Dietary Strategies for the Treatment of Cadmium and Lead Toxicity

Cadmium (Cd) and lead (Pb) are toxic heavy metals that cause adverse health effects in humans and animals. Chelation therapy, the conventional treatment for heavy metal toxicity, is reported to have a number of safety and efficacy issues. Recent studies have shown that dietary supplements play important roles in protecting against Cd and Pb toxicity. This paper reviews the evidence for protective effects of essential metals, vitamins, edible plants, phytochemicals, probiotics and other dietary supplements against Cd and Pb toxicity and describes the proposed possible mechanisms. Based on these findings, dietary strategies are recommended for people at risk of Cd and Pb exposure. The application of these strategies is advantageous for both the prevention and alleviation of Cd and Pb toxicity, as such supplements can be added easily and affordably to the daily diet and are expected to have very few side effects compared to the chelation therapy.

ingredients can easily and affordably be added to the daily diet and can overcome the negative side effects of the chelation therapy.
Herein we review the potential dietary strategies for Cd and Pb toxicity of essential metal, vitamin, edible plant and dietary phytochemical supplementation and probiotics, among others.

Essential Metals
Many studies in both animals and humans have shown that a deficiency in essential metals such as zinc [33], calcium [34] or iron [35] can lead to greater absorption and toxicity of Cd and Pb. Therefore it is logical to suggest that the supplementation with essential metals can provide protective effects against Cd and Pb intoxication. A selection of such studies listed in Table 1 show the benefits of essential metals in this context.
Zinc is one of the most well studied essential metals for the alleviation of heavy metal toxicity. As zinc has similar chemical and physical properties to Cd and Pb, it competes for the binding sites of metal absorptive and enzymatic proteins [36]. Intake of zinc also induces the synthesis of metallothionein (MT) [37], a low molecular weight protein that has a high affinity for Cd and causes detoxification by binding Cd [38]. Zinc supplementation effectively protects the activity of blood δ-aminolevulinic acid dehydratase (ALAD), a zinc-dependent enzyme that is very sensitive to Pb toxicity [39]. Moreover, zinc intake has been reported to alleviate the oxidative stress caused by Cd and Pb exposure [40,41], which may be due to zinc's functionality as a cofactor of the antioxidant enzyme copper zinc-superoxide dismutase (Cu/Zn SOD).
A considerable number of studies have shown that selenium administration is protective against Cd and Pb toxicity within a range of different organs of mice including the brain, lungs, liver, kidneys and blood. Selenium is a cofactor of the antioxidant enzyme glutathione peroxidase (GPx) and it contributes to the antioxidant defence system, which enables it to alleviate Cd and Pb toxicity by reducing the Cd/Pb-induced oxidative stress and enhancing the antioxidant capacity of the host [42,43]. It is also believed that selenium may form inactive complexes with heavy metals which can further enhance their detoxification [44].
Iron competes with Cd for access to intestinal metal uptake transporters including divalent metal transporter-1 (DMT1) and metal transporter protein 1 (MTP1), which may explain the decrease in intestinal Cd absorption after iron supplementation [45]. Moreover, the expression of these transporters is often modulated by nutritional status of essential minerals such as iron and zinc [28]. For instance iron deficiency has been reported to up-regulate the expression of DMT1 in intestinal epithelium [45,46]. Hence iron supplement can prevent or limit Cd absorption by reducing the expression of such transporters. On the other hand, as iron is a component of the heme complex, the deficiency of iron will enhance Pb toxicity to the heme synthesis system [47]. Other essential metals, such as calcium and magnesium, have also been reported to be effective against Cd and Pb toxicity (Table 1). These essential metals can reduce the heavy metal burden by competing with Pb or Cd for intestinal absorption and prevent heavy metal induced tissue damage by competitive binding to active sites of the enzymes [48,49]. Magnesium 20 mg/kg b.w. Mg orally 1 or 2 weeks Male mice exposed to 10 mg/kg b.w. Cd Testes and kidneys Mg pre-treatment was efficient in restoring the renal and testis GSH levels. [49] AChE, acetylcholinesterase; b.w., body weight; CAT, catalase; GD, gestational day; GPx, glutathione peroxidase; GSH, glutathione; LTP, hippocampal long-term potentiation; LP, lipid peroxidation; MAO, monoamine oxidase; PND, postnatal day; s.c., subcutaneously; SOD, superoxide dismutase; XO, xanthine oxidase.
In summary, these essential metals decrease intestinal Cd and Pb absorption, recover the essential metal homeostasis and alleviate the oxidative stress caused by Cd and Pb toxicity. Diet associated essential metal supplementation should be regarded as important for essential metal-deficient people, such as children and pregnant women. Because without sufficient essential metal stores to prevent heavy metal absorption, these people are especially susceptible to heavy metal toxicity [16,21,50,51]. It should also be noted that Cd and Pb exposure cause the loss of essential metals, which leads to complications such as iron-deficiency anaemia and osteoporosis [52,53]. Appropriate concentrations of essential metal supplementation is therefore also beneficial for preventing these complications.

