Dietary Calcium Intake and Bone Loss Over 6 Years in Osteopenic Postmenopausal Women

Sarah M. Bristow; Anne M. Horne; Greg D. Gamble; Borislav Mihov; Angela Stewart; Ian R. Reid

Disclosures

J Clin Endocrinol Metab. 2019;104(8):3576-3584. 

In This Article

Abstract and Introduction

Abstract

Context: Calcium intakes are commonly lower than the recommended levels, and increasing calcium intake is often recommended for bone health.

Objective: To determine the relationship between dietary calcium intake and rate of bone loss in older postmenopausal women.

Participants: Analysis of observational data collected from a randomized controlled trial. Participants were osteopenic (hip T-scores between −1.0 and −2.5) women, aged >65 years, not receiving therapy for osteoporosis nor taking calcium supplements. Women from the total cohort (n = 1994) contributed data to the analysis of calcium intake and bone mineral density (BMD) at baseline, and women from the placebo group (n = 698) contributed data to the analysis of calcium intake and change in BMD. BMD and bone mineral content (BMC) of the spine, total hip, femoral neck, and total body were measured three times over 6 years.

Results: Mean calcium intake was 886 mg/day. Baseline BMDs were not related to quintile of calcium intake at any site, before or after adjustment for baseline age, height, weight, physical activity, alcohol intake, smoking status, and past hormone replacement use. There was no relationship between bone loss and quintile of calcium intake at any site, with or without adjustment for covariables. Total body bone balance (i.e., change in BMC) was unrelated to an individuals' calcium intake (P = 0.99).

Conclusions: Postmenopausal bone loss is unrelated to dietary calcium intake. This suggests that strategies to increase calcium intake are unlikely to impact the prevalence of and morbidity from postmenopausal osteoporosis.

Introduction

Calcium is an essential element in the diet, but there is ongoing controversy regarding the optimal intake for bone health. This is reflected in the variety of calcium intakes currently recommended, for example, 700 mg/day in the United Kingdom[1] and 1300 mg/day in the United States, Australia, and New Zealand.[2,3] Calcium intakes fall below the recommended levels in many areas of the world,[4] and increasing calcium intake remains a widely promoted strategy for preventing osteoporosis.[5] The negative renal,[6] gastrointestinal,[7] and cardiovascular[8] effects of calcium supplements have turned the focus to an increase of calcium intake through the diet.[9] However, whether calcium intake across the typical dietary range influences the preservation of bone mass has not been established.

Recommendations regarding calcium intake have been based on calcium-balance studies, in which calcium balance is used as a surrogate for bone balance. An early analysis of calcium-balance data indicated that intakes of 1500 mg/day achieved neutral balance in postmenopausal women,[10] implying that this intake would completely prevent postmenopausal bone loss. Subsequently, the modest bone-density benefit in randomized controlled trials (RCTs) of calcium supplements has been interpreted as evidence of this requirement for higher intakes of calcium. In contrast, a more recent analysis of calcium-balance data found that neutral balance was achieved for all adults at 741 mg/day and that balance was highly resistant to a change in intake across the typical dietary range (415 to 1740 mg/day).[11] Furthermore, the increase in bone mineral density (BMD) in RCTs of calcium is now understood to be a one-off gain that occurs during the first year of supplementation, independently of dietary calcium intake (i.e., it is not greater among those with lower intakes).[12,13] Thus, evidence from calcium balance studies and RCTs does not clearly support the concept of calcium deficiency across the typical dietary range.

These methods have limitations in assessing the relationship between dietary calcium and bone loss. RCTs study the effects of a substantial increase in calcium intake (usually$1000 mg/day), in addition to usual dietary intake, and do not compare intakes within the typical dietary range. The precision of calcium balance in reflecting bone balance is uncertain,[2] and calcium balance has not been shown to predict fracture risk. An alternative method to assess this relationship is to assess directly bone balance through sequential measurements of BMD [which is predictive of fracture risk[14]] and relate this to an individual's regular diet. Many previous observational studies may have been confounded by high rates of hormone replacement therapy (HRT) use, as users of HRT have higher calcium intakes than nonusers.[15,16] Therefore, we have used data from the placebo group of an RCT to study the relationship between dietary calcium intake and BMD at baseline and change in BMD over 6 years in osteopenic postmenopausal women. This cohort was free from calcium supplements and other medications known to influence calcium or bone metabolism at baseline and throughout follow-up.

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