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Silicon: Facts and fiction
Supplemental Silicon for horses
In the last period we have received several questions regarding Silicon (Si) supplementation to horses. Some horse owner’s belief that Si supplementation would have a beneficial effect on bone health in horses. However, we wonder if present knowledge about Si, and more specific, the knowledge regarding the beneficial effect of feeding supplemental Si to horses does support this belief?
What is Silicon?
Silicon is the second most abundant element of the Earth’s crust. Due to its high affinity for oxygen, it is not found in its elemental form in nature but readily forms solid phase silicates in which e.g. aluminum (Al), magnesium (Mg), etc. may also be part of the complex. Some silicates that are known in our everyday life are glass and sand. Further, rocks and soil contain environmental silicates, which are gradually leached by weathering to release soluble silicates, or “silicic” acids into the soil solutions, rivers or other natural water bodies. The major form of soluble silicic acids in natural water is orthosilicic (or monosilicic) acid.
In the human body silicon appears to be the third most abundant trace element after iron and zinc. The highest concentrations of Si are found in tissues in the aorta, trachea, tendons, bones and skin. Researchers have proposed that the reason for their high Si content is the binding of Si to glycosaminoglycans and their protein complexes in connective tissues.
In order to be able to have any nutritional or clinical effect, the (supplemented) Si should be bioavailable (the specie must be able to absorb the nutrient in the gastrointestinal tract). In humans, orthosilicic (or monosilicic) acid is considered the most readily available source of Si but knowledge regarding human or other animals may not directly apply horses. Serum Si levels will increase after ingestion of foods rich in silicon but the mechanism of absorption of Si is not known. It is known from human studies that the bioavailability of Si may vary between foods. Unfortunately, there have been only limited studies published regarding Si bioavailability or efficacy in horses. These studies used Sodium zeolite A as silicon source. In one study executed by Texas University, sixty Quarter Horse weanlings were fed four different dietary levels of sodium zeolite A (0, 0.66, 1.32 and 2.0%). Plasma silicon concentrations were higher in the supplemented groups. Thus, it appears that Si in sodium zeolite A is available for horses. In the follow-up study similar effects on the plasma Si concentration were observed. A study from Michigan State University determined the effect of supplemental dietary Si (Sodium zeolite A) on plasma and milk Si concentrations of lactating mares and the subsequent effect on plasma Si concentrations in nursing foals. All supplemented mares had higher plasma Silicon concentrations compared to the control group (day 30) and on day 45 supplemented mares had higher Si concentrations in the milk. This resulted also in higher plasma Si concentrations in foals at day 45. Thus, it appears that Sodium zeolite A provides bioavailable silicon for horses. Studies regarding the bioavailability of other Si-sources fed to horses have not been reported yet. Based on studies executed in other species it may be suggested that the bioavailability of Si is not only influenced by dietary factors (source, mineral interactions) but also by the physiological state of the animal.
In general, animals need to ingest nutrients according to their requirement. If the intake of a nutrient is lacking or deprived, deficiency symptoms may occur. For example, silicon deprived (growing) animals may have abnormal collageneous tissues, such as bone, joints and skin (or feathers). The possible beneficial effect of Si on bone health is probably the reason for Si supplementation in the equine community. Indeed, current knowledge obtained from animal studies suggest that the primary effect of silicon on bone appears to be through stimulation of bone formation. Probably due to its effects on collagen syntheses and possibly in matrix mineralisation. Few data also suggest that Si may inhibit bone resorption. There is also some evidence that hormones may be involved in these interactions. However, it may be questioned if there is indeed evidence that supplemental Si (i.e. supplemental Si above requirement) may exert a beneficial effect on bone health. There are limited animal studies available that observed a clear relationship between Si supplementation and bone health, and those available, report conflicting results. A recent study in humans concluded that dietary silicon intake is positively associated with bone mineral density in men and premenopausal women but not in post-menopausal women. Further, there is evidence that Si-based gels and cements used in orthopaedic surgery facilitate healing.
Less information is available regarding the efficacy of supplemental Si for improving the health (and performance) of horses. In the Texas A&M University study, feed intake, weight gain, heartgirth circumference, or height gain at the withers and hips was measured. Bone density was also measured and expressed as radiographic aluminum equivalents (RBAE). Supplementing Si-containing Sodium zeolite A did not result in significant differences in these parameters although the gain in RBAE during the first 56 days of the trial tended to be largest in the horses fed the highest concentrations of Si. However, there were no significant differences in RBAE between dietary treatments over the 168-day trial. In the follow-up study 53 horses of the initial population of 60 weanlings were placed in training at 18 months and remained in the same treatment group but with a different level of Si (0, 0.92, 1.86 and 2.8%). The authors concluded that Sodium zeolite A was not detrimental to performance as supplemented horses had faster average race times. They also concluded that their results suggested that horses supplemented with Si were less prone to injury. However, the number of horses per treatment was unequal and relatively small. In the Michigan State University study the researchers concluded some trends for altered bone metabolism were observed in postpartum mares. Besides one briefly mentioned “clinical” case study no data regarding the efficacy or prophylactic effect of Si in equine bone disease have been published. In general, studies using Sodium zeolite A as Si source may be more indicative for the effect of the sodium zeolite A complex rather than the effect of supplementing a single Si source (e.g. orthosilicic acid). Sodium zeolite A breaks also down into Al, and Al overloading may increase bone mineral density.
Requirement and excess
The Si requirement for horses is not known. For other livestock species it has been suggested that Si is likely to present problems of nutritional excess rather than deficiency. High intakes of Si may favour complexation of elements such as magnesium, iron and manganese. Further, an increased risk for urolithiasis (“urinary stones”) has been suggested for grazing animals when ingesting high amounts of Si.
Grasses and cereal grains (especially oats) may contain several grams of Si kg-1 DM and leguminous forages contain less silicon (lower level of about 1.8 g kg-1 DM) but these amounts are still high when Si is considered to be a trace element.
Small particles of silica present in plants may not be utilizable. As most horse diets contain cereals or have grass/pasture available, it seems unlikely that horses are fed Si deficient diets and need Si supplementation (the control diet in the Michigan State University study contained 10.8 g Si in the basal diet which consisted of a concentrate and a hay/alfalfa mixture). As Si requirements for horses have not been established, it cannot be excluded that supplemental Si can be of occasional benefit for horses with bone density / connective tissue problems. Si may be, similar as other trace elements, a nutrient for which an adequate level in the diet is necessary. When supplementing Si alone, it should be noted that this might affect availability of other minerals. Currently, there is not much evidence to support the use of additional Si in horses.
· Frey et al. 1992. Plasma silicon and radiographic bone density in weanling quarter horses fed sodium zeolite A. Eq. Vet. Sci. 12: 292-296.
· Lang et al. 2001. Supplemental silicon increase plasma and milk silicon concentrations in horses. J. Anim. Sci. 79: 2627-2633.
· Nielsen et al. 2005. Training distance to failure in young racing Quarter Horses fed sodium zeolite A.
· Sripanyakorn et al. 2005. Dietary silicon and bone health. Br. Nutr. F. Nutr. Bull 30:222-230.
· Underwood, E.J. and Suttle, N.F. 1999. The mineral nutrition of livestock (3rd Edition). CABI Publishing.