# The Double Face of Metals: The Intriguing Case of Chromium
## 3 Bioavailability, Absorption and Excretion of Cr(III) and Cr(VI)  
Brewer's yeast, sea food, oysters, liver, meat, cheese, fruits, green beans, spinach and broccoli are dietary sources rich in Cr(III) (Table 3). Chromium content in food is influenced by its presence in the soil in which the plants and vegetables grow and by feedstuffs fed to animals and contamination during processing or cooking methods in Cr-containing stainless-steel equipment [48; 49]. The estimated safe and adequate daily dietary intake for Cr(III), established by the National Research Council, is 50-200 \(\upmu\)g/day corresponding  
Figure 1: **Pathway of chromium metabolism in the cell. ** Cr(III) is unable to cross the cellular membrane, while Cr(VI), being structurally similar to phosphate and sulfate, crosses the cellular membrane via a nonspecific anion channel. In the cell, Cr(VI) undergoes metabolic reduction to Cr(III) in the presence of Asc, GSH and Cys [38; 41]. Figure 2: **Reduction reaction of Cr(VI) to Cr(III) and Fenton reaction. ** The reaction process of Cr(VI) to Cr(III), especially in the presence of GSH, produces hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)), hydroxyl radicals (**OH**) and superoxide anion radicals (O\({}_{2}\)\({}^{\star}\)), due to Fenton reaction. These ions are very unstable, causing lipid peroxidation, DNA and protein damage [37]. to 0.71-2.9 \(\upmu\)g/kg/day for a 70 kg adult [10]. In 2001, the Food and Nutrition Board at the Institute of Medicine of the National Academies of Sciences (US) specified the daily chromium intake at a value of 25 \(\upmu\)g and 35 \(\upmu\)g for women and men, respectively [50]. Maximum intake levels are up to 250 \(\upmu\)g/day for supplemental intake as suggested by the World Health Organization (WHO); these are in the same order of magnitude as the exposure resulting from normal dietary intake [51]. Cr(III) is a natural constituent of the diet and may be found in a multiplicity of foods and supplements, whereas Cr(VI) mostly derives from industrial processes and may be found in drinking water as a result of anthropogenic contamination [52; 53]. Cr(III) is absorbed in the gut through the unsaturated passive transport. The absorption intake depends on the chromium quantity in the food and the chemical form of this element (i.e., chloride, picolinate, nicotinate, etc.). The absorption of chromium in human beings is much higher in the form of yeast chromium (5-10%) than CrPic (2.8%) or CrCl\({}_{3}\) (0.1-0.4%) (European Commission 2003) [54]. Organic chromium (i.e., picolinate, nicotinate, methionine, histidine) is better absorbed than inorganic [55]. In 2007, [PERSON] and co-authors reported that the highest correlation dose/accumulation in the tissues occurred when chromium nanoparticles were used (average size 40-50 nm) [56]. However, other factors are related to the absorption of this element: ascorbic acid, aspirin, oxalic acid, simple sugar, nicotinic acid and some amino acids may enhance the absorption of chromium, whereas calcium, magnesium, zinc, titanium, iron and phosphate reduce the level of absorption [57]. A chromium deficiency has been noted in athletes after strenuous exercise, pregnant women, and the elderly because of their difficulty in absorbing inorganic chromium in an adequate amount to convert into the active form [58]. Chromium absorbed by passive diffusion represents only about 1% [59]. After being absorbed from the intestine, Cr(III) is released into the bloodstream, and its transportation is mediated by transferrin, a \(\upbeta\)-globulin, followed by receptor-mediated endocytosis, and then transferred to cells of various tissues, first of all, liver and kidneys [60]. Transferrin is a serum protein, involved in iron metabolism and responsible for transporting Fe(III) in the bloodstream [61]. This protein is also involved in the transport of Cr(III) because of its similarity to the ferric ion both in size and charge [62]. Transferrin holds two binding sites for iron with different affinities depending on pH.