From Fellrnr.com, Running tips
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== 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
. (Dehydration of 5% does impact performance .) 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 . 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 .
== 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
. 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.
== 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
The cause of Hyponatremia is poorly understood, but believed to be related to excessive water intake
. (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 . In a study of the 2002 Boston Marathon, 13% of finishers had some level of Hyponatremia, and 0.6% had critical Hyponatremia . 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 . Healthy kidneys can excrete about 2 pints (1 liter) of fluid per hour, but this may be reduced by exertion or illness . 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"
, combined with a general concern about salt intake.
== 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
. 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.
== 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
. 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.)
== 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 . (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 .
== Muscle Cramps ==
The common wisdom that muscle cramps are caused by lack of electrolytes or dehydration does not appear to be supported by science
== Blisters and black toe nails ==
== References ==