A. Adela Hruby, PhD, MS, MPH, who recently completed an American Heart Association Predoctoral Fellowship at Tufts Friedman School, responds:
According to the USDA Nutrient Database, per gram, both raw spinach and frozen spinach (unprepared or prepared by boiling and draining), contain around the same amount of magnesium (about 75-82 milligrams per 100 grams of spinach). However, if you go by a typical serving of one cup, rather than by weight, then frozen spinach, because its less bulky than raw spinach, is a richer source of magnesium. That is, a cup of prepared frozen spinach weighs in at about 190 grams and contains 156 milligrams of magnesium, while a cup of raw spinach weighs in at 30 grams and contains 24 milligrams of magnesium. So, if you want the same magnesium boost, get about six cups of raw spinach (which makes for a great salad!) for every one cup of frozen spinach.