Other than Vitamin D, there is probably no vitamin or mineral that is so critcal to so many functions within the body. Of course, the modern diet high in processed foods leaves the great majority of people deficient in Magnesium. This leads to a host of illnesses that research has uncovered in the last fifteen years.
NEWS FLASH: A recent study of senior men found that magnesium was very correlated with testosterone levels.  Yes, this means that optimizing your magnesium levels could actually boost your testosterone. More research needs to be done, but it looks like this all-important mineral ramps up androgen production as well. CAUTION: The study showed that IGF-1 levels are also raised with magnesium and some researchers believe that raising IGF-1 could increase cancer risk.
Look at the nasty list of syndromes, diseases and conditions below. All of these can be avoided by taking a simple Magnesium supplement.
1. Heart arrhythmias 
2. Heart disease 
3. High blood pressure 
4. Metabolic Syndrome 
5. Diabetes 
6. Low bone density 
7. Migraines 
8. Insulin Resistance 
9. TNF-Alpha / Inflammation [See my link on TNF Inhibitors for more information.]
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2) Int J Epidemiol,1999,28:645-651
3) Magnesium Res,1989,2:229;PNAS,Oct 15 1984,81(20):6511-15
4) Am J Med,1992,93(2A):11S-20S;J Am Soc Nephrol,1992,3(4Suppl):S78-85
5) Magnes Bul,2000,22:53-59
6) Eur J Nutr,Oct 2001,40(5):231-7
7) Cephalalgia,1996;16:257-63;Pediatr Med Chir,1993,15(5):481-8
8) J Clin Endocrinol Metab,1997,82:3783-3785
9) Int J Androl, 2011 Dec, 34(6 Pt 2):e594-600, "Magnesium and anabolic hormones in older men"