Usual Adult Dose for Vitamin/Mineral Supplementation during Pregnancy/Lactation. Recommended DIETARY intake: Women 19 to 30 years: 508 mg magnesium oxide (MgOx) (310 mg elemental magnesium) orally daily. Women 31 years and older: 525 mg MgOx (320 mg elemental) orally daily. Men 19 to 30 years: 656 mg MgOx (400 mg elemental) orally daily.