The Science Of Hydration

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1 Introduction

The advice given to runners on hydration has changed over time and looks set to continue to change. There are competing forces at work - sports drink manufacturers, event organizers (often sponsored by the manufacturers) and scientists (some also sponsored by the manufacturers). One thing is clear about hydration - it is important. Incorrect hydration can lead to impaired performance, and in extreme cases, death.

A condition related to dehydration is Hyponatremia, which is where the sodium (salt) level in the blood becomes too dilute. This is a dangerous condition that has killed a number of runners. More on this later.

This blog entry is a follow on to Practical Hydration which should be read first.

2 Effects of dehydration

Everyone knows that dehydration is bad. But how bad? Current research indicates that some level of dehydration (up to 3%) does not impact performance, or impacts performance much less than expected [7]. (Dehydration of 5% does impact performance [11].) This may be due to the fact that carbohydrate (glycogen) is stored with water, in the ratio of about 1g glycogen to 2.5g water [8]. This means that 2000 calories of glycogen depletion that are likely to occur in marathon distance runs would result in about 4lb weight loss with no reduction in hydration (2000Kcal/4=500g glycogen + 1250g water = 1750g). In practice moving from a high carbohydrate to high fat diet can see 6lb weight loss, believed to be glycogen + water depletion [8].

3 Salt loss through sweat

The amount of salt that is lost through sweating varies a lot. It varies from individual to individual, and for an individual it will vary depending on fitness and heat acclimation [9]. This means that you may have to experiment with your salt intake, both during and after exercise. Anecdotal tip: If your skin is crusty with salt after a run, you are probably someone who sweats out a lot of salt.

4 More on Hyponatremia

Hyponatremia is where the sodium (salt) levels becomes too dilute. Initial symptoms tend to be a gain in weight and a general swelling and 'puffiness', most noticeable in the hands. More severe symptoms are caused by a swelling of the brain (cerebral edema) including nausea, vomiting, headache and malaise [10].

The cause of Hyponatremia is poorly understood, but believed to be related to excessive water intake [1]. (I believe that this is excessive fluid intake in the absence of sufficient electrolytes.) Hyponatremia can be common in endurance athletes. In a 1997 Ironman triathlon, almost 4% of competitors received attention for Hyponatremia [4]. In a study of the 2002 Boston Marathon, 13% of finishers had some level of Hyponatremia, and 0.6% had critical Hyponatremia [2]. The study revealed that the risk factors for Hyponatremia include a slow finish time (>4 hour) and consumption of >6 pints (3 liters) of water during the race; BAA suggests a 'slight build' is also a risk factor[12]. Healthy kidneys can excrete about 2 pints (1 liter) of fluid per hour, but this may be reduced by exertion or illness [3]. So drinking >6 pints in 4 hours could easily exceed the kidneys capacity to cope.

The recent rise in Hyponatremia may be due to earlier advice to athletes to "drink as much as possible" [13], combined with a general concern about salt intake.

5 HypERnatremia - the opposite of HypOnatremia

Generally, Hypernatremia (too much sodium in the blood) seems to be a result of dehydration rather than excessive salt intake [17]. It should be noted that taking electrolyte capsules bypasses the body's taste. This sense of taste seems to reflect our body's internal sensors; our desire for salty foods reflects our salt requirements.

6 Salt and High Blood Pressure

If you have high blood pressure, you may need to be careful with your salt intake. There is evidence that increased salt intake can increase blood pressure [4]. If you have high blood pressure, discuss these issues with your doctor. If your doctor is not an athlete, I would highly recommend changing to one that is. If you don't know what your blood pressure is, get it checked. (As an aside, if you have low blood pressure, which I do, increasing your salt intake can really help.)

7 Caffeine and Alcohol

The scientific evidence shows that caffeine is generally not a diuretic [5, 14, 15]. Previous studies have shown that if you don't normally take caffeine and then get a large dose, there is some diuretic effect. However normal intakes of caffeine by non-users and use by regular users is not a diuretic [16]. (If you urinate more because you drink a 20oz Latte, it is because of the 20oz of fluid, not the caffeine.) Alcohol is another story; drinking anything stronger than 2% will cause dehydration. Because alcohol takes 36 hours to clear the body, it should be avoided for 48 hours before you wish to avoid impaired performance [5].

8 Muscle Cramps

The common wisdom that muscle cramps are caused by lack of electrolytes or dehydration does not appear to be supported by science [6].

9 Blisters and black toe nails

Dehydration reduces body weight, which can reduce the size of your feet. This in turn changes the fit of your shoes, causing blisters. Hyponatremia can cause swelling, which increases the size of your feet and can cause blisters. Both conditions can also increase the chance of black toe nails.

10 References

[1]Exercise-Associated Hyponatremia

[2]Hyponatremia among Runners in the Boston Marathon

[3]Water Intoxication

[4]Micronutrient Information Center - Sodium

[5]Caffeine dehydration : Caffeine and alcohol - just how dehydrating are they?

[6]Muscle Cramps : No link between hydration and cramps

[7]Hydration - fluid intake advice and tips

[8]The Relation Of Glycogen To Water Storage In The Liver

[9]Cracking the Code on Hydration


[11]Dehydration reduces cardiac output and increases systemic and cutaneous vascular resistance during exercise,%20J%20Appl%20Physiol%2079,%201487-96,%201995.pdf


[13]USATF Announces Major Changes in Hydration Guidelines for Long Distance Runners

[14]Metabolic and exercise endurance effects of coffee and caffeine ingestion

[15]Effects of caffeine ingestion on body fluid balance and thermoregulation during exercise

[16]Caffeine ingestion and fluid balance: a review.;jsessionid=KNhWhGQSZnXhY11p2f7qnnmn1Q7z376shvhsK7hTWDLVGQhWpGGJ!811725889!181195628!8091!-1

[17]Sodium Status of Collapsed Marathon Runners