A UL That Works Differently
Magnesium has an unusual upper limit. The NIH Office of Dietary Supplements sets the Tolerable Upper Intake Level at 350 mg/day, but with a crucial caveat: 'the ULs only include magnesium from dietary supplements and medications; they do not include magnesium found naturally in food and beverages' [1].
In other words, you can eat plenty of magnesium-rich food without counting it toward the 350 mg limit — the cap is specifically about *supplemental* magnesium. (For how ULs work in general, see Upper Intake Levels.)
Why Food Magnesium Isn't Capped
In healthy people, the kidneys efficiently excrete excess magnesium absorbed from food, so dietary magnesium rarely causes problems. Supplemental magnesium, taken as concentrated salts, is more likely to overwhelm the gut and cause symptoms before it's excreted.
The Main Side Effect: Diarrhea
Per NIH, 'high doses of magnesium from dietary supplements or medications often result in diarrhea that can be accompanied by nausea and abdominal cramping' [1]. This is essentially an osmotic laxative effect, and it depends heavily on the form: poorly absorbed forms like magnesium oxide and citrate are the most laxative, while glycinate is among the gentlest (see Magnesium Types Compared).
When to Be Extra Cautious
The people most at risk from excess magnesium are those whose kidneys can't clear it well. Reduced kidney function can allow magnesium to build up in the blood, which in severe cases is dangerous. If you have kidney disease, don't take magnesium supplements without a clinician's guidance.
Practical Guidance
- Keep supplemental magnesium at or below 350 mg/day unless a clinician directs otherwise.
- If you get loose stools, switch to a gentler form or lower the dose.
- Food magnesium doesn't count toward the 350 mg supplement limit.