Optimizing calcium and vitamin D intake through diet and supplements

Although calcium and vitamin D are often recommended for prevention and treatment of osteoporosis, considerable controversy exists in terms of their safety and efficacy.¹ This article highlights the issues, referring readers to reviews and meta-analyses for details and providing some practical advice for patients requiring supplementation.

CALCIUM INTAKE AND BONE DENSITY

Calcium enters the body through diet and supplementation. If intake is low, blood calcium levels fall, resulting in secondary hyperparathyroidism, which has 3 main effects:

• Increased fractional absorption of the calcium that is consumed
• Reduced urinary excretion of calcium
• Increased bone resorption, which releases calcium into the blood,² which explains the potential for the deleterious effect of deficient intake of calcium on bone.³

Based on the simple physiology outlined above, it seems logical that insufficient intake of calcium over time could lead to mobilization of calcium from bone, lower bone mineral density, and higher fracture risk.³ This topic has been reviewed by the European Society for Clinical and Economic Aspects of Osteoporosis, Osteoarthritis, and Musculoskeletal Diseases and the International Foundation for Osteoporosis.¹

Many lines of evidence suggest that low calcium intake adversely affects bone mineral density.¹ Low calcium intake has been associated with lower bone density in some cross-sectional studies,^4–6 though not all.⁷ Interventions to increase calcium intake in postmenopausal women have shown beneficial effects on bone density,^8–10 though in some studies the benefit was small and nonprogressive.¹¹ The question is whether this improvement in bone mineral density translates into fewer fractures.

Results from individual studies looking at fracture prevention through calcium supplementation have been conflicting,^10,12–14 and reviews and meta-analyses have summarized the data.^1,3,15 A recent review of these meta-analyses showed a small but significant reduction in some types of fracture.¹

Some speculate that the difficulty in demonstrating fracture efficacy might be due to imperfect compliance with calcium intake, and that the participants in the placebo groups often had fairly robust calcium intake from diet and off-study supplemental intake, which could reduce the sensitivity of studies to demonstrate the fracture benefit.^1,16

The US Preventive Services Task Force¹⁷ recommends that the general public not take supplemental calcium for skeletal health, but emphasizes that this recommendation does not apply to patients with osteoporosis. Most other official guidelines (eg, those of the Endocrine Society,¹⁸ American Association of Clinical Endocrinologists,¹⁹ Institute of Medicine,²⁰ and National Osteoporosis Foundation²¹) recommend adequate calcium intake to optimize skeletal health.

CALCIUM INTAKE AND CORONARY ARTERY DISEASE

Patients often wonder if the calcium in their supplements ends up in their coronary arteries rather than their bones. Although we once dismissed such concerns, several studies and meta-analyses have reported higher rates of cardiovascular disease with supplemental calcium use.^22–24 A proposed mechanism to explain this increased risk is that taking calcium supplements transiently raises the serum calcium level, resulting in calcium deposition in coronary arteries, accelerating atherosclerosis formation.²⁵

On the other hand, some studies and meta-analyses have not shown any increased risk of cardiovascular disease with calcium and vitamin D supplementation.^26,27 This subject has been reviewed by Harvey et al.¹

Our conclusions are as follows:

Patients should be told that the National Osteoporosis Foundation and the American Society for Preventive Cardiology released a statement in 2016 adopting the position that calcium intake from food and supplements should be considered safe from a cardiovascular perspective.²⁸

If patients want to avoid the possible increase in risk of cardiovascular disease due to calcium supplementation, they can optimize their calcium intake with dietary calcium. Observational studies that showed increased risk with supplemental calcium found no such increase in cardiovascular disease with a robust dietary intake of calcium.²⁹

This is not to say that patients should be encouraged to boost their dietary calcium intake and avoid heart disease by eating more cheese and ice cream, as these foods are high in saturated fats and cholesterol. Many dairy and nondairy sources of calcium do not contain these undesirable nutrients.

CALCIUM SUPPLEMENTATION AND NEPHROLITHIASIS

High dietary calcium intake has not been shown to increase the risk of kidney stones.

In the Nurses’ Health Study, the multivariate relative risk of stone formation was 0.65 (95% confidence interval [CI] 0.5–0.83) in those in the highest vs the lowest quintiles of dietary calcium intake.³⁰ In contrast, the relative risk of stones in those taking calcium supplements was 1.2 (CI 1.02–1.41),³⁰ although this higher risk was not seen in younger women (ages 27 to 44).³¹

Similar results were seen in the Women’s Health Initiative, in which calcium carbonate and vitamin D supplements resulted in a relative increased risk of stone formation of 1.17 (95% CI 1.02–1.34) compared with women on placebo.¹²

Data from male stone-formers also suggests that high dietary calcium intake does not increase the risk of stones.³²

A theory to explain the difference between dietary and supplemental calcium with respect to stone formation is that dietary calcium binds to oxalate in the gut and reduces its absorption. The most common type of kidney stones are composed of calcium oxalate, and the oxalate, not the calcium, may be the real culprit. In contrast, calcium supplements are often taken between meals and therefore do not exert this protective effect and may be absorbed more rapidly and raise the serum calcium level more, which could lead to higher urinary calcium excretion.³³