Calcium

Calcium is the chemical element brought to you by the symbol Ca and atomic number 20. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust.

Calcium and health
It is best known for being essential for healthy bones and teeth.

The US FDA recommends that most adult men and women need around 1000 mg of calcium per day, or roughly 3 cups of milk.

Since kidney stones consist primarily of calcium, folk wisdom held that excess calcium intake would increase the likelihood of getting kidney stones. It turns out, that this is only the case for calcium supplements; dietary calcium seems to have the opposite effect, i.e. calcium deficiency carries a marginally greater risk of kidney stones than excess dietary calcium intake does.

Hazards
Unlike magnesium, the metal itself reacts violently with water with the release of heat, if not as vehemently as sodium, potassium, rubidium, or cesium. Calcium metal tarnishes in an oxygen-rich atmosphere to form calcium oxide, itself a strong and corrosive alkali, that itself reacts with water to form a highly alkaline solution of calcium hydroxide. This may not be quite as dangerous as sodium hydroxide (lye), but it is caustic. Calcium salts otherwise are generally no more toxic than the ions attached. An exception is calcium chloride, which absorbs water even from flesh and can cause burns from such.

Cancer cure
Robert Barefoot (among others) has pushed his "Coral Calcium" cure for cancer since the late 1990s. His claim is that undetectably low levels of calcium can impair the body's immune system, and "Over 200 degenerative diseases are caused by calcium deficiency. That includes cancer, heart disease, diabetes, Alzheimer's, you name it." The FDA issued a warning to Barefoot in 2008 to cease and desist marketing his products as cures for cancer or anything else.

As with most woo, there is some kernel of "fact" that underlines the woo. There are preliminary studies that suggest a link between higher intake of calcium and reduced risks of cancer. There are also preliminary studies that suggest calcium and magnesium help with the reduction of neurological side effects from chemotherapy. This is a far cry from "curing" cancer.

Fat absorption
Preliminary studies on milk drinkers and weight loss suggest calcium might have a role in how much consumed fat is stored by the body, leading to a plethora of weight-loss products using calcium. Other studies suggested extra intake of calcium offered limited if any benefit to dieters.

Other woo
Calcium the Wonder Drug cures everything else as well: Gum disease, insomnia, menopause and arthritis, and PMS. There is no scientific evidence to back up the notion that calcium "cures" any of these ailments, and no studies are compelling enough to warrant medical intake beyond the daily recommended amounts.

Possible dangers of calcium supplements
As with any supplement, consuming too much or consuming it in a form unnatural to the body can pose risks. Studies have suggested that consuming calcium in an "over the counter form" may pose a greater risk for heart attacks. As with any drug, it is important to let medical professionals know what and how much calcium you are taking, especially if it is greater than the US FDA recommendations.