People on bone-building supplements have often wondered if they should be taking Strontium and Calcium supplements together. In a purely technical sense, the answer is ‘No’. The two supplements are to be had apart. In the strict sense Strontium should be had an hour before breakfast time and approximately three hours after dinner.  In other words, it should be had on an empty stomach. In almost a contrast of dosage timings, Calcium is taken with food so that when food nutrients are being metabolised, the calcium absorption happens along with the other nutrients in the same digestive phase.
However, it seems almost impractical to follow the schedule mentioned above, without making it the follow-up/tracking and focus of the whole day on which supplement is to be had at what time. So why do the doctors set apart the timings of the two supplements? The answer lies in the way Strontium and Calcium are absorbed in the gastrointestinal tracts.
It is understood by the medical fraternity that both the Strontium and the calcium supplements are absorbed in our gastrointestinal tracts using the same mechanism. Thus, if we take both the supplements together or at the same time, the two will compete for absorption by the body. Studies run on this issue have demonstrated that when the two supplements are taken together, the system absorbs twice as much Calcium than it does Strontium. This makes the bioavailability of Strontium an issue despite taking the supplement.
A report published by the European Medicines Agency (EMA) titled Osseor, INN-strontium ranelate Summary of Product Characteristics) – clearly says that intake of food, milk, derivative products as well as medicines reduces the bioavailability of Strontium by as much as 60% – 70%. For this reason, it goes on to recommend that strontium products should be separated by at least two hours from food intake time. 
In fact, the dynamics of the two supplements can also be looked at from another perspective – from that of how they work once they are absorbed into the blood stream. Both Strontium and Calcium supplements work in very similar ways, they excite the calcium receptors which in turn trigger bone formation. This means the production of osteoblasts (i.e bone cells responsible for bone formation) now begins and there is a reduction in the production of osteoclasts (i.e. bone cells responsible for removal of bone tissue and bone minerals) Together the process helps to increase bone density.  When we take both these supplements together, both the minerals will compete for ‘x’ number or volume of calcium receptors with calcium overtaking Strontium and thus rendering the intake of strontium supplement less than optimum. Since it is important that Calcium and Vitamin D be taken along with Strontium for it to be able to deliver the maximum bone health benefits, it is advisable that the intake of Calcium and Strontium be taken apart from one another in the way prescribed by the doctors.
- Can You Take Calcium with Strontium?; End Fatigue; December, 2011; http://www.endfatigue.com/health_articles_o-q/Osteoporosis-calcium_with_strontium.html
- Summary of Product Characteristics (Page 4); European Medicines Agency; December, 2011; http://www.ema.europa.eu/docs/en_GB/document_library/EPAR_-_Product_Information/human/000561/WC500051121.pdf
- Strontium Support II (Video); Innovative Research Scientific Integrity; December, 2011; http://www.aor.ca/html/products.php?id=97
For more technical/scientific information on Strontium and Calcium absorption, you may read the Clinical Chemistry report at:
- Intestinal Strontium Absorption: From Bioavailability to Validation of a Simple Test Representative for Intestinal Calciurn Absorption; Clin Chem; December, 2011; http://www.clinchem.org/content/41/10/1446.full.pdf