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Abnormal calcium metabolism has been implicated in human hypertension. Caffeine consumption may contribute to hypertension since it increases urinary calcium excretion. Nineteen hypertensive subjects (HTN) and nineteen age and gender matched normotensive controls (NTC) who habitually consumed at least 175 mg caffeine daily were studied before and after abstinence from all caffeine (CAF) consumption for 2 weeks. Caffeine abstinence (CAF-) increased fasting serum ultrafiltrable calcium in HTN and NTC, but not serum total calcium. Parathyroid hormone (PTH) levels decreased after CAF abstinence in 14 of 18 HTN subjects, including all seven subjects consuming less than 700 mg calcium daily. Three day dietary calcium intakes and 72 h urinary excretion of calcium were not different between CAF+ and CAF- or between HTN and NTC. A morning caffeine dose of 6 mg/kg lean body mass increased urinary Ca/creatinine ratios similarly for 2 h after beverage consumption in both HTN and NTC. Caffeine consumption stresses calcium metabolism in hypertensive individuals, especially those consuming less than 700 mg calcium daily.

Objective: Our purpose was to determine the effect of 2 weeks of caffeine abstinence on calcium (Ca) and bone metabolism in women habitually consuming caffeine and either low or moderate amounts of Ca., Methods: Participants were 25 women, aged 39-76 years (mean 65 years, median 57 years) habitually consuming at least 200 mg caffeine daily. Three days of dietary records and 24-hour urine collections were made immediately prior to collection of fasting blood and 1-hour urine in a metabolic unit. Women were classified as low Ca consumers (414-584 mg daily) or moderate Ca consumers (662-1357 mg daily) based on 6 days of diet records., Results: Women in the low Ca group had higher levels of serum ultrafiltrable Ca (UFCa) after caffeine abstinence (1.40 mmol/L CAF+ vs 1.52 mmol/L CAF-, p < 0.01), while there were no differences between experimental periods for UFCa in the moderate Ca group (1.35 mmol/L CAF+ vs 1.38 mmol/L CAF-, ns). Women in the low Ca group also had lower serum bone isoenzyme alkaline phosphatase levels after caffeine abstinence (9.3 U/L CAF+ vs 8.8 U/L CAF-, p < 0.05), while no significant changes were seen in bone alkaline phosphatase in the moderate Ca consumers (8.7 U/L CAF+ vs 8.9 U/L CAF-, ns). Fasting total serum Ca, urinary hydroxyproline/creatinine, and Ca/creatinine ratios were unchanged in both dietary groups. Three-day mean 24-hour urinary Ca excretion decreased after caffeine abstinence in the moderate Ca group only. There were no differences in dietary intakes of Ca from dairy products between CAF+ and CAF- in either the low or moderate Ca groups., Conclusion: Abstinence from moderate caffeine intake (mean 5.8 mg/kg lean body mass, 383 mg/day caffeine) raises ultrafiltrable Ca and decreases bone alkaline phosphatase in older women consuming < 600 mg Ca daily.

Thirty-seven women, aged 31-78 years, on two separate mornings consumed a decaffeinated beverage to which 6 mg caffeine/kg lean body mass or no caffeine were added. Total urine output of water, calcium, magnesium, sodium, chloride, potassium and creatinine increased in the two hours following caffeine ingestion when compared to the control beverage. Increased urinary mineral (mg)/urinary creatinine (g) ratios were seen for calcium (120 to 200), magnesium (70 to 110), sodium (3,800 to 6,200) and chloride (9,200 to 14,800), following the caffeinated beverage. Creatinine clearance did not change significantly. The percent reabsorption of calcium (98.6% to 97.5%, p less than .001) and magnesium (97.0% to 94.2%, p less than .0001) decreased significantly during the post-caffeine period. The calcium and magnesium filtered loads did not differ significantly between the caffeine and no caffeine beverages. Therefore, caffeine-induced urinary loss of calcium and magnesium is largely attributable to a reduction in calcium and magnesium renal reabsorption, although the physiological mechanism and tubular segment affected remain to be established.