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 <ref name="ref7"/>. (Dehydration of 5% does impact performance <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 <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 <ref name="ref8"/>.
== Salt loss through sweat ==