Caffeine calcium excretion effects

Massey, L. K., Opryszek, M. S., No effects of adaptation to dietary caffeine on calcium excretion in young women, Nutrition Research, 10, 741, 1990. 

Certain foreign compounds may cause the retention or excretion of water. Some compounds, such as the drug furosemide, are used therapeutically as diuretics. Other compounds causing diuresis are ethanol, caffeine, and certain mercury compounds such as mersalyl. Diuresis can be the result of a direct effect on the kidney, as with mercury compounds, which inhibit the reabsorption of chloride, whereas other diuretics such as ethanol influence the production of antidiuretic hormone by the pituitary. Changes in electrolyte balance may occur as a result of excessive excretion of an anion or cation. For example, salicylate-induced alkalosis leads to excretion of Na+, and ethylene glycol causes the depletion of calcium, excreted as calcium oxalate. 

Many observational studies have found an association between high caffeine intake and decreased BMD. This finding, however, could be a surrogate marker for decreased calcium-containing beverage and food intake. Although 2 to 5 cups of caffeine produce small increases in calcium excretion (about 4 to 5 mg calcium per cup of coffee), the effect can be offset with adequate calcium intake. 

However, a well-controlled clinical study by Heaney and Rafferty using calcium-balance methods found no impact of carbonated soft drinks containing phosphoric acid on calcium excretion. The study compared the impact of water, milk, and various soft drinks (two with caffeine and two without two with phosphoric acid and two with citric acid) on the calcium balance of 20- to 40-year-old women who customarily consumed 3 or more cups (680 mL) of a carbonated soft drink per day. They found that, relative to water, only milk and the two caffeine-containing soft drinks increased urinary calcium, and that the calcium loss associated with the caffeinated soft drink consumption was about equal to that previously found for caffeine alone. Phosphoric acid without caffeine had no impact on urine calcium, nor did it augment the urinary calcium loss related to caffeine. Because studies have shown that the effect of caffeine is compensated for by reduced calcium losses later in the day, Heaney and Rafferty concluded that the net effect of carbonated beverages—including those with caffeine and phosphoric acid—is negligible, and that the skeletal effects of carbonated soft drink consumption are likely due primarily to milk displacement. 

Dietary calcium has a relatively small impact on urinary calcium (e.g., only 6-8% of an increase in dietary calcium intake will appear in the urine). The major food components that affect urinary calcium are protein, phosphorus, caffeine, and sodium. For each 50-g increment in dietary protein, approximately 1.5 mmol (60 mg) of additional calcium is lost in urine. The higher amounts of phosphorus consumed concurrently with a high-protein diet can blunt, but not eliminate, this phenomenon. Dietary phosphorus (as well as intravenously administered phosphorus) increases PTH synthesis and subsequently stimulates renal calcium reabsorption and reduces the urinary excretion of calcium. Caffeine causes a reduction in renal reabsorption of calcium and a subsequently increased loss of urinary calcium soon after it is consumed. It has been shown repeatedly in animals and humans that dietary sodium, in the form of salt (NaCl), increases urinary calcium excretion. On average, for every 100 mmol (2300 mg) of sodium excreted in urine, there is an approximately 0.6-1 mmol (24-40 mg) loss of calcium in free-living healthy populations of various ages. Because most of the urinary calcium is of bone origin, it is commonly hypothesized that those nutrients or food components that are hypercalciuretic are also detrimental to the skeleton. On the other hand, thiazide medications are hypocalciuric and, as such, may have modest positive effects on bone. 

Massey, L. K., Wise, K. J., The effect of dietary caffeine on urinary excretion of calcium, magnesium, sodium and potassium in healthy young females, Nutrition Research, 4, 43, 1984. 

Caffeine is also a potent stimulant of gastric juice, hydrochloric acid, and pepsin secretion. The serum gastrin concentration may be increased by as much as five times after the ingestion of three cups of coffee. Coffee has a diuretic effect and also increases the excretion of erythrocytes and renal tubular cells in the urine. Caffeine increases the absolute amounts of sodium, potassium, calcium, and magnesium in urine—an effect not observed with decaffeinated coffee. 

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