Hello. As Jim says like Vitamin D, we are supposed to be commonly deficient in magnesium. Symptoms:
MAGNESIUM
Muscle cramps, tremors or spasms
Muscle weakness
Insomnia or nervousness or hyperactivity
Constipation
Fits or convulsions
Breast tenderness or water retention
Depression or confusion
As you can see these are fairly vague symptoms but the cramps are supposed to be a classic sign. No mention of 'low energy' though.
You could invest the money spent on supplement on eating more organic veg. For what it is worth here is a list of magnesium rich whole foods. The last line indicates which vitamins help the magnesium be absorbed (ciggies, booze and sugar tend to do the opposite):
MAGNESIUM
Almonds and cashew, brazil nuts
Buckwheat flour, cooked beans and peanuts
Garlic, green peas and raisins
HELPERS: Vitamins B1, B6, C and D, zinc and calcium phosphorus