## 1 Introduction
### Sources and Bioavailability of Silicon from Foods  
Silicon compounds of greatest nutritional importance include silicic acids and their salts (silicates). Among them, OSA is considered to be the best form due to its high absorption rate and bioavailability [19]. OSA is characterized by a high absorption rate equal to about 43%; however, at a concentration of more than 0.1%, polysilicic acids are formed, which show a much lower bioavailability [12; 20]. It is believed that for the majority of the population, the main source of Si is food. This is because Si is naturally present in the diet in the form of silicic acid, silicon dioxide (silica), and silicates, and higher levels are found in food of plant origin than in food of animal origin [21]. Siliciic acid is naturally present in food, mainly in water and beverages, particularly beer [22; 23]. Plants naturally absorb silicic acid, where it is polymerized as solid silica bodies, silica cells, or phytoliths [24]. In these phytoliths, hydrogen and cellulose bond within the cell walls as silicon dioxide, SiO\({}_{\text{2n}}\)H\({}_{\text{2O}}\)[25]. Tables 1 and 2 show the content and average intake of silicon from various common sources. Drinking water is considered to be an important source of silicon; however, silicon content depends on its geological origin. The consumption of silicon in water can account for up to 20% of the total consumption [26]. In vivo and in vitro studies of the bioavailability of silicon in water have shown that at least 50% of the silicon present in mineral water is absorbed; therefore, it is a good source of this compound [27]. Beer is also important in the dietary pool of silicon sources due to the presence of barley and hops, which are abundant in silicon. In addition, the processes involved in beer production cause the decomposition of silicon compounds into OSA, an easily absorbable form [26]. As a result, up to 55% of the silicon in beer can be absorbed. However, it should be noted that increasing the consumption of alcoholic beer should not be considered a sensible way of increasing the dietary intake of Si; however, nonalcoholic beer might be considered [28]. Cereals, such as barley, oats, wheat, and rice bran, as well as rice and some herbaceous plants, account for 30% of dietary silicon intake, followed by fruits (especially apple and banana), vegetables (particularly carrot, potato, green beans), beverages, nuts, and some dried fruits [26; 29; 30]. Collectively, these foods provide more than 75% of dietary silicon intake [31]. Grain products such as bread, cake, rice, pasta, breakfast cereals, and flour are also high dietary sources of silicon. Other sources of silicon include fish and animal meat, eggs, milk, juices, drinking water, alcoholic beverages, and many pharmaceutical supplements [32]. However, food processing can cause significant losses of silicon (up to 50%), so unprocessed whole-grain products contain much higher quantities. It is generally accepted that after consuming silicon-rich foods, the level of this element in blood serum increases significantly [33]. Silicon is most effectively absorbed from cereal products and, to a lesser extent, green beans and dried fruits. On the other hand, Si that is present in bananas in significant quantities (5 mg/portion) is only 5% absorbed. Bananas are thought to contain a highly polymerized form of silicon that is not effectively hydrolyzed and absorbed in the gut [34]. [PERSON] et al. (2004) [28] and [PERSON] et al. (1994) [35] studied the absorption of silicon from beer, where 80% of the overall silicon content is in the form of OSA. The absorption of silicon from beer was 55%, and 42-72% of Si was excreted in urine. [PERSON] et al. (1999) [36] studied Si intake as OSA from water and reported that OSA excretion in urine was between 21% and 74%.