Directly quoted from Volek and Phinney , The Art and Science of Low Carb Living, pages 243-244:

"9. Muscle Cramps: Unnatural Complications of a Highly Refined Diet"

A distressing number of otherwise healthy people have frequent muscle cramps, and in the worst case, a muscle cramp of the heart equals sudden death. Physicians don't lie to deal with muscle cramps because the only effective medication we had to stop them was banned in 1992 due to unacceptable side effects.

Muscle cramps are the end result of many contributing factors, including overuse, dehydration, and mineral inadequacies. Low serum potassium is not uncommon in people with frequent cramps, so physicians often try potassium supplements. However there is a daisy-chain leading back from muscle cramps to low blood potassium to intracellular magnesium depletion. Low carbohydrate diets don't cause muscle cramps per se (meat and leafy greens are good sources of magnesium), but neither do they miraculously get better on low carb regimens unless the underlying problem is dealt with. This is just one more reason why leafy greens and home-made broths (good sources of magnesium
0 are desirable components of a healthy low carb diet.

So here's the shortcut to ending most nocturnal or post-exercise muscle cramps. Take 3 slow-release magnesium tablets daily for 20 days. The proprietary brand-name product is 'Slow-Mag', but there are a number of equally effective generics at a fraction of the brand-name price (e.g.Mag-64 or Mag-Delay). Most people's cramps cease within 2 weeks of starting 'Slow-Mag', but you should continue to take the full 20-day course (60 tabs per bottle at 3 per day lasts 20 days). If the cramps return, do it again, and then continue taking one tab per day. If the cramps return, take 2 tabs per day. Most people can be titrated to remain cramp-free by this method. Why use a more expensive slow-release magnesium preparation like Slow-Mag? Because magnesium oxide preparations like 'milk of magnesia' cause diarrhea, passing through the small bowel before they can be effectively absorbed.

WARNING: The only contraindication to oral magnesium supplements is severe renal failure (e.g., a GFR <30). If you have any history of kidney problems or known loss of kidney function check with your doctor before taking Slo-Mag or its generic equivalents. "