Calcium supplements and 24-hour blood pressure

Many people take calcium supplements to improve their bone health. However, studies have shown that people who take calcium supplements long-term have a slightly higher chance of having a heart attack. How calcium supplements increase the risk of a heart attack is not known, but it might be through an effect on our blood pressure. In a recent study we found that when women took a calcium supplement in the morning; their blood pressure was higher for the next several hours than when they took a placebo (a pill containing no calcium). If this increase in blood pressure is repeated every time a calcium supplement is taken long-term it could be what is causing the increased risk of a heart attack. Calcium supplements are usually taken twice a day, in the morning and evening. The aim of this study is to find out how calcium supplements affect our blood pressure over a full day and night. To do this we need to measure blood pressure for 24 hours in women taking a morning and evening dose of a calcium supplement. We also want to find out if any effects of calcium supplements on blood pressure persist when people take them long-term. To do this we want to repeat the 24-hour measurement of blood pressure after women have taken a calcium supplement for 4 weeks. We would like to enrol 50 healthy women who have been through the menopause for at least 5 years. This study will compare two treatments: a. 0.5 grams of calcium as citrate (a calcium supplement) in the morning and evening b. A placebo (pills containing no calcium) in the morning and evening At each visit to our clinic participants will be given either the calcium supplement or the placebo along with a standardised breakfast meal. They will take their second dose of calcium or placebo pills that evening at home. During each visit to our clinic an ambulatory blood pressure monitor will be fitted on participants by study staff. This monitor will measure blood pressure every 30 minutes during the day and every 60 minutes during the night. Participants will be asked to keep an activity diary during the blood pressure monitoring periods. The first and second visits will be separated by at least 7 days during which participants will not be required to take any supplements. The second and third visits will be separated by 4 weeks during which half of the participants in the study will take the calcium supplement in the morning and evening every day and half will take the placebo. Participants will not know whether they are taking calcium or placebo at any of your visits or during the 4 weeks of supplementation. This study is awaiting ethical approval from the New Zealand Health and Disability Ethics Committee.