The purpose of this study is to measure the effect of omeprazole on intestinal calcium absorption in postmenopausal women. Because older adults frequently take PPI, it is critical to understand the impact of PPI therapy on calcium homeostasis. In this study, I hypothesize that PPI therapy does not alter intestinal calcium absorption in postmenopausal women.

Existing literature makes it unclear whether PPI therapy truly decreases intestinal calcium absorption. Up to 25 postmenopausal women will participate in this two-month study. The primary study outcome is the change in intestinal calcium absorption following omeprazole therapy. The secondary outcomes include the change in urine n-telopeptide, the month-to-month variability in intestinal calcium absorption and the establishment of a DNA bank for genetic studies of calcium homeostasis.

We will interview women and review their medical records to determine eligibility. Eligible subjects will undergo three GCRC studies. The first 2 studies will determine the monthly variation in calcium absorption, while the 3rd study will occur after taking 40 mg of omeprazole daily for 30 days. Women will present to the GCRC in the early morning and receive an oral and intravenous calcium tracer with breakfast. Over the next 24 hours, we will collect all urine for measurement of its calcium content. During the first or second GCRC stay, we will measure each subject's gastric pH by collecting gastric fluid from a temporary nasogastric tube. In consenting subjects we will collect one tube of blood, isolate its DNA and save it indefinitely within the locked GRECC Drezner Laboratory (5th Floor, VA Hospital) permitting future genetic studies related to calcium homeostasis.

• Osteoporosis
• Achlorhydria
• GERD
• Hip Fracture

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Inclusion Criteria:

• Women at least 5 years past menopause, defined as the last date of menses

Exclusion Criteria:

• Allergy/Intolerance to orange juice
• Allergy/Intolerance to omeprazole or other PPI therapy
• Use of drugs that interact with omeprazole including oral anti-fungal agents, coumadin, diazepam, phenytoin & tacrolimus
• Use of antacids, PPI or H2-blocker therapy within the past two months
• Intestinal conditions associated with malabsorption or low gastric acid levels including Crohn's Disease, ulcerative colitis, pernicious anemia, bacterial overgrowth, celiac sprue, chronic diarrhea or use of antibiotics within the past month
• Known Stage 4 or 5 Chronic Kidney Disease, defined as an estimated GFR <30 cc/minute
• Use of medications known to interfere with calcium metabolism, including oral steroids or anticonvulsants