Calcium Intake and the Risk of Nephrolithiasis
Aim 1: To determine whether the propensity to form renal stones in healthy postmenopausal women differs between dietary calcium from milk and supplemental calcium from calcium carbonate.
Aim 2: To compare the effect of two different forms of calcium, milk vs. calcium carbonate, on calciotropic hormones and bone turnover status in healthy postmenopausal women.
In this short-term metabolic study, we will compare two commonly used calcium sources, calcium from milk and calcium carbonate supplements, in 35 healthy postmenopausal women. Surrogate endpoints will be urinary calcium, urinary saturation with respect to calcium oxalate, bone turnover markers and serum PTH suppression. 400 mg twice daily elemental calcium from milk or calcium carbonate will be added to a basal diet containing 400mg elemental calcium, for a total daily intake of 1,200mg elemental calcium (the recommended daily allowance for postmenopausal women). Vitamin D intake will be maintained near 1,000IU daily across phases.
In this crossover trial, subjects will undergo three phases of study, the order of which will be randomized by utilizing 3x3 Latin Squares. The phases will be comprised of a Milk Phase, Calcium Carbonate Phase, and Placebo Phase. Each phase will be 3 weeks in duration. A washout period of 1 week will be imposed between the phases. During this period, no additional calcium will be given apart from subject[AND]amp;apos;s customized diet.
* During the Milk Phase, subjects will drink 320mL (10.7 oz) of skim milk once with breakfast and once with dinner (giving a total of 400mg of elemental calcium and 133IU of vitamin D3 per serving). Eight hundred international units of additional vitamin D3 will be given each day during the Milk Phase (giving a total of 1066IU vitamin D3 per day).
* During the Calcium Carbonate Phase, subjects will take in calcium carbonate in the form of Tums (RegisteredTM) (200mg of elemental calcium per tablet) 2 tablets with breakfast and with dinner plus 1,000IU of supplemental vitamin D3 will be given per day.
* During the Placebo Phase, subjects will be given 2 tablets similar in shape and size to the calcium carbonate tablets with breakfast and with dinner. One-thousand international units of vitamin D3 will be given each day.
Twenty-four hour urine will be obtained on days 20 and 21 of each study phase. The urine collection on day 21 will be collected in one hour intervals for the first 4 hours of collection. Blood for a basic metabolic panel and iPTH will also be collected hourly with the hourly urine collections. This data will be used to study the acute effects of milk versus calcium carbonate in terms of absorption and urinary parameters. On the morning of days 21 and 22 of each phase, a fasting venous blood sample will also be obtained for secondary endpoints.
Postmenopausal women, less than 80 years of age