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Answer:

The caffeine in coffee can cause a slight increase in magnesium excreted in the urine, but because coffee provides more magnesium than is lost, it is not necessary to supplement with magnesium, or get more of it from your diet, just because you drink coffee.

Interestingly, there is some evidence that using water containing some magnesium to brew coffee may improve its flavor. Coffee prepared with water without minerals such as magnesium, tends to be sour, while coffee prepared with water containing low amounts of minerals may have a milder taste (Pangborn, J Food Sci 1971). It’s thought that magnesium and other minerals bind to acids and other compounds in ground coffee that affect flavor, modifying their levels in brewed coffee (Herndon, J Agric Food Chem 2014).

Most water already includes some magnesium. One study in North American cities found tap water to average 10 to 20 mg/L, spring water 8 mg/L, and mineral water 24 mg/L, although amounts varied considerably by source (Azoulay, J Gen Intern Med 2001). A coffee company, Five Senses Coffee, found that coffee brewed with water containing 12 to 24 mg/L of magnesium (from magnesium chloride) was reportedly sweeter, stronger and more acidic than coffee brewed with higher amounts of magnesium (36 to 48 mg/L), which was "grassy and chalky."

A competitor of Starbuck’s reported that Starbucks appears to add potassium to some of its coffees — probably to reduce the acidity.

Although some websites suggest that adding ground eggshell (which naturally contains calcium carbonate) to coffee might reduce its acidity and improve flavor, the benefit, if any, is likely very small because ground eggshells have low solubility in coffee or water, which limits its buffering capacity. As shown in a laboratory study, pouring brewed coffee (at room temperature) over ground eggshells (with egg membrane removed before grinding) did not significantly affect its acidity (Aditya, J Food Process Eng 2020).

Learn more about magnesium, including how much you should be getting each day and foods that contain magnesium, plus our Top Pick among supplements in our Magnesium Supplements Review.

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