Abstract: We included 16 trials with 3048 participants. None of the studies reported hypertension as a dichotomous outcome. The effect on systolic and diastolic blood pressure was mean difference (MD) -1.43 mmHg (95% confidence interval (CI) -2.15 to -0.72) and -0.98 mmHg (95%CI -1.46 to -0.50) respectively. The effect on systolic and diastolic blood pressure for those younger than 35 years (7 trials with 399 participants) was -2.11 mmHg (95%CI -3.58 to -0.64) / -2.61 mmHg (95% CI -3.74, -1.49). The effect on systolic and diastolic blood pressure for those 35 years or more (9 trials with 2649 participants) was -0.96 mmHg (95%CI -1.83 to -0.09) / -0.59 mmHg (95%CI -1.13 to -0.06). The effect on systolic and diastolic blood pressure for women (6 trials with 1823 participants) was -1.45 mmHg (95% CI -2.78 to -0.12) / -0.92 mmHg (95% CI -1.71 to -0.14). The effect on systolic and diastolic blood pressure for men (5 trials with 617 participants) was -2.07 (95%CI -3.56 to -0.59] / -1.91 (95%CI -2.80 to -1.02).The quality of evidence for each of these outcomes was high. The effect is consistent in both genders regardless of baseline calcium intake. The effect on systolic blood pressure was 0.08 mmHg (95% CI -2.16 to 2.32) with doses less than 1000 mg, -1.14 mmHg (95% CI -2.01 to -0.27) with 1000 - 1500 mg, and -2.79 mmHg (95% CI -4.71 to -0.86) with more than 1500 mg. The effect on diastolic blood pressure was -0.54 mmHg (95% CI -2.23 to 1.15), -0.71 mmHg (95% CI -1.37 to -0.06) and -1.43 mmHg (95% CI -2.22 to -0.64) respectively. The quality of evidence for each of these outcomes was high. None of the studies reported adverse events. An increase in calcium intake slightly reduces both systolic and diastolic blood pressure in normotensive people, particularly in young people, suggesting a role in the prevention of hypertension. These results should be interpreted with caution, since the proposed biological mechanism explaining the relationship between calcium and blood pressure has not been fully confirmed. The effect across multiple prespecified subgroups and a possible dose response effect reinforce this conclusion. Even small reductions in blood pressure could have important health implications for reducing vascular disease. There is a great need for adequately-powered clinical trials randomising young people. Subgroup analysis should involve basal calcium intake, age, sex, basal blood pressure, and body mass index. We also require assessment of side effects, optimal doses and the best strategy to improve calcium intake.

Plain-language summary: We selected studies that assessed the effect of dietary calcium interventions such as supplementation or food fortification on blood pressure in normotensive people of all ages. The last search date was October 2014. This review analysed information from 16 trials (3048 participants). We found that an increase in calcium intake slightly reduces both systolic and diastolic blood pressure 1.43 mmHg lower and 0.98 mmHg lower respectively. This effect was higher with doses of calcium above 1000 mg/day. Systolic blood pressure was reduced by 1.14 mmHg with doses of calcium 1000 to 1500 mg/day and by 2.79 mmHg with doses of calcium equal to or over 1500 mg/day. We noted a reduction in blood pressure in both men and women and at ages from 11 to 82 years old, but the reduction was greater among younger people. Systolic blood pressure was reduced by 2.11 mmHg among those less than 35 years and by 0.96 mmHg among those 35 years or older. None of the studies reported adverse events. We need further research to determine the ideal dosage of supplementation and whether it is more effective and safer as part of the diet or as a supplement. We found high quality of evidence for systolic and diastolic blood pressure in both men and women. The quality of evidence was also high for participants 35 years or older and moderate for younger people. The quality of evidence was high for doses of calcium of 1000 to 1500 mg/day and was moderate for lower or higher doses. Five of the 16 trials were industry funded.