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When the body is under stress, the need for Vitamin C goes up. Unlike other creatures, we can't make our own Vitamin C and do not have an appetite for it.
A study done years ago demonstrated that using Vitamin C will prevent heat exhaustion. Best bet: use sodicum ascorbate because it is buffered and won't hurt your stomach.
See: http://www.seanet.com/~alexs/ascorbate/194x/index.htm#Weaver-1948
