Magnesium is naturally present in a variety of foods, available as a supplement, and an ingredient in antacids and laxatives. The mineral plays an important role in assisting more than 300 enzymes to carry out various chemical reactions in the body such as building proteins and strong bones, and regulating blood sugar, blood pressure, and muscle and nerve functions. Magnesium also acts an electrical conductor that contracts muscles and makes the heart beat steadily.
More than half of the magnesium in our body is stored in bones, and the remaining in various tissues throughout the body.
RDA: The Recommended Dietary Allowance (RDA) for adults 19-51+ years is 400-420 mg daily for men and 310-320 mg for women. Pregnancy requires about 350-360 mg daily and lactation, 310-320 mg.
UL: The Tolerable Upper Intake Level is the maximum daily intake unlikely to cause harmful effects on health. The UL for magnesium is 350 milligrams from supplements only. High-dose supplements can lead to diarrhea, nausea, and cramping in some people. Extra magnesium from food is safe because the kidneys will eliminate excess amounts in urine.
Magnesium and Health
Magnesium is a key factor in making several parts of the body run smoothly: the heart, bones, muscles, nerves, and others. Without enough magnesium, these areas malfunction. This is summarized in research, which finds that a magnesium deficiency or low magnesium diet leads to health problems. Although epidemiological studies show that higher magnesium diets are associated with lower rates of disease, results are mixed from clinical trials showing that magnesium supplementation can correct these conditions. It may be because a magnesium-rich diet is often higher in other nutrients, which collectively work together in disease prevention as opposed to a supplement containing a single nutrient.
For disease prevention, a good rule of thumb is to eat a daily diet that includes some magnesium-rich foods and take a supplement if directed by a physician to correct a deficiency if blood levels are low.
Magnesium is found in plant foods like legumes, dark green leafy vegetables, nuts, seeds, whole grains, and fortified cereals. It is also in fish, poultry, and beef.
- Almonds, peanuts, cashews
- Pumpkin seeds
- Peanut butter
- Beans (black, kidney)
- Soybeans, soymilk
- Cooked spinach, Swiss chard
- White potato with skin
- Brown rice
- Oatmeal (instant, whole oats)
- Dark chocolate (at least 70%)
- Milk, yogurt
A magnesium supplement may be prescribed if the body is having problems absorbing the nutrient. Over-the-counter magnesium supplements come in different forms; liquid types like magnesium citrate or chloride may be better absorbed than solid tablets like magnesium oxide and sulfate.
Magnesium can have a laxative effect at high doses; in fact, it is sold as a laxative in the form of magnesium hydroxide. Magnesium hydroxide is also an ingredient in some popular antacids to treat heartburn and upset stomach; it is important to be aware of the laxative effect when using magnesium hydroxide tablets for an upset stomach.
Signs of Deficiency and Toxicity
Although magnesium is naturally found in a variety of foods and some fortified foods, national dietary surveys show that most Americans of all ages eat less than recommended amounts.  However, these deficiency levels are marginal and not likely to produce symptoms. The body also helps to preserve magnesium levels when stores are low by limiting the amount excreted in urine and absorbing more magnesium in the gut. 
A true deficiency occurs with a long-term low magnesium diet, malabsorption, and large losses from alcohol abuse or use of medications that deplete magnesium (some diuretics, proton pump inhibitors, and antibiotics).
- Signs of deficiency include:
- Fatigue, weakness
- Poor appetite
- Nausea, vomiting
- Numbness or tingling in skin
- Muscle cramps
- Abnormal heart rate
- Risk factors for deficiency include:
- Alcohol abuse. A long-term excessive intake of alcohol is associated with a poor diet low in magnesium, digestive upset that leads to malabsorption, and problems with various organs that can flush out too much magnesium through urine.
- Older ages. The elderly have lower magnesium intakes according to national dietary surveys. Aging also causes decreased absorption of magnesium in the gut and increased excretion in urine. Furthermore, older adults are more likely to be on medications for chronic diseases that can lower magnesium stores.
- Conditions with malabsorption. Diseases that interfere with digestion can lower the amount of magnesium absorbed. Most magnesium is absorbed in the largest segment of the small intestine, the ileum, which may be compromised in conditions like celiac and Crohn’s disease. Surgery that removes the ileum, which is sometimes needed with colon cancer, ulcerative colitis, or Crohn’s disease, further adds to a deficiency risk.
- Type 2 diabetes mellitus. Insulin resistance or uncontrolled diabetes can cause the kidneys to make extra urine to rid the body of high levels of blood sugar. This increased amount of urine may also flush out magnesium.
Toxicity is rare from food sources because the kidneys will remove excess magnesium in the urine. However toxic levels may occur with long-term use of high-dosage supplements. People with kidney disease have a higher risk of toxicity because their kidneys are not working properly and cannot flush out extra magnesium.
- Signs of toxicity include:
- Nausea, vomiting
- Low mood, depression
- Muscle weakness
- Low blood pressure, abnormal heartbeat
- Heart attack
Did You Know?
Magnesium supplements are a popular remedy for leg and foot cramps, a bothersome condition that may jolt you awake in the middle of the night or during exercise. A deficiency of magnesium can certainly cause muscle cramping, but these supplements are often used whether or not people know they are truly deficient.
Evidence so far does not support this treatment for muscle cramps. A Cochrane review of seven randomized controlled trials looking at the effects of magnesium supplements on muscle cramps did not find a significant difference in intensity or duration of cramps in people using the supplements versus a placebo for one month.  Information was not available on the participants’ blood levels of magnesium at the start of the trials, so it is not known if the muscle cramps were related to a deficiency versus other factors.
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