium followed by the same amount 10-15 minutes later. The 2003 Boston Marathon medical team used "a hot salty broth" with concentration approaching that of the 3% sodium chloride IV to successfully treat hyponatremia[34]. Anecdotal reports (including my personal experience) suggest quick recovery, within 5-20 minutes. Slow down on sodium ingestion as soon as symptoms abate.

Do not start a normal (0.9% sodium chloride) IV and do not give a sport drink, both of which will only increase the fluid overload[2]. Do not resume drinking until you have urinated the overload[34].

Dehydrated and HYPERnatremic

If you replace nothing while exercising, neither water nor sodium, you will initially become dehydrated and eventually have a too high sodium plasma concentration (hypernatremic). Sweat is a filtrate of blood plasma, filtered by the remarkably sophisticated sweat glands. Sweat is more dilute than blood plasma (see Table 2). You lose relatively less sodium than water in sweat, so relatively more sodium than water stays in the blood plasma. Hence, sodium concentration in blood plasma increases (you are dividing a somewhat smaller number, amount of sodium, by a much smaller number, volume of plasma.)

Non-bloating HYPOnatremia

You can make yourself dehydrated and hyponatremic by replacing water somewhat inadequately and sodium way too inadequately. What water you are ingesting is not enough to keep you properly hydrated, but enough to dilute the plasma sodium concentration to where you will become hyponatremic.

If you get in this state, you may know that you are dehydrated (your weight will have decreased). But there may be no specific symptoms to tell you that you are hyponatremic. It makes sense to re-hydrate with sodium; ingest salty snacks and/or sodium capsules or tablets as you re-hydrate.

Summary

The key is prevention. Determine your fluid and sodium requirements and follow them. Remember

Be wary of generalized prescriptions, especially those that begin with "an athlete should consume no more than . . . " or "at least . . . Let careful observations be the touchstone for your needs. If you find you require more/less water, or more/less salt than you expected after reading some treatise on the topic, then so be it.

More on electrolyte and fluid requirements in endurance athletes.


Warning: include_once(../../../includes/join_box.php): failed to open stream: No such file or directory in /home/worldult/public_html/sections/MOVEDarticles/nutrition/hyponatremia2.php on line 347

Warning: include_once(): Failed opening '../../../includes/join_box.php' for inclusion (include_path='.:/usr/local/php71/pear') in /home/worldult/public_html/sections/MOVEDarticles/nutrition/hyponatremia2.php on line 347

References

1. Barr SI. Effects of dehydration on exercise performance. Can J Appl Physiol 1999;24(2):164-72.

2. Speedy DB, Noakes TD, Schneider C. Exercise-associated hyponatremia: a review. Emerg Med (Fremantle) 2001;13(1):17-27.

3. Barr SI, Costill DL, Fink WJ. Fluid replacement during prolonged exercise: effects of water, saline, or no fluid. Med Sci Sports Exerc 1991;23(7):811-7.

4. Dennis SC, Noakes TD, Hawley JA. Nutritional strategies to minimize fatigue during prolonged exercise: fluid, electrolyte and energy replacement. J Sports Sci 1997;15(3):305-13.

5. Speedy DB, Rogers IR, Noakes TD, et al. Exercise-induced hyponatremia in ultradistance triathletes is caused by inappropriate fluid retention. Clin J Sport Med 2000;10(4):272-8.

6. Olsson KE, Saltin B. Variation in total body water with muscle glycogen changes in man. Acta Physiol Scand 1970;80(1):11-8.

7. Montain SJ, Sawka MN, Wenger CB. Hyponatremia associated with exercise: risk factors and pathogenesis. Exerc Sport Sci Rev 2001;29(3):113-7.

8. Haldeman L. Personal Communication, 2003.

9. Costill DL. Sweating: its composition and effects on body fluids. Ann N Y Acad Sci 1977;301:160-74.

10. Dill DB, Horvath SM, Van Beaumont W, Gehlsen G, Burrus K. Sweat electrolytes in desert walks. J Appl Physiol 1967;23(5):746-51.

11. Mao IF, Chen ML, Ko YC. Electrolyte loss in sweat and iodine deficiency in a hot environment. Arch Environ Health 2001;56(3):271-7.

12. Shirreffs SM, Maughan RJ. Whole body sweat collection in humans: an improved method with preliminary data on electrolyte content. J Appl Physiol 1997;82(1):336-41.

13. Nielsen B, Hales JR, Strange S, Christensen NJ, Warberg J, Saltin B. Human circulatory and thermoregulatory adaptations with heat acclimation and exercise in a hot, dry environment. J Physiol 1993;460:467-85.

14. Nielsen B. Heat acclimation--mechanisms of adaptation to exercise in the heat. Int J Sports Med 1998;19 Suppl 2:S154-6.

