sentence: following are some foods and the amount of magnesium in them : black-eyed peas ( 1/2 cup ) = 45 mg buckwheat flour ( 100g ( 4 oz ) ) = 250 mg halibut ( 100g ( 4 oz ) ) = 107 mg milk : low fat ( 1 cup ) = 40 mg oats ( 100g ( 4 oz ) ) = 235 mg peanut butter ( 2 tablespoons ) = 50 mg spinach ( 1/2 cup ) = 80 mg wholemeal bread ( 1 slice ) = 25 mg biological chemistry mg 2 + is the fourth-most-abundant metal ion in cells ( per mole ) and the most abundant free divalent cation -- as a result , it is deeply and intrinsically woven into cellular metabolism .