The reason for the cramps is a combo of dehydration and loss of electrolytes. As mentioned water is great to prep for a long day in the heat. However, water will not replace these electrolytes. When I say water I mean pure water not smart water or sports water.
Drinking only water in intense heat and being active is harmful! The creation of Gatoraid was for a legit purpose! Now, walking a mile and drinking a Gatoraid is not needed at all, you actually potentially consume more calories than just burned depending on what kind you drink. In the OPs case prep with water, while in the sun be sure to drink water in conjunction with any electrolyte replenishing drink of your choice.
Not mentioned yet, pedialyte is another great choice. It's kind of gross, but pickle juice is a great way to prevent and treat muscle cramps cause by electrolyte imbalances. If you don't want the sugary drinks, consume electrolyte rich foods such as bananas, pickles, canned soups, tomatoes etc.
For an example: in an hour it's not uncommon to sweat out up to 600mg of potassium, this is critical for heart and cell function. If you sweat for 8 hours and don't replenish, your down up to 4800 mg for the day. Your body needs about 3500mg per day in normal circumstances. Like everything necessary for health, not sure most people are meeting this amount.
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