There are basically two ways to develop low magnesium levels. Like potassium, magnesium is excreted in the urine, so diuretics, which often cause hypokalemia (low potassium), also generally cause hypomagnesemia (low magnesium). In fact hypokalemia and hypomagnesemia often go together. Patients who abuse alcohol are often hypomagnesemic as well because alcohol can cause a temporary dysfunction in the renal tubules causing magnesium to be excreted in the urine.
The other main way to lose magnesium is through the gastrointestinal tract. Essentially diarrhea is a major way many people lose magnesium. People who take proton pump inhibitors (PPIs) to suppress stomach acid can also have low magnesium levels because these medications may block magnesium absorption in the stomach.
Magnesium and calcium are in the same family in the periodic table, meaning that they have similar chemical properties. Both are instrumental in bone formation (most of the magnesium in your body is in your bones) and anything that disrupts calcium metabolism will also potentially have an effect on magnesium levels.
People who develop magnesium deficiency can suffer both neurologic and cardiac problems if their levels fall low enough.
Patients with low magnesium levels can develop muscle spasms, presumably because a deficiency in magnesium lowers the electrical threshold at which nerve cells become depolarized, which makes them hyper-excitable. The same is true for calcium, and low calcium levels can also cause muscle spasm. Since low calcium and magnesium levels often co-exist, it can sometimes be hard to know how much each is contributing to the problem.
In peripheral nerves, low magnesium causes spasms in the muscles, but when it happens in the central nervous system, it can cause seizures. In fact, magnesium is still used today to prevent seizures in pregnant women with pre-eclampsia, although it isn’t used much in the general population because more effective treatments exist.
Low magnesium can predispose to irregular heart beats. That’s because low magnesium (and low calcium) affect how quickly or slowly cardiac cells depolarize. This disruption causes a specific pattern on the ECG called a “long QT interval” which puts individuals at risk for an arrhythmia called Torsades des Pointes. In fact, much like in pre-eclampsia, a rapid infusion of magnesium is the mainstay treatment for this arrhythmia. Low magnesium levels can also increase the number of skipped beats and worsen others arrhythmias like atrial fibrillation.
Treating magnesium deficiency is both very easy and very hard. In fact, the amount of time and effort spent replacing magnesium deficiency on the medical ward is somewhat mindboggling. It’s also not entirely clear that it makes a difference.
For very severe cases of magnesium deficiency with symptoms, such as hemodynamically unstable arrhythmias, seizures like eclampsia, or severe muscle spasms like tetany, treating people with intravenous magnesium is clearly necessary.
But treating patients with IV infusions of magnesium is inefficient. When you give someone an IV bolus of magnesium, their blood levels spike temporarily and then about 50% of that magnesium is subsequently excreted in their urine within a few hours.
As such, when possible, oral magnesium replacement is preferred. Since oral magnesium pills are absorbed more slowly, you do not have that sudden peak of magnesium levels in the blood followed by its rapid excretion in the urine.
A problem is that oral magnesium replacement is often not tolerated by hospitalized patients. Many cannot take medications orally, for example after a surgery or because they are in the ICU. Also, oral magnesium has side effects like stomach upset. Magnesium is also a laxative (think milk of magnesia), so oral magnesium tablets often cause diarrhea in higher doses.
The good news is that replacing magnesium usually happens pretty quickly. Since most magnesium is stored inside cells, it is generally recommend to keep taking magnesium for one or two days after blood levels normalize to make sure you have adequately replaced all the magnesium you lost. Most people don’t really need to make magnesium supplements long term… unless of course they happen to be constipated.
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