Magnesium. Another day, another conversation with someone about how their “health issues” are really just a magnesium deficiency. Aching lower back. Trouble sleeping. Constipation. Here, take this magnesium citrate. The next day: Wow! How did you know that would help so much? Estimates are that 50% of the U.S. population is deficient in magnesium but I argue that it’s much higher.