The inorganic magnesium forms are generally absorbed at a lower rate and because of this will draw water to the large intestine more strongly, which is more likely to induce bowel movements. These forms are usually less expensive and contain higher amounts of magnesium per weight so fewer pills are needed.

• Magnesium Oxide is magnesium bound to oxygen and is poorly absorbed. It is commonly used as a laxative and sometimes as an antacid.
• Magnesium Carbonate is magnesium bound to a mineral group containing carbon and oxygen and is sometimes used as an antacid. It is moderately well absorbed and can have a bowel loosening effect.¹
• Magnesium Chloride consists of one magnesium ion bound to two chloride ions.  It is often extracted from water with a high salt content like the Dead Sea or the Great Salt Lake. Elemental magnesium only makes up about 12% of magnesium chloride, but the magnesium is generally believed to be well absorbed. This form is also frequently used in topical magnesium products.  

The organic magnesium forms are generally well absorbed and will have a less pronounced effect on the bowels. They tend to be bulkier, though, so more pills or powder may be necessary to obtain the desired amount.

• Magnesium Citrate is the most commonly used form in supplements. It easily dissolves in liquid and is well absorbed. Citrate is involved in energy production and may actually enhance this process.² Especially in higher amounts, magnesium citrate can have a stool loosening effect.
• Magnesium Gluconate is magnesium bound to the naturally occurring compound gluconic acid. It is well absorbed and does not cause loose stools. Magnesium gluconate has been shown to fight free radicals and the oxidative stress they cause.³ In this capacity magnesium gluconate may be protective of the cardiovascular system.^{4 5}
• Magnesium Glycinate is magnesium bound to the amino acid, glycine. It is well absorbed and does not exert a laxative effect on the bowels. Glycine is a building amino acid and also a calming neurotransmitter in the brain. The combination of magnesium and glycine is calming and relaxing.
• Buffered Magnesium Glycinate contains approximately 66% magnesium glycinate buffered by 34% magnesium oxide. This formulation is easy on the stomach and has a high elemental magnesium content, meaning smaller capsules and/or fewer pills to take. It also tends to be more budget friendly than unbuffered magnesium glycinate.  
• Magnesium L-threonate is bound to threonic acid and appears to cross the blood-brain barrier more efficiently than other forms of magnesium. There it supports memory by improving the mechanisms necessary for brain cells to communicate with each other.^{6 7}
• Magnesium Malate is magnesium bound to malic acid, which is found naturally in apples and other fruits and vegetables and plays an important role in energy production. Magnesium malate may be especially beneficial to athletes and those suffering from fatigue and musculoskeletal problems such as fibromyalgia.
• Magnesium Orotate is magnesium bound to orotic acid. Orotates are used by the body to create DNA and RNA and when bound to magnesium may support improved athletic performance as well as protect an already weakened heart.⁸ Magnesium orotate does not have a laxative effect.⁹
• Magnesium Taurate is magnesium bound to the amino acid, taurine. Taurine has a calming effect on the brain and body.¹⁰ It also positively influences heart health by slowing the progression of plaque formation in the arteries, modulating blood pressure and heart rate, and supporting healthy blood clotting.¹¹ Magnesium taurate does not affect the bowels.

Other terms you may see when buying a magnesium supplement are:

• Chelated Magnesium. In chemistry, chelation refers to a particular type of bond, but in dietary supplements the term is used more loosely to describe a mineral complex that has been formulated with amino acids or other organic acids. Magnesium complexes with glycinate, aspartate, and taurine may all be called chelated, even though they may or may not be technically chelated. True chelated minerals tend to be easier on the stomach and bowels and are generally well absorbed.¹²
• Ionic Magnesium is magnesium that has been dissolved in a solution (usually water) to free the magnesium from the substance it was bound to. Many claim that ionic magnesium is better absorbed than other forms, but there is currently not much research available to verify this claim. Ionic magnesium can have a bowel loosening effect.

In addition to the pills and powders for internal use, topical magnesium is also available. People have been absorbing magnesium (and other minerals) into the body through the skin for hundreds of years through mineral hot springs or Epsom Salt (magnesium sulfate) baths. Recently several companies have introduced topical magnesium oils and sprays. These products allow you to deliver magnesium directly to sore muscles to take advantage of its relaxing properties or simply as a way to get magnesium into the body while bypassing the digestive system.

References

1. Group EF.  9 Common Types of Magnesium Explained. Global Healing Center Website. September 10, 2013. Available at: http://www.globalhealingcenter.com/natural-health/types-of-magnesium/
2. Bitner Price L. Anaplerotic Therapy in Propionic Acidemia. Master of Science Thesis presented to The University of Utah. May 2013.
3. Turner RJ, DaSilva KW, O’Connor C, van den Heuvel C, Vink R. Magnesium gluconate offer no more protection than magnesium sulfate following diffuse tramatic brain injury in rats. Journal of the American Collage of Nutrition. 2004;23(5):5415-5445.
4. Murthi SB, Wise RM, Weglicki WB, Komarov AM, Kramer JH. Mg-gluconate provides superior protection against postischemic dysfunction and oxidative injury compared to Mg-sulfate. Molecular and Cellular Biochemistry. 2003; 245(1-2):141-148.
5. Mak IT, Komarov AM, Kramer JH, Weglicki WB. Protective mechanisms of MG-gluconate against oxidative endothelial cytotoxicity. Cellular and Molecular Biology. 2000;46(8):1337-1344.
6. Alessio M. Novel Magnesium Compound Reverses Neurodegeneration. Life Extension Magazine. February 2012.
7. Wang J, Liu Y, Zhou LJ, Wu Y, Li F, Shen KF, Pang RP, Wei XH, Li YY, Liu XG. Magnesium L-threonate prevents and restores memory deficits associated with neuropathic pain by inhibition of TNF-α. Pain Physician. 2013;16(5):E563-75.
8. Rosenfeldt FL. Metabolic supplementation with orotic acid and magnesium orotate. Cardiovasc Drugs Ther. 1998;12 Supp:147-152.
9. Classen HG. Magnesium orotate—experimental and clinical evidence. Rom J Intern Med. 2004;42(3):491-501.
10. Ross, J. The Mood Cure. New York, NY: Penguin Books; 2002.  
11. McCarty MF. Complementary vascular-protective actions of magnesium and taurine: a rationale for magnesium taurate. Med Hypotheses. 1996;46(2):89-100. 
12. Gerber J. A review of mineral absorption with special consideration of chelation as a method to improve bioavailability of mineral supplements. Integrative Practitioner Website. Available at: http://www.integrativepractitioner.com/article_ektid6418.aspx Accessed: March 21, 2014.