Anxiety is associated with urinary biomarkers such as urinary phosphate and magnesium excretion. More particularly, anxiety is related to low magnesium availability as the direct result of an increase in urinary magnesium excretion (i.e., magnesium in urine).
Previous studies support this conclusion as mice reportedly exhibit an increase in anxiety-like behaviors under reduced magnesium availability. The partial loss of magnesium is attributed to the increase in urinary phosphate excretion that occurs following anxiety-related muscular tension, which consumes high amounts of energy.
By contrast, morning urinary concentration and excretion of both magnesium and phosphate is low which is reasoned to be attributed to the relaxation induced by sleep. Hence, it was proposed to use magnesium for the treatment of anxiety.
Reference:
Grases G, Pérez-Castelló JA, Sanchis P, et al. Anxiety and stress among science students. Study of calcium and magnesium alterations. Magnes Res. 2006;19(2):102-106.