Vitamins
Vitamins are vital nutrients for humans and can easily be obtained from the diet. Vitamin C, B1 and B6 deficiencies have been reported to enhance sensitivity towards Cd and Pb toxicity [54,55]. Vitamin supplementation has proved to be effective against Cd and Pb toxicity in both human and animal studies.
Vitamins C and E are natural non-enzymatic antioxidants that are able to scavenge free radicals and decrease lipid peroxidation. Many studies on the effects of vitamins C and E on Cd and Pb intoxication have been performed. Vitamin C attenuates the oxidative damage and histopathological changes induced by CdCl2 in the lungs and brain of rats [56]. It has similar protective effects in the liver, kidney, brain and the testes of Pb-exposed rats [57]. Apart from its well-established antioxidant properties, vitamin C has been reported to act as a chelating agent of Pb, with a similar potency to that of EDTA [58]. Probably due to this chelating capacity, a decrease of blood Pb levels from 1.8 ± 0.05 μmol/L to 0.4 ± 0.05 μmol/L (p ≤ 0.01) was observed in a study of 75 adult smokers receiving 1 g vitamin C daily for one week [59]. However, it is noteworthy that very few animal studies can confirm the positive impact of vitamin C on reducing blood Pb levels. Indeed a human clinical study with 52 adult male subjects found that 3 months of vitamin C supplementation had no impact on the levels of Pb in blood or hair [60]. Pre-treatment with vitamin E exhibits protective effects against Cd toxicity, as measured by the haematological values, lipid peroxide concentration and antioxidant defence system in the blood, liver and brain of rats [61,62]. The combination of vitamins C and E also resulted in reduction of oxidative stress-related damage to spermatogenesis in Cd-exposed mice [63] and protects steroid production in Cd-exposed rats [64]. In a recent study of workers exposed to Pb (73 μg Pb/dL blood), after one year of oral vitamin C and E supplementation (1 g daily vitamin C and 400 IU daily vitamin E), lipid peroxidation in erythrocytes was reduced to values between 47.1% and 69.4%, which were no longer statistically different to those of the non-Pb exposed workers. The total antioxidant capacity in erythrocytes was also reversed to values between 58.9% and 67.7% in Pb-exposed workers after treatment, a level that was similar to those in non-Pb exposed workers [65].
Dietary vitamin B1 supplement has been reported to decrease Pb levels in the liver, kidneys, bone and blood, and recover ALAD activity in the blood in animal studies [66][67][68]. Vitamin B1 influences the absorption of Pb and its pyrimidine ring mediates its interaction with Pb, which may cause an increase in Pb excretion and the alleviation of its toxicity [68,69]. Vitamin B6 has also been found to be effective in reducing accumulation of Pb in tissues and in reduction of inhibition of ALAD activity. This function is likely to be attributed to the ring nitrogen atom in its structure, which can chelate Pb before it is absorbed [70].

