This is a common misconception. Remember — coffee is also a source of magnesium. A 200 mL cup of coffee provides around 7 mg of magnesium. So if you drink 3 cups of coffee, this amounts to around 21 mg of magnesium. Interestingly, how you brew your coffee affects the amount of magnesium in a cup. Every day, we lose around 4 mg of this mineral, so you do not need to be afraid of coffee — when you drink it, you get more of this mineral than you lose.