THE EFFECT OF CAFFEINE ON URINARY MINERAL LOSS AND BONE METABOLISM

Thirty-seven women, aged 31 to 78 yr (mean = 60.1), had greater urinary calcium, magnesium, sodium, and chloride to creatinine ratios in the two hours following ingestion of a beverage containing caffeine when compared to the two hours following a decaffeinated beverage. Potassium and phosphorus to creatinine ratios did not increase. The increases in calcium and magnesium were due to decreased percent reabsorption of these minerals by the kidney. Calcium reabsorption dropped from 98.77% to 97.43%, p < .0001; magnesium from 96.99% to 94.19%, p < .0001. This resulted in an increased total urinary calcium loss.;A study employing 6 day old mouse calvaria in tissue culture found that added caffeine at concentrations ranging from 2.3 x 10-5 M to 2.3 x 10-3 M does not directly act upon bone to release calcium.;These findings suggest that moderate caffeine consumption (about 2 cups of coffee) leads to increased urinary loss of calcium. Moderate caffeine intake coupled with less than 600 mg calcium intake/day leads to decreased serum calcium, decreased serum ultrafiltrable calcium and increaed alkaline phosphatase bone isoenzyme suggesting increased bone turnover. Therefore, caffeine consumption may increase the risk of bone loss and development of osteoporosis in women consuming very low intakes of calcium.;The eleven lowest calcium consumers (<600 mg/day) had reduced serum (88.30 to 85.75 mg/dl, p < .025) and ultrafiltrable (60.56 to 55.95 mg/dl, p < .025) calcium concentrations during three days of usual, moderate caffeine consumption (mean = 5.74 mg caffeine/kg lean body mass) when compared to three similar days following two weeks abstinence from caffeine. They also had increased serum alkaline phosphatase bone isoenzyme activities (8.80 to 9.29 U/l, p = .033) indicating increased bone turnover during caffeine consumption. Women consuming 600 mg or more of calcium daily showed no such changes between the caffeine and no-caffeine periods.