15. Ayus JC, Varon J, Arieff AI. Hyponatremia, cerebral edema, and noncardiogenic pulmonary edema in marathon runners. Ann Intern Med 2000;132(9):711-4.

16. Noakes TD. Hyponatremia in distance athletes: pulling the IV on the 'dehydration myth'. Phys Sportsmed 2000;28(9):71-76.

17. O'Brien KK, Montain SJ, Corr WP, Sawka MN, Knapik JJ, Craig SC. Hyponatremia associated with overhydration in U.S. Army trainees. Mil Med 2001;166(5):405-10.

18. Frizzell RT, Lang GH, Lowance DC, Lathan SR. Hyponatremia and ultramarathon running. Jama 1986;255(6):772-4.25.

19. Ayus JC, Arieff AI. Pathogenesis and prevention of hyponatremic encephalopathy. Endocrinol Metab Clin North Am 1993;22(2):425-46.

20. Speedy DB, Noakes TD, Rogers IR, et al. Hyponatremia in ultradistance triathletes. Med Sci Sports Exerc 1999;31(6):809-15.

21. Almond CS, Fortescue EB, Shin AY, Mannix R, Greenes DS. Risk Factors for Hyponatremia among Runners in the Boston Marathon. Acad Emerg Med 2003;10(5):534-5.

22. Glace BW, Murphy CA, McHugh MP. Food intake and electrolyte status of ultramarathoners competing in extreme heat. J Am Coll Nutr 2002;21(6):553-9.

23. Sharwood K, Collins M, Goedecke J, Wilson G, Noakes T. Weight changes, sodium levels, and performance in the South african ironman triathlon. Clin J Sport Med 2002;12(6):391-9.

24. Speedy DB, Faris JG, Hamlin M, Gallagher PG, Campbell RG. Hyponatremia and weight changes in an ultradistance triathlon. Clin J Sport Med 1997;7(3):180-4.

25. Speedy DB, Campbell R, Mulligan G, et al. Weight changes and serum sodium concentrations after an ultradistance multisport triathlon. Clin J Sport Med 1997;7(2):100-3.

26. Speedy DB, Rogers IR, Noakes TD, et al. Diagnosis and prevention of hyponatremia at an ultradistance triathlon. Clin J Sport Med 2000;10(1):52-8.

27. Speedy DB, Noakes TD, Kimber NE, et al. Fluid balance during and after an ironman triathlon. Clin J Sport Med 2001;11(1):44-50.

28. Stuempfle KJ, Lehmann DR, Case HS, et al. Hyponatremia in a cold weather ultraendurance race. Alaska Med 2002;44(3):51-5.

29. Armstrong LE, Curtis WC, Hubbard RW, Francesconi RP, Moore R, Askew EW. Symptomatic hyponatremia during prolonged exercise in heat. Med Sci Sports Exerc 1993;25(5):543-9.

30. Heer M, Baisch F, Kropp J, Gerzer R, Drummer C. High dietary sodium chloride consumption may not induce body fluid retention in humans. Am J Physiol Renal Physiol 2000;278(4):F585-95.

31. Hew TD, Chorley JN, Cianca JC, Divine JG. The incidence, risk factors, and clinical manifestations of hyponatremia in marathon runners. Clin J Sport Med 2003;13(1):41-7.

32. Noakes TD, Wilson G, Gray DA, Lambert MI, Dennis SC. Peak rates of diuresis in healthy humans during oral fluid overload. S Afr Med J 2001;91(10):852-7.

33. Speedy DB, Noakes TD, Boswell T, Thompson JM, Rehrer N, Boswell DR. Response to a fluid load in athletes with a history of exercise induced hyponatremia. Med Sci Sports Exerc 2001;33(9):1434-42.

34. Siegel AJ. Hyponatremia at the 2003 Boston Marathon, 2003 (personal communication).

35. Moran DS, Mendal L. Core temperature measurement: methods and current insights. Sports Med 2002;32(14):879-85.

36. Nielsen B, Nybo L. Cerebral changes during exercise in the heat. Sports Med 2003;33(1):1-11.

37. Noakes TD, Myburgh KH, du Plessis J, et al. Metabolic rate, not percent dehydration, predicts rectal temperature in marathon runners. Med Sci Sports Exerc 1991;23(4):443-9.

38. Adrogue HJ, Madias NE. Hyponatremia. N Engl J Med 2000;342(21):1581-9.

39. Gross P. Treatment of severe hyponatremia. Kidney Int 2001;60(6):2417-27.

40. Han DS, Cho BS. Therapeutic approach to hyponatremia. Nephron 2002;92 Suppl 1:9-13.

More on electrolyte and fluid requirements in endurance athletes.

page design modified: August 4, 2014