Edible Plants and Dietary Phytochemicals
Vegetables, fruits and other edible plants are important dietary sources of vitamins and essential metals. Edible plant supplementation at sufficient levels can promote the levels of the vitamins and essential metals in the human body, which in turn can decrease the risks of Cd and Pb toxicity. Moreover, edible plants provide a great variety of other nutrients, such as dietary protein and phytochemicals, which have been reported to have beneficial effects against Cd and Pb toxicity (Tables 2 and 3).
A selection of studies on the protective effects of edible plants against Cd and Pb toxicity is presented in Table 2. Soybean for example has been a part of the Southeast Asian diet for millennia. Two recent animal studies showed that dietary soybean supplementation helped to prevent arterial and cardiac injury by alleviating the oxidative stress induced by Cd toxicity [71,72]. The authors suggested that the soybean protein and soybean isoflavones provided the observed antioxidant effects.
Garlic, ginger and onion are used as ingredients for flavour, aroma and taste enhancement all over the world. Garlic is also a well known medicinal plant. Garlic extract alleviates Pb-induced neural, hepatic, renal and haematic toxicity in rats and protects against Cd-induced mitochondrial injury and apoptosis in tissue culture models [73][74][75][76]. Based on these studies, garlic's protective property against Cd and Pb toxicity can be attributed to (1) its antioxidative ability, provided by organo-sulphur compounds such as diallyl tetrasulfide; (2) its chelation ability, provided by sulphur-containing amino acids and compounds with free carboxyl and amino groups, which in turn promotes the excretion of Pb or Cd from the body; and (3) the prevention of Cd and Pb intestinal absorption, by its sulphur-containing amino acids such as S-allyl cysteine and S-allyl mercaptocysteine. Ginger and onion have similar antioxidant capacities to garlic, and supplementation with these food ingredients gave protection against Pb-induced renal and developmental toxicity and Cd-induced gonadotoxic and spermiotoxic effects in rats [77][78][79].
Green tea and curry leaves are commonly used in Asian cooking and are endowed with numerous potential benefits to human health including alleviating the oxidative stress induced in diabetes [80] and protecting liver from ethanol induced toxicity [81]. These plants are also gaining popularity in the West. The protective effect of green tea against Cd and Pb toxicity is mainly due to its active constituent, catechins, which are discussed later in this section. The flavonoids and phenols in curry leaves can function as antioxidants and as potential chelators, which offer protection against Cd-induced cardiac toxicity [82]. Fruits such as grapes are also effective against Cd toxicity [83]. Besides the function of vitamins and essential metals in grapes, the abundant polyphenols such as anthocyanins may also alleviate the oxidative stress caused by Cd and Pb toxicity. Tomato is regarded as one of the most powerful natural antioxidants [84] and can prevent renal toxicity induced by Pb exposure in rats [85]. Moreover, tomato has been reported to produce metal chelating proteins and phytochelatins when exposed to heavy metal ions [86,87]. In fact the oral administration of tomato has been shown to significantly reduce the accumulation of heavy metals (Cd, Pb and Hg) in the liver of rats [88].  Other plants, such as ginseng (Panax ginseng Meyer) [107], liquorice (Glycyrrhizae radix) [108], torch ginger (Etlingera elatior) [109] and tossa jute (Corchorus olitorius) [110] are also reported to have protective effects against Cd and Pb toxicity. Some of these plants such as tossa jute (used as a vegetable and food ingredient common to the people of Eastern Asia and Africa) or torch ginger (used in Malaysian local dishes) are popular dietary components in certain areas, whereas the others are routinely added in in candies and beverages (such as liquorice). They can therefore be recommended as dietary supplements for the prevention and alleviation of heavy metal intoxication to populations that are at risk of heavy metal exposure and who regularly consume these plants.
Some studies designed to explore the protective mechanisms have investigated the effects of specific plant-derived phytochemicals against Cd and Pb toxicity, rather than the intact plant itself. Table 3 presents a selection of related phytochemicals, their protective mechanisms and their food sources. Most of these phytochemicals are phenolic or isoflavone in nature and are found in commonly consumed fruit and vegetables. These bioactive compounds can act as oxygen free radical scavengers or metal chelators, which enables them to be used as natural antagonists to Cd and Pb toxicity.

