Intake of Boron in Humans

In 1923, boron was accepted as being an essential nutrient initially for broad beans (Viciafaba) and subsequently for all plants. Boron is required for the development of zebraflsh (Danio rerio) (Rowe and Eckert, 1999) and for reproduction and development in frogs (Xenopus laevis) (Fort et al 1999). Experimental studies suggest that boron is a potentially essential nutrient for humans also (WHO, 1996 Nielsen, 2008). The aim of the present review is to assess the literature on dietary sources and intake of boron in humans and to evaluate the impact of boron on metabolism in health and disease. 

The average concentration of boron in the earth s CTust is 17 ppm, and most soils fall within the range of 3-100 ppm. Boron occurs in high concentrations in sedimentary rocks and in clay-rich marine sediment due to the relatively high concentration of boron in seawater. Deposits of boron are found in association with volcanic activity and where marshes or lakes have evaporated under arid conditions. Coastal soils contain up to 50 times as much boron as do inland soils while, generally, humid soils are lower in boron content than slightly leached soils. The concentration of boron in soil is influenced also by the presence of other minerals, and soil pH and texture (Butterwick et al., 1989 Steinnes, 2009). 

2 BORON IN THE FOOD SUPPLY 4.2.1 Boron Content of Food 

3 ppm (USA) (Allen et al., 1989). Examples of the boron content of foods are shown in Table 4.1. The concentration of boron within plants varies considerably. Geological influences, in addition 

The preferred method for the analysis of boron is inductively coupled plasma atomic emission spectroscopy (ICP-AES). Inductively coupled plasma mass spectroscopy (ICP-MS) is the most widely used nonspectrophotometric method for analysis of boron, as it uses small volumes of sample, is fast, and applies to a wide range of materials. When ICP equipment is unavailable, colorimet-ric/spectrophotometric methods can be utilized. However, many of these methods are subject to interference and should be used with caution (WHO, 1998). Part of the discrepancy in the nutritional composition of boron may be related to differences in methods of analysis. 

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Terrestrial plants, especially nuts and some fruits and vegetables, are rich sources of boron (Table 29.3). Honey is another good source of boron, and concentrations up to 7.2 mg/kg dry weight have been reported (Nielsen 1986). Boron concentrations are also elevated in marine plants, zooplankton, and corals, but are low in hsh and certain marine invertebrates (Table 29.3). No data were found on boron levels in terrestrial mammalian wildhfe. The average daily intake of boron in humans ranges between 1 and 25 mg however, populations residing in areas of the western... 

The concentration of boron in human milk is approximately 30-40 pg/L (Hunt et al., 2004, 2005), which is similar to concentrations of boron that are found in cow s milk (Anderson, 1992). The boron concentrations in hnman milk do not change significantly over time in mothers of term infants, but are reported to decline gradually in mothers of preterm infants (Hunt et al., 2004). The intake of boron is 0.5 mg/day in infants and toddlers (Hunt and Meacham, 2001), and 1 mg/day for preschool children (Pieczyriska et al., 2003). School lunches in the United States provide approximately 0.5 mg (Murphy et al., 1971). 

The intake of boron from the food by human adults ranges from 10 to 20 mg. per day according to the data of Kent and McCance. The higher levels are associated with the consumption of large amounts of fruit. This food boron is rapidly and almost completely absorbed and excreted, mostly in the urine. Boric acid taken in by mouth is also rapidly excreted in the urine. The position is very similar in the cow. ... 

The recommended daily intake of boron is <20 mg. Actual amounts are dependent on food composition and local conditions. It is estimated that people ingest about 2-10 mg of boron a day, but boron deficiency in humans has not yet been recorded. Boron contained in foods is easily absorbed in the gastrointestinal tract, but 30-92% of boron ingested is excreted in the urine. At higher doses of boric acid, boron accumulates in the nervous system. 

The usual dietary intake of borate is about 0.1 mmol/day. Borate occurs as B(OH)3 and B(OH), and also occurs as weak, covalent complexes with sugars. Borate never occurs in biology as free boron atoms. Dietary borate is readily absorbed by the gut, and most of this chemical is excreted in the lu-ine. Studies wifii humans have revealed that 88-97% of dietary borate is excreted in the urine (Himt et al., 1997). There is some evidence that borate deficiency provokes a further decline in health during vitamin D deficiency. 

Much evidence exists to support the contention that boron has beneficial effects on bone. The effects of boron, however, are most evident in the presence of suboptimal status of another nutrient important in bone formation or remodeling. In chicks, boron deprivation (0.465 mgkg diet) exacerbates the distortion of marrow sprouts (location of calcified scaffold erosion and new bone formation) and delay in imtiation of cartilage calcification in bones during marginal vitamin D deficiency (Hunt 1996). In humans, estrogen therapy to maintain bones increases serum 17P-estra-diol this increase is depressed when dietary boron intake is low (0.25-0.35 g per day) (Nielsen 1996, 1997). Boron deprivation also can exacerbate the increase in serum calcitonin and osteocalcin caused by low dietary copper and magnesium in humans. 

Limited evidence is available regarding the molecular mechanism of boron homeostasis. Based on analysis of plasma boron concentrations in seven sibships, Barr et al. (1996) observed smaller variances within than between families and hypothesized that boron levels in humans are under genetic control. The data presented by Barr et al. (1996) need to be interpreted with caution as the blood samples were collected from individuals living in a rural region of northern Chile, and their dietary intakes were not determined. Also, the study was retrospective aud utilized blood samples that had been stored for over 20 years. More recent evidence suggests the involvemeut of a sodium-coupled boron transporter in animal cells, expressed in the basolateral manbrane, which determines the steady-state concentration of borate in the cytoplasm and hence maintains borate homeostasis (Park et al., 2004). 

The quality of the experimental evidence for nutritional essentiality varies widely for the ultratrace elements. The evidence for the essentiality of three elements, iodine, molybdenum and selenium, is substantial and noncontroversial specific biochemical functions have been defined for these elements. The nutritional importance of iodine and selenium are such that they have separate entries in this encyclopedia. Molybdenum, however, is given very little nutritional attention, apparently because a deficiency of this element has not been unequivocally identified in humans other than individuals nourished by total parenteral nutrition or with genetic defects causing disturbances in metabolic pathways involving this element. Specific biochemical functions have not been defined for the other 15 ultratrace elements listed above. Thus, their essentiality is based on circumstantial evidence, which most often is that a dietary deprivation in an animal model results in a suboptimal biological function that is preventable or reversible by an intake of physiological amounts of the element in question. Often the circumstantial evidence includes an identified essential function in a lower form of life, and biochemical actions consistent with a biological role or beneficial action in humans. The circumstantial evidence for essentiality is substantial for arsenic, boron, chromium, nickel, silicon, and vanadium. The evidence for essentiality for the... 

New advances in boron nntrition research shonld include better characterization of the mechanisms through which boron modulates immune function and insulin release (Hunt 1998). Epidemiological studies should be initiated to identify health conditions associated with inadequate dietary boron (Sutherland et al. 1998). Finally, Dourson et al. (1998) recommend more research on uncertainty factors used in establishing tolerable daily intake values for the protection of human health, with emphasis on variations in interspecies and intraspecies differences in resistance to boron. 

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