Strontium (Sr) is a naturally occurring element found throughout the Earth’s crust, in seawater, and in many common foods. It belongs to the alkaline earth metal group, positioning it chemically near calcium on the periodic table. This close chemical relationship is the fundamental reason strontium interacts with the human body, particularly with bone tissue. Stable strontium is the form relevant to human biology and health. However, it is often confused with its radioactive isotope, Strontium-90, a product of nuclear reactions that poses a public health concern. This article clarifies the biological function of stable strontium and distinguishes its role from the hazards associated with its radioactive counterpart.
Strontium’s Role in Human Physiology
Stable strontium is classified as an ultra-trace element in the human body, typically found in very small amounts, around 320 milligrams in a 70 kg adult. Nearly all of the body’s strontium, approximately 99%, is concentrated within mineralized tissues like bone and teeth. The element is a “bone-seeker” because its ion, Sr²⁺, is chemically similar to the calcium ion, Ca²⁺, the primary mineral component of the skeleton.
This similarity allows strontium to be incorporated directly into the crystalline structure of hydroxyapatite, the mineral matrix of bone, where it replaces a small percentage of calcium ions. When strontium is absorbed through the digestive tract, the body handles it in a manner similar to calcium. Strontium absorption is less efficient than calcium, typically ranging from 20% to 30%. Strontium is distributed throughout the body via the bloodstream and is eventually excreted by the kidneys.
Therapeutic Use in Bone Health
The ability of stable strontium to incorporate into bone has been leveraged for therapeutic use, primarily in the management of osteoporosis. In pharmacological doses, strontium exerts a dual action on bone remodeling. Specifically, it stimulates the activity of osteoblasts, the cells responsible for forming new bone matrix, which increases bone formation. Simultaneously, strontium inhibits the function of osteoclasts, the cells that resorb old bone tissue, thereby reducing bone loss.
This unique mechanism is largely mediated by strontium’s interaction with the calcium-sensing receptor (CaSR) found on both osteoblasts and osteoclasts. The prescription medication, Strontium Ranelate, was developed to exploit this dual action and has been shown to reduce the risk of vertebral and non-vertebral fractures. However, the use of Strontium Ranelate has been restricted in many countries due to concerns over potential side effects, including an increased risk of cardiovascular events, such as deep vein thrombosis.
A different form, Strontium Citrate, is widely available as an over-the-counter dietary supplement, particularly in the United States. Strontium Citrate is not regulated as a drug and is not interchangeable with the prescription medication, Strontium Ranelate. Studies on the efficacy and long-term safety of the supplemental form are limited. Its incorporation into bone can artificially inflate bone mineral density readings on standard diagnostic scans, making it difficult to accurately assess true bone health and fracture risk.
Dietary Sources and Intake
Humans acquire stable strontium primarily through the diet. The element is present in varying concentrations in soil and water globally, and its concentration in food depends heavily on the soil composition where crops are grown. Common dietary sources include grains, leafy vegetables, and seafood, with dairy products also contributing.
Typical daily intake of stable strontium in North America and Europe is estimated to be low, often ranging from 1 to 10 milligrams per day. Deficiency is not a recognized health concern, as the body’s need for this trace element is minimal for normal physiological function. The strontium content in plant-based foods can be notably higher in items like spinach, lettuce, and celery, while filter-feeding shellfish are also concentrated sources.
The Hazards of Radioactive Strontium-90
The radioactive isotope, Strontium-90 (\(^{90}\)Sr), is a distinct substance from the stable element and represents a serious health hazard. This isotope is not naturally occurring; it is a byproduct of nuclear fission, generated by nuclear reactors or atomic weapons explosions. Strontium-90 is a public health concern because it is absorbed by the body through contaminated food and water, behaving like calcium.
Once ingested, the isotope is directed to the skeleton, where it is deposited in the bone and bone marrow. Since Strontium-90 has a relatively long half-life of nearly 29 years, it remains in the bone tissue for decades, continuously emitting high-energy beta radiation. This internal exposure can damage surrounding cells and DNA, significantly increasing the risk for serious health issues such as bone cancer and leukemia.