# Potential Metal Contamination in Foods of Animal Origin--Food Safety Aspects
## 2 Major Toxic Metals in Food
### Cadmium (Cd)  
Cadmium can be measured in the Earth's crust at levels ranging from 0.1 to 1 mg/kg [34]. It can be found in rocks and ores at concentrations 10- to 20-times higher [35; 36], mainly in zinc-containing ores and, to a lesser extent, in lead- and copper-bearing ores [37]. Similarly, cadmium can be detected in water but at lower concentrations (surface and groundwater: <1 \(\upmu\)g/L; oceans: 0.001-0.1 \(\upmu\)g/mL). However, it may occur in higher amounts in areas rich in phytoplankton [35; 36]. Plants can take up and accumulate cadmium, which may increase due to phosphate-containing fertilizers, acidic soils, and industrial wastewater used for irrigation [38]. Feeding and grazing on such forage crops can result in significant exposure for food-producing animals [39]. Calcium phosphate used as a feed additive can also contain cadmium, with concentrations high enough to pose a risk to humans through consumption of the liver and kidneys of food-producing animals [35]. In aquatic environments, cadmium is evenly distributed in the food web and does not undergo biomagnification. Its level in freshwater organisms depends on their capacity for absorption [35]. Based on structural similarity and physicochemical properties, cadmium can replace zinc, calcium, and other metals in various metal-containing proteins (enzymes), altering their structure and function [35]. It can affect the antioxidant system by inducing oxidative stress and lipid peroxidation and by modifying the membrane lipid layer [40; 41]. It has genotoxic potential (alteration of DNA synthesis, DNA strand break) due to the generation of reactive oxygen species. The International Agency for Research on Cancer classifies cadmium as a Category 1 carcinogen, a well-known human carcinogen. It also induces the expression of various genes (e.g., metallothione, heme oxygenase, heat shock proteins) [42]. Cadmium can bind to the thiol groups of membrane proteins, resulting in depolarization of the mitochondrial membrane, leading to ATP deficiency and cytotoxic effects, or it can trigger the release of mitochondrial enzymes through damaged membranes, causing apoptosis [43]. It also has estrogen-like effects, disrupting sexual maturation and accelerating puberty in mammals [43]. Cadmium compounds are generally poorly absorbed from the intestinal tract (0.5-3% in monkeys, 2% in goats, 5% in pigs and sheep, and 16% in cattle). The extent of absorption depends on their solubility in the digestive tract. The absorption of highly soluble cadmium salts (e.g., chloride, nitrate, acetate, sulfate) is significantly higher than that of poorly soluble salts (e.g., sulfide) [35]. Cadmium in foods of animal origin is less readily absorbed than soluble salts, but at low concentrations, the absorption of organic and inorganic compounds is similar [44; 45]. In the blood, it is transported primarily bound to albumin and, to a lesser extent, to globulin, metallothioneein, cysteine, glutathione, or directly to cells [46]. It is distributed throughout the body, but more than half of the total cadmium load accumulates in the kidneys and liver. Initially, it is found at the highest concentrations in the liver, then, after a few days, it moves to the kidneys, which serve as the final storage site. Cadmium is excreted very slowly from the body, partly in urine and partly in feces. Its biological half-life is very long (7-30 years) in the animal body. Due to its low molecular weight, the Cd-metallothioneein complex is filtered through the glomeruli but is reabsorbed by cells of the S1 and S2 segments of the proximal tubules, thus concentrating in the renal cortex [47]. The biological availability of different cadmium compounds varies, depending on the chemical form. In aquatic environments, cadmium is likely present in free ionic form, but in foods, it is usually found in complexes bound to proteins (e.g., metallothioneein).