## 1 Introduction  
In recent decades, there has been a strong interest in silicon (Si) as a trace element, which plays an important role in physiological and metabolic function in both plants and humans. Silicon is present in all organisms (including plants and humans) and also in soil [1]. Silicon is involved in the metabolism of connective tissue and is important for the biosynthesis of collagen and glycosaminoglycan, which are needed for bone formation [2; 3; 4]. Silicon is also required for the function of polyhydroxylase, which is responsible for the formation of collagen, elastane, cartilage, and other connective tissues. Si deficiency causes weak and malformed bones, osteoporosis, Alzheimer's disease, bone decalcification, cardiovascular disease, and atherosclerosis. Silicon is also involved in the maintenance and growth of hair and nails [5; 6; 7]. There is approximately 1 g of Si in the human body, which is the third most abundant trace element after iron and zinc [8]. Si is deposited and stored in the connective tissue of the aorta, trachea, tendons, bones, and skin, and stored silicon has been found to decline with age [9; 10; 11]. One of the most important silicon compounds is silica (silicon dioxide, SiO\({}_{2}\)), which is the basic component of many minerals used in the industry. Silicon in the form of orthosilicic acid (OSA) and salts (silicates) have the greatest importance in nutrition and are considered the best sources of silicon. Natural sources of silicon include cereals, fruits, and vegetables. The essentiality and precise functional roles of Si for humans have not been fully established, and a recommended intake for this element has not been set [12; 13; 14; 15]. In 2004, the European Food Safety Authority (EFSA) was unable to establish a tolerable upper level (UL) for silicon intake as there was no suitable dose-response data nor data on adverse effects of silicon available [14]. However, given the available research results, a silicon intake level was set at 20-50 mg per day, and a safe upper level was set at 700 mg per day for adults over a lifetime. Such quantity should not cause any undesirable effects [12; 16]. The consumption of silicon by people living in Europe and North America is much lower than in societies whose diets are based on vegetables, fruits, and unprocessed cereals, e.g., in India and China, where the consumption is 140-204 mg/day. In these populations, a lower incidence of bone fractures has been observed, suggesting that increasing the intake of silicon above the recommended intake may have a beneficial effect. The primary sources of silicon in the diet are food products of plant origin, water and beverages, and to a lesser extent, animal products. Silicon may also appear in the diet as part of processed products in which the additive (excipient) E551, an anticaking and antifoaming agent that is high in silicon, is used [17; 18]. In recent years, dietary supplements and functional foods containing silicon compounds have become available on the market in various forms, with varying bioavailability, and have become increasingly popular, claiming numerous health-promoting effects. However, these are frequently poorly documented and sometimes not confirmed by the EFSA. Therefore, the purpose of this publication is to present up-to-date data on the various sources of silicon, their bioavailability and safety, and to characterize the silicon-containing chemical compounds used in the production of dietary supplements and functional foods. This data will help to guide consumer selection of silicon-enriched products and may also help in the selection of appropriate compounds by functional food manufacturers. This paper may also provide guidance to nutritionists and physicians who are involved in the prevention and complementary therapy of widely occurring osteoarticular diseases and people interested in the promotion of healthy skin, hair, and nails.