Question 1

What's the difference between glycinate, threonate, and citrate?

Accepted Answer

Glycinate (14% elemental Mg, 40–50% bioavailability) is the best general-purpose form — gentle on the GI tract, ideal for sleep and repletion. Threonate (8% elemental) is BBB-penetrating and the only form with cognitive RCT data, but you need it alongside a whole-body form. Citrate (16%, 30–40% bioavailability) is the budget pick; it can cause loose stools over 400 mg/d. Oxide (60% elemental) is mostly a laxative — absorption is 4–10%.

Question 2

Why is the upper limit only 350 mg/day?

Accepted Answer

350 mg/day is the IOM's tolerable upper intake level (UL) for *supplemental* magnesium — it does not apply to magnesium from food. The UL is set to avoid osmotic diarrhea, which is the dose-limiting side effect at chronic higher intakes in people with normal renal function. For confirmed deficiency, 400–600 mg/day under clinician guidance is standard; the UL is an exemption, not a hard ceiling.

Question 3

Do I need to separate magnesium from other supplements or meds?

Accepted Answer

Yes in a few specific cases: separate from calcium by 2 hours (competitive absorption), from tetracyclines/quinolones/bisphosphonates by 2–4 hours, and from levothyroxine by 4 hours. PPIs reduce Mg absorption chronically — consider higher-end dosing if you're on long-term omeprazole, pantoprazole, etc.

Question 4

How long before I notice an effect?

Accepted Answer

Sleep and anxiety effects: often within 1–2 weeks. Migraine prevention: give it 12 weeks minimum before judging. Hypertension: 12–24 weeks for maximum BP drop (meta-analyses show SBP −2.81 mmHg overall, −7.68 mmHg in treated hypertension). Repletion of tissue magnesium in confirmed deficiency: 8–12 weeks.

Question 5

Can I take magnesium if I have kidney issues?

Accepted Answer

GFR 30–60: use cautiously, start at the low end, and check serum Mg monthly for the first 3 months. GFR under 30: avoid oral magnesium supplements entirely — hypermagnesemia is a real risk when the kidneys can't clear it. IV magnesium is a separate medical decision.

Question 6

Is serum magnesium a useful test?

Accepted Answer

Not really. Only 1% of total body magnesium sits in serum — 99% is intracellular (50–60% bone, 39% soft tissue). Serum Mg stays normal even when tissue stores are depleted. RBC magnesium is a better indicator for day-to-day clinical use, and ionized Mg is better still but harder to source. For practical purposes, base dosing on goal + dietary intake gap rather than serum.