Probiotics as Functional Food Supplements
Probiotics are defined as "live micro-organisms which, when administered in adequate amounts, confer a health benefit on the host" (WHO 2001). Most commercial probiotics contain species of Bifidobacterium, Bacillus, Lactobacillus as well as the yeast Saccharomyces boulardii [111]. Probiotics is now a multibillion dollar industry. There is significant number of studies indicating the benefits of probiotics in relation to antibiotic associated diarrhoea, allegy, lactose intolerance, reduction of cholesterol as well as development of immune system and protection against gut pathogens [112,113]. Some species of lactic acid bacteria (LAB) including Lactobacillus rhamnosus, L. plantarum, and Bifidobacterium longum are capable of binding heavy metals in vitro [114,115]. Moreover, LAB are known to have antioxidative properties in human subjects [116,117], which may be another important characteristic for heavy metal toxicity protection. On the basis of these functions, specific LAB have the potential to be developed as probiotics for alleviation and treatment of heavy metal toxicity. This hypothesis was also proposed in a recent review by Monachese et al. [118].
Our work has demonstrated that two lactobacilli strains exhibit protective effects against Cd and Pb toxicity in mice. L. plantarum CCFM8610, a probiotic with a good Cd binding capacity, is able to protect mice from acute and chronic Cd toxicity via its intestinal sequestration and antioxidant effects [119,120]. The oral administration of this strain effectively decreased intestinal Cd absorption, reduced Cd accumulation in tissue, alleviated tissue oxidative stress, reversed hepatic and renal damage, and ameliorated the corresponding histopathological changes of Cd-exposed mice. L. plantarum CCFM8661 protects against Pb toxicity by recovering the blood ALAD activity, decreasing the Pb levels in the blood and tissues and preventing Pb-induced oxidative stress [121]. Several recent reports confirmed that other probiotics may also be protective against heavy metal toxicity. A mixture of L. rhamnosus Rosell-11, L. acidophilus Rosell-52 and B. longum Rosell-175 significantly reduced Cd-induced genotoxicity both in vitro using liver tissue culture and in rats [122]. Another study investigated the potential of L. rhamnosus GR-1 supplemented yogurt to lower heavy metal levels in at-risk populations of pregnant women and in children in Tanzania [123]. Their results showed that blood levels of mercury and arsenic of pregnant women increased in the control groups (p < 0.05) but remained stable in the probiotic group, indicating a protective effect of L. rhamnosus GR-1 consumption. This means that with confirmed protection against heavy metal toxicity in animal studies, probiotics also have the potential to prevent or treat heavy metal toxicity in humans. However, it is worth pointing out that the strain L. rhamnosus GR-1 does not significantly reduce blood levels of Pb and Cd in pregnant women or children. These studies indicate that specific probiotic or cocktails of probiotic mixes may be required for protection against different types of heavy metal toxicity.
Lactobacilli are widely used in the food industry and are generally regarded as safe. The use of these probiotic lactobacilli can be considered a new dietary therapeutic strategy against heavy metal toxicity.

Other Dietary Supplements
Other nutrients also have the potential to alleviate Cd and Pb-induced pathogenic effects. For example, royal jelly protects against Cd-induced genotoxicity and oxidative stress in mice, due to its antioxidant effects. Algae such as Spirulina and Chlorella can attenuate Cd or Pb toxicity in the liver, kidneys and brain of animals [124][125][126][127]. Spirulina also has marked anti-teratogenic effects in Cd-injected pregnant mice. Oral administration of a high dose of Spirulina significantly decreased the frequency of foetuses with exencephaly, micrognathia, and skeletal abnormalities induced by Cd [128]. Moreover, Spirulina has been reported to reduce the quantity of micronucleated polychromatic erythrocytes and micronucleated normochromatic erythrocytes in blood cells of Cd-exposed mice (both the mother and the foetus) [129]. These algae possess many dietary antioxidants, such as vitamin C, vitamin E, phycocyanobilin and carotenes, which enable them to alleviate toxic metal-induced oxidative stress [130].

Conclusions and Perspectives
We have summarised the literature on potential dietary supplements for Cd and Pb toxicity. Based on these published reports, we recommend that people who are at risk of exposure to toxic metals ensure a sufficient intake of essential elements and vitamins and enhance their consumption of vegetables and fruit ( Figure 1). Some edible plants, such as tomatoes (rich in iron, calcium, selenium, zinc, vitamins B and C, quercetin and naringenin), berries (rich in essential elements, vitamin C, anthocyanin and catechin), onions (rich in selenium, quercetin and vitamins B and C), garlics (rich in sulphur-containing compounds, essential elements and vitamins C and E) and grapes (rich in vitamins, essential elements and anthocyanin) are of special importance as natural antagonists to Cd and Pb toxicity and should be consumed on a regular basis. These dietary supplements are an affordable option, with fewer side effects than chelation therapy, for the billions of people around the world who are inadvertently exposed to toxic metals on a daily basis [118]. In addition, with the increasing contamination of the food chain, the accumulation of Cd and Pb in edible animals can present an indirect route of heavy metal poisoning in humans [1]. Therefore, providing livestock and farmed fish with the above-mentioned food interventions may also be helpful to reduce Cd and Pb exposure in humans. While we have focused on the dietary strategies for treatment of heavy metal toxicity, intake of the suggested dietary regimes in people that are at high risk of Cd and Pb toxicity may be helpful in preventing these heavy metals from being absorbed in the body in the first place thus limiting or entirely preventing the exposure of these metals to body tissues. We need to mention that although the protective effects of essential elements, vitamins and probiotics have already been investigated in human trials, further confirmation is still necessary. It should be also noted that the studies mentioned above do not provide sufficient information on the appropriate doses of the dietary supplements in humans. It is possible that excessive consumption of essential metals, vitamins or phytochemicals may cause adverse effects in humans [65,[131][132][133]. Long-duration epidemiological studies are required to determine the optimal doses of the dietary supplements, singly and in combination, to provide safe and effective dietary strategies against Cd and Pb toxicity.