Magnesium is an extraordinarily helpful supplement for almost all of us. Unless you happen to eat lots and lots (more than seven servings a day) of greens, you happen to drink magnesium-rich spring water, or your diet emphasizes stone-ground, nictalized hominy (the kind used to make traditional Mexican tortillas), you will probably have a magnesium deficiency from time to time.
The most practical way to prevent magnesium deficiency isn't to eat lots of greens (you can run into problems with oxalic acid with spinach and some of the more exotic greens, like lamb's quarters), or to drink imported spring water (too expensive), or even to eat lots of tortillas (tortilla chips usually aren't made from corn that has been treated with the traditional processes that make magnesium bioavailable).
There is one major problem with taking magnesium supplements. Too much, and you will get diarrhea. That isn't because magnesium stimulates the bowel the way some of the herbal laxatives like Senna and aloe bitters do. It just absorbs water. The more water there is in your bowel, the more water there is in your stool, and the runnier your bowel movements will be. For some people, this effect is not noticeable even when they take 1,000 mg. For others, it is after taking 300 mg.
That's why I recommend that you don't take more than 500 mg of magnesium at a time. It's better to take a 200 or 250 mg tablet twice a day if you can find it. It's generally OK to take up to 1,000 mg a day, but not more. The reason larger doses of magnesium taken orally don't work is they make it easier to "go" and the magnesium simply gets flushed down the toilet.
How can you know whether you need magnesium supplements? As a general rule, nearly all of us do, and it doesn't hurt to take 250-500 mg a day even if you don't. If you are admitted to a hospital (at least in the US), chances are your doctor will run a blood test for magnesium and give you an IV drip if your levels are 1.7 mg/dl or lower.
The same principle applies if you are not in the hospital. Get a blood test to know for sure. This will also tell you if you have taken too much and developed hypermagnesemia, a condition of too much magnesium.
Very, very, very few people develop hypermagnesemia. It's most common in people who have kidney failure who took too much magnesium (including antacids that contain magnesium) in their oliguric phase, when their kidneys were producing too little urine. In the polyuric phase, when the kidneys produce lots of urine, but just don't remove certain proteins, hypermagnesemia is almost unheard of.
Hypermagnesemia can also happen to:
- People who have hypothyroidism.
- People who have suffered major tissue loss due to burns, sepsis, or crushing injuries.
- People who take lithium for bipolar disorder (lithium interferes with the kidneys' ability to clear magnesium).
- Women who have received IV magnesium for preeclampsia.
- People who have taken large amounts of Milk of Magnesia for megacolon, a condition in which impacted fecal matter essentially stops bowel movement, thinking they had constipation.
- People who have a rare genetic disease called familial hypocalciuric hypercalcemia, which also limits the release of excess calcium.
It is far more common to suffer hypomagnesemia than hypermagnesemia, and no one who does not have kidney disease, thyroid disease, or recent major tissue loss (more than cutting your finger or even breaking a bone, more like being hit by a truck) is likely to take too much, at least not for very long.
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