Michael Roizen, MD, Internal Medicine, answered
Women should get at least 400 milligrams (mg) of magnesium a day, and men should get at least 333 mg a day. Current studies show that the average intake of Americans is less than 300 mg. People who need to be extra careful to get the right amounts of magnesium include those who are pregnant or lactating, those with kidney disease, diabetics, those on low-calorie diets, and those taking digitalis preparations and diuretics. All of these people should consult their physician before beginning any new regimen.
Government guidelines suggest 420 milligrams (mg) per day for men, 320 mg for women. We recommend that both men and women aim to get 400-500 mg of magnesium per day. It may be tough to get all the magnesium you need from food, so top up your daily intake with a multivitamin that contains at least 100 mg of magnesium.
If you are pregnant or breastfeeding, have kidney disease or diabetes, are on a low-calorie diet, or are taking digitalis preparations or diuretics, talk to your doctor about how much magnesium you should be getting.