The latest LEF magazine hypes magnesium as the "new vitamin D". What they mean is that magnesium & vitamin D are each involved in many aspects of human health. The article is not yet available online, but here's one LEF did in Dec, 2014: .
From PCa study :
"Magnesium is the second most abundant intracellular cation in the body, involved with over 300 biological activities, and calcium and magnesium levels in the body are jointly regulated through a negative feedback system, and through competition for intestinal absorption and renal reabsorption. Calcium and magnesium also compete for membrane binding sites within the cell, and previous in vitro and in vivo studies indicate that magnesium inhibits calcium activity or that magnesium deficiency enhances the physiologic effects of calcium."
"The 1999–2000 National Health and Nutrition Examination Survey found 79% of U.S. adults have a magnesium intake below the Recommended Dietary Allowance. Magnesium deficiency in Western societies has been linked to insulin resistance, type II diabetes, metabolic syndrome, coronary heart disease, colorectal cancer, and colorectal adenoma. Similar to the relationships between obesity and most chronic diseases, systemic inflammation may be a unifying pathway by which magnesium deficiency contributes to such a broad range of morbidity. Chronic inflammation may also play a key role in the progression from normal tissue to prostatic intraepithelial neoplasia (PIN) and prostate cancer, and it is also possible that the elevated inflammatory response associated with magnesium deficiency is dependent on concurrent calcium levels."
In the U.S. we have an obsession with calcium [Ca] - it is even added to orange juice. As though calcium alone will ensure bone health. We largely ignore magnesium [Mg] & create an unfavorable Mg:Ca ratio when we supplement with calcium.
My well water, or rather, something in the water acting on the copper pipes, leaves blue stains in the sinks. Since excess copper should be avoided (tumors need it for angiogenesis), I drink bottle water. The best-tasting has a high mineral content, IMO.
I remember, years ago, that the San Pellegrino label contained a warning not to use it as one's sole source of drinking water. No explanation. Perhaps due to the high calcium level (179mg/L).? I avoid it because of the unfavorable magnesium:calcium ratio. One liter has 52 mg magnesium Mg:Ca= 0.29.
Apollinaris was my brand: Ca=90mg/L, Mg=120mg/L, Mg:Ca=1.33. & best tasting of all the well-known brands, IMO.
Apollinaris was bought by Coca-Cola from Cadbury-Schweppes in 2006, but Coke killed the brand in the U.S. a few years ago. Odd, since it seemed not to be competing with another Coke brand. Perhaps its market share was too small to bother with? So why not sell it to another beverage company?
Gerolsteiner has double the minerals of San Pellegrino: Ca=348mg/L, Mg=108mg/L, Mg:Ca=0.31 & actually leaves a chalky deposit on one's drinking glass.
Perrier is the worst: C=155mg/L, M=6mg/L, Mg:Ca=0.04.
My integrative medicine doctor says: "Calcium will make you old; magnesium will make you young."
With PCa, we try to avoid excess calcium, because of the studies associating high calcium with aggressive disease. But mostly, no-one cares about magnesium & PCa.
 A 2000 study from Taiwan looked at PCa deaths & "the levels of calcium and magnesium in the drinking water of these residents ..." "Data on calcium and magnesium levels in drinking water throughout Taiwan were obtained from the Taiwan Water Supply Corporation (TWSC)."
"The adjusted odds ratios for prostate cancer death for those with higher magnesium levels in their drinking water, as compared to the lowest tertile, were 0.73 ... and 0.64 ..., respectively."
Unfortunately, the Abstract doesn't state the mg/L of the three tertiles. All we see is more is better. Nor is the Mg:Ca ratio examined.
 It took until 2011 for a U.S. study to look at magnesium & PCa, & in particular, the Ca:Mg ratio.
"Ionized calcium (Ca) and magnesium (Mg) compete as essential messengers to regulate cell proliferation and inflammation. We hypothesized that inadequate Mg levels, perhaps relative to Ca levels (e.g. a high Ca/Mg ratio) are associated with greater prostate cancer risk." Bravo!
"In this biomarker sub-study of the Nashville Men's Health Study (NMHS), we included 494 NMHS participants, consisting of 98 high-grade (Gleason≥7) and 100 low-grade cancer cases, 133 prostate intraepithelial neoplasia (PIN) cases, and 163 controls without cancer or PIN at biopsy."
"Serum Mg levels were significantly lower, while the Ca/Mg ratio was significantly higher, among high-grade cases vs. controls"
"Elevated Mg was significantly associated with a lower risk of high-grade prostate cancer (OR = 0.26 ...)"
"An elevated Ca/Mg ratio was also associated with an increased risk of high-grade prostate cancer (OR = 2.81 ...)"
 (2013 - U.S.)
"TRPM7 is a novel magnesium-nucleotide-regulated metal current ... channel that is regulated by serum Mg(2+) concentrations. Changes in Mg(2+) concentration have been shown to alter cell proliferation in various cells ..."
"... an increase in the extracellular Ca(2+)/Mg(2+) ratio led to a significant increase in cell proliferation of prostate cancer cells when compared with control cells. Consistent with these results, age-matched prostate cancer patients also showed a subsequent increase in the Ca(2+)/Mg(2+) ratio and TRPM7 expression. Altogether, we provide evidence that the TRPM7 channel has an important role in prostate cancer and have identified that the Ca(2+)/Mg(2+) ratio could be essential for the initiation/progression of prostate cancer."