When the summer comes, and you’re working hard, salt gets easily excreted from the sweat glands. What about adding sodium into the game? I quote briefly:
HEAT CRAMPS - These are usually the first warning of heat exhaustion and occur in the muscles which are doing the most work.
Symptoms: Shallow breathing, vomiting, dizziness.
The book later talks about that you need to have about a pinch of salt in a half litre. Though, sodium(salt) is also found in meat, etc.
SAS Survival Handbook Third Edition P. 449