Changes

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]<ref name="ref7"/>. (Dehydration of 5% does impact performance [11]<ref name="ref11"/>.) 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]<ref name="ref8"/>. 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]<ref name="ref8"/>.

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]<ref name="ref9"/>. 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.

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]<ref name="ref10"/>.

The cause of Hyponatremia is poorly understood, but believed to be related to excessive water intake [1]<ref name="ref1"/>. (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]<ref name="ref4"/>. In a study of the 2002 Boston Marathon, 13% of finishers had some level of Hyponatremia, and 0.6% had critical Hyponatremia [2]<ref name="ref2"/>. The study revealed that the risk factors for Hyponatremia include a slow finish time (&gt;4 hour) and consumption of &gt;6 pints (3 liters) of water during the race; BAA suggests a 'slight build' is also a risk factor[12]<ref name="ref12"/>. Healthy kidneys can excrete about 2 pints (1 liter) of fluid per hour, but this may be reduced by exertion or illness [3]<ref name="ref3"/>. So drinking &gt;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]<ref name="ref13"/>, combined with a general concern about salt intake.

Generally, Hypernatremia (too much sodium in the blood) seems to be a result of dehydration rather than excessive salt intake [17]<ref name="ref17"/>. 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.

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]<ref name="ref4"/>. 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.)

The scientific evidence shows that caffeine is generally not a diuretic [5, 14, 15]<ref name="ref5"/><ref name="ref14"/><ref name="ref15"/>. 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]<ref name="ref16"/>. (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]<ref name="ref5"/>.

The common wisdom that muscle cramps are caused by lack of electrolytes or dehydration does not appear to be supported by science [6]<ref name="ref6"/>.

[1]<references><ref name="ref1">Exercise-Associated Hyponatremia

[2]<ref name="ref2">Hyponatremia among Runners in the Boston Marathon

[3]<ref name="ref3">Water Intoxication

[4]<ref name="ref4">Micronutrient Information Center - Sodium

[5]<ref name="ref5">Caffeine dehydration : Caffeine and alcohol - just how dehydrating are they?

[6]<ref name="ref6">Muscle Cramps : No link between hydration and cramps

[7]<ref name="ref7">Hydration - fluid intake advice and tips

[8]<ref name="ref8">The Relation Of Glycogen To Water Storage In The Liver

[9]<ref name="ref9">Cracking the Code on Hydration

[10]<ref name="ref10">Hyponatremia

[11]<ref name="ref11">Dehydration reduces cardiac output and increases systemic and cutaneous vascular resistance during exercise

[12]<ref name="ref12">BAA - HYDRATION, DEHYDRATION AND HYPONATREMIA

[13]<ref name="ref13">USATF Announces Major Changes in Hydration Guidelines for Long Distance Runners

[14]<ref name="ref14">Metabolic and exercise endurance effects of coffee and caffeine ingestion

[15]<ref name="ref15">Effects of caffeine ingestion on body fluid balance and thermoregulation during exercise

[16]<ref name="ref16">Caffeine ingestion and fluid balance: a review.

[17]<ref name="ref17">Sodium Status of Collapsed Marathon Runners