The recommended dietary allowance (RDA), including magnesium from daily food sources and all dietary supplements, ranges from 400-420 mg per day for males and 310-360 mg per day for females. It’s safe to get high doses of magnesium from food (>350 mg/day) as long as it is in traditional food form and spread throughout your daily meals. The upper limit -- the highest dose a person should take of magnesium in a supplement form is:
• 65 mgs/d for children ages 1-3
• 110mgs/d for children ages 4-8
• 350 mg/d for adults and children ages 9 and up
Higher doses in supplements can cause gastrointestinal (stomach) distress such as mild diarrhea.
Magnesium intakes in the US fall well below the recommendation for the majority of non-supplement users. Recently studies have link low magnesium intake to higher risk of diabetes, insulin resistance and other cardiovascular disease risk markers. A recent study tracked men and women for 12 years and found those who consumed the most magnesium (~350 mg per day for women and 450 mg per day for men) from both food and supplements had a 33% lower risk of diabetes and less inflammation markers (both indicators of heart disease) than those consuming between 220-270 mg per day.
The prudent approach is to eat magnesium rich foods (e.g. halibut, nuts, soy beans, spinach, etc.) and take a daily multivitamin mineral supplement that contains no less than 100 mg and no more than 300 mg of magnesium to ensure you hit the daily target.