Does Your Calcium Supplement Contain Strontium?

When it comes to maintaining strong, healthy bones, calcium supplements are popular. But did you know some calcium supplements also contain a mineral called strontium?

Why is strontium added to some supplements? Strontium is a naturally occurring mineral found in small amounts in sedimentary rock, soil, and groundwater. Calcium (Ca), strontium (Sr), and barium (Ba) are grouped on the periodic table for their similar chemical behaviors, so strontium can interact with the body's bone-building processes and is sometimes added to bone health supplements.

Strontium ranelate was once available in the US by prescription for osteoporosis treatment, but production stopped in 2017, largely due to serious cardiac-related adverse effects. This post focuses on the form typically found in supplements: strontium citrate.

What are the concerns? Strontium's molecular weight is heavier than calcium. Strontium absorbed into bone (similar to calcium) causes DXA scan results to be inaccurate. Per Mirza and colleagues: "Much of the bone density increase being seen with SrC [strontium citrate] is likely due to the replacement of calcium with strontium in the bone. Strontium has a larger atomic weight than calcium (38 vs 20), which causes it to attenuate X-rays more strongly than calcium. Hence the readings on the DEXA scan appear artificially inflated with bone density looking better than is the case. Comparisons of Ca hydroxyapatite (natural bone crystal) and Sr hydroxyapatite (Sr-substituted bone crystal) have revealed that for every 1% of strontium incorporated into bone crystal, BMD [bone mineral density] is overestimated by 10%." Strontium citrate's biological half-life can be as long as 18 years, meaning the DXA overestimation persists for a very long time.

A note about AlgaeCal. AlgaeCal strongly promotes strontium for bone building and has a website section addressing "myths" about the mineral. They cite 3 studies to support their position — the same 3 studies for years:

1. Michalek JE, Preuss HG, Croft HA, et al. Changes in Total Body Bone Mineral Density Following a Common Bone Health Plan with Two Versions of a Unique Bone Health Supplement. Nutrition Journal 2011;10,32. — Not a randomized control trial; partly funded by Integrative Health Technologies, Inc. (a company with strong connections to AlgaeCal); one author (GR Kaats) is on AlgaeCal's advisory board.
2. Kaats GR, Preuss HG, Croft HA, Keith SC, Keith PL. A Comparative Effectiveness Study of Bone Density Changes in Women Over 40... International Journal of Medical Sciences 2011;8(3):180-191. — Not double-blind/randomized; 3 of 5 authors were employed by Integrative Health Technologies at publication time.
3. Kaats GR, Preuss HG, Stohs S, Perricone N. A 7-Year Longitudinal Trial of the Safety and Efficacy of a Vitamin/Mineral Enhanced Plant-Sourced Calcium Supplement. Journal of the American College of Nutrition 2016;35(2):91-99. — Not randomized controlled; funded by a grant from AlgaeCal Inc., as disclosed at the study's end.

Caution recommended. Despite the claims, there is no high-quality research showing strontium citrate to be effective against osteoporosis. Low-quality, company-funded studies may show promising results, but must be validated with rigorously designed, double-blind, placebo-controlled trials with large populations before establishing efficacy and safety. If you take any form of strontium, tell your healthcare provider.

References

1. Mirza FS, et al. Change in Bone Mineral Density with Strontium Citrate: An Illusion or Reality. Journal of Nutritional Health and Food Science 2016; 4(3): 1-3.