Sr – Strontium is found in igneous rocks at 375 ppm; shale at 300 ppm; sandstone at 20 ppm; limestone at 610 ppm; fresh water at 0.08 ppm; sea water at 8.1 ppm; soils at 300 ppm; marine plants at 260 to 1 400 ppm; land plants at 26 ppm; marine animals at 20 to 500 ppm; land animals at 14 ppm (highest in mammalian bone).
Strontium can replace calcium in many organisms (including man) – there is considerable evidence for essentiality in mammals including man.
Deficiencies of Sr are associated with certain types of Ca and B resistant osteoporosis and arthritis.
Strontium 90, the man made product of fission atomic explosions and the greatest biohazard fear during the cold war, does not occur in nature.
Strontium is a metallic element, in the same chemical family as calcium and magnesium. The biological behavior of strontium resembles that of calcium. Strontium occurs in high-calcium foods like milk and milk products. It is stored in bones and teeth. Notoriety surrounding strontium focuses on the radioactive isotope, strontium-90, a product of nuclear fallout. This radioactive element is stored in the bones of all vertebrates.
Presently, strontium might be an essential nutrient for humans. Omission of strontium from a purified diet for experimental animals led to slowed growth. The incidence of dental caries has been found to be lower in areas with high strontium levels in drinking water. Chronic consumption of highly refined foods, deficient in strontium and other nutrients, correlates with decreased bone strength. Ingesting pharmacological doses (gram amounts) of strontium for periods up to three years has been reported to reduce bone pain and increase bone density in a group of osteoporotic patients. Typical U.S. consumption is several milligrams daily.
The possibility that strontium or barium are essential has not been confirmed (or denied) despite the claim of Rygh (1949). He reported that the omission of either strontium or barium from the diet resulted in depressed growth and reduced calcification of bones and teeth in rats and guinea pigs. Both Sr and Ba can spare calcium and are relatively nontoxic. They also show some stimulatory action.
Gerlach and Muller reported that the strontium concentration of a wide variety of animal tissues ranged from 0.01 to 0.10 mcg/g, with no evidence of accumulation in any particular species, soft organ, or tissue. Subsequent analyses showed similar concentrations in a variety of human organs.
Blood or serum strontium apparently is depressed in patients with cholecystitis or myocardial infarction, and elevated in patients with primary carcinoma of the liver.
In general, the metabolism and distribution of strontium mimics that of calcium. The major sites of retention of both elements are the skeleton and teeth, with aorta a distant third. Strontium is incorporated mainly in the mineral phase of bones and teeth. The strontium content of bone has attracted much interest because of the possible problem of strontium-90 (90Sr) retention from radioactive fallout. The total strontium content of the standard reference man has been reported to be 323 mg, of which 99% is present in the bones.
Strontium occurs in the enamel and dentin of teeth in concentrations that parallel the levels in the bones of these same individuals and of those from similar geographical locations. Strontium is deposited primarily before eruption, during tooth calcification, is mostly permanently retained, and is not affected by fluoride in the drinking water.
Foods and Feedstuffs
In general, foods of plant origin are appreciably richer sources of strontium than are animal products, except where the latter include bone. Strontium tends to be concentrated in the bran rather than in the endosperm of grains and in the peel of root vegetables. Brazil nuts are an especially rich source of strontium. Like calcium, milk and milk products can contribute a major percentage of strontium intake; values between 11% and 32% of total dietary intake have been reported.
Early studies showed that the metabolism of strontium was similar to but not identical with that of calcium. However, wherever there is a metabolically controlled passage of ions across a membrane (e.g., gastrointestinal absorption, renal excretion, lactation, and placental transfer), calcium apparently is transported more effectively than strontium. Nonetheless, physiological and nutritional variables that affect strontium metabolism are similar to those that affect calcium metabolism and usually operate in the same direction. These variables include the following:
1. Calcium and strontium are better absorbed by young than old animals.
2. The stresses of pregnancy and lactation increase the efficiency of absorption of calcium and strontium.
3. The intestinal site of greatest absorption of both calcium and strontium is the duodenum, but the site of most effective (efficiency X residence time) absorption is the ileum.
4. Factors that enhance calcium and strontium absorption are vitamin D, lactose, and specific amino acids such as lysine and arginine.
5. The inclusion of alginates, or fiber such as cellulose, in the diet depresses both strontium and calcium absorption.
6. Parathyroid hormone accelerates the resorption of bone strontium, as it does calcium.
7. Magnesium deficiency depresses both strontium and calcium absorption.
In addition, the absorption of strontium is increased under fasting conditions and is decreased in the presence of food. Raising dietary calcium intakes from low to normal reduces strontium retention and supplementation with calcium plus phosphorus is more effective in reducing strontium retention than calcium alone. Increasing the dietary levels of the alkaline earth elements also depresses radiostrontium retention, with strontium being the least effective. Intestinal strontium absorption by adults of various mammalian species ranges from 5 to 25%, with age changes in the same species varying from >90% in very young to 0% in old individuals.
Strontium is poorly retained by humans. In the adult individual, net retention is essentially zero, or a steady state exists. Excretion occurs via the kidneys and apparently the bile
Absorbed strontium is carried in the blood to the tissues, where the little that is retained is preferentially deposited by two distinct processes in the bones and teeth. Hodges et al. have described these as a rapid incorporation phase attributed to the blood strontium deposited by ionic exchange, surface absorption, and/or preosseous protein binding, and a slow incorporation of strontium into the lattice structure of the bone crystals during their formation. Both processes are believed to depend mainly on the strontium concentration of the blood, including the cord blood in view of the early appearance of strontium in the bones and teeth of human fetuses.
2. Essentiality, Function, and Interactions
There is no conclusive evidence that strontium is essential for living organisms, although in 1949 Rygh reported that the omission of this element from the mineral supplement fed to rats and guinea pigs consuming a purified diet resulted in growth depression, an impairment of the calcification of bones and teeth, and a higher incidence of carious teeth. This report has been neither confirmed nor invalidated. However, findings of Colvin and co-workers indicated that some of the calcium requirement of growing chicks could be spared by strontium. The possibility that strontium can partially fulfill the calcium requirement is supported by findings such as that of MacDonald, who found that the inhibition of beef brain cortex adenylate cyclase activity in vitro by a chelating agent EGTA, though reversed completely by calcium, was partially reversed by strontium. This suggests that strontium can substitute for calcium in some enzyme systems.
The suggestion that strontium deprivation decreases the incidence of carious teeth has not been supported well by subsequent research. Only epidemiological data suggest strontium is cariostatic. Curzon and co-workers described a curvilinear association between caries incidence and the strontium concentration in drinking water that was related to the enamel concentration of strontium. Schamschula et al. found that plaque strontium concentration was inversely related to caries incidence. However, when strontium was specifically administered orally to rats, no evidence was obtained that indicated strontium is a cariostatic agent. Instead, relatively high oral ingestion of strontium during the pre-eruptive period caused an increase in dental caries, and during the period of amelogenesis eliminated the cariostatic action of fluoride, in rats. Feeding strontium to female rats during gestation and lactation resulted in increased caries incidence in offspring. Chaudhri found that the strontium content was higher in decayed than in healthy teeth of Australian children. Furthermore, Beighton and McDougall (found that strontium had no significant effect on plaque bacterial composition.