5.1.4. Chinese Population

In a Chinese population study (*n* = 8429), intake levels of dietary Cd were inversely associated with eGFR, and the risk of CKD rose with Cd intake levels in a dose-dependent manner: Cd intake levels of 23.2, 29.6 and 36.9 μg/day were associated with 1.73-, 2.93- and 4.05-fold increments of CKD risk, compared with a Cd intake level of 16.7 μg/day [15]. [Pb]b of 10 μg/dL was associated with tubular injury in Chinese men who were exposed to Pb in the workplace [295]. In a coexposure analysis of residents in Cd-polluted and control areas in China [296], the risk of tubular injury, as assessed with [NAG]u, was highest in subjects who had [Cd]b ≥ 2 μg/<sup>L</sup> and [Pb]b ≥ 10 μg/dL, while the risk of a decrease in eGFR was highest in those with ECd ≥ 3 μg/g creatinine and EPb ≥ 10 μg/g creatinine.

#### 5.1.5. Korean and Belgian Populations

The prevalence of CKD in a representative Korean population (*n* = 1797) was 7.1% and the population means for [Pb]b, [Hg]b and [Cd]b were 2.37, 4.35 and 1.17 μg/L, respectively [297]. Elevated [Cd]b levels were associated with 1.52- and 1.92-fold increases in the risk of CKD in those with diabetes and hypertension, respectively. Neither [Pb]b nor [Hg]b showed such a relationship [297]. Supporting Cd as a risk factor for CKD is another study of 2992 Koreans, 20–65 years of age; [Cd]b > 1.74 μg/<sup>L</sup> was associated with a 1.97-fold increase in odds for CKD in women [298]. In a study of a subset of participants (*n* = 2005, aged ≥20 years) in a nationwide survey (*n* = 8641), Cd exposure was again found to be an important risk factor for CKD in Korea: [Cd]b levels in the highest quartile (mean, 2.08 μg/L) were associated with a 1.93-fold increase in the risk of CKD [283]. In contrast, [Pb]b levels in the highest quartile (mean, 4.13 μg/dl) were not associated with a significant increase in CKD risk in this subset analysis [299].

Intriguingly, although Pb exposure levels in Korea did not seem to a ffect CKD risk, evidence that Pb coexposure may enhance Cd toxicity in kidneys has emerged from another Korean population study (*n* = 1953, aged 18–83 years) in which [Pb]b and [Cd]u both correlated positively with [β2MG]u, a marker of tubular dysfunction [300]. However, the correlation between [Cd]b and [β2MG]u was

strengthened in those who had [Pb]b above the median of 2.20 μg/dL. Similar to Korean findings, a study of Belgian metallurgic refinery workers suggested that there is a Cd–Pb interaction [301]. The associations between [Cd]u, [NAG]u and [RBP]u were only seen in workers who had high levels of [Pb]b (≥21.9 μg/dL), corresponding to the 75th percentile or higher. In addition, the associations between [Cd]b, [NAG]u and [intestinal alkaline phosphatase]u only became statistically significant in workers who had [Pb]b ≥ 21.9 μg/dL [285]. This Cd–Pb interaction was seen although blood Pb in Belgian workers of 21.9 μg/dL was 6-fold higher than the 90th percentile blood Pb level of 3.66 μg/dL in a Korean study [300]. Of note, [Pb]b of ~20 μg/dL did not exceed the exposure limit for neurotoxicity in adults of 25 μg/dL [71]. Further epidemiologic research is required to examine the mechanisms underlying the interaction of Cd and Pb in chronic Cd and Pb exposure conditions.

#### *5.2. Environmental Exposures and Mortality from All Causes*

In the previous section, exposures to Cd and Pb have been identified as risk factors for CKD across populations. In this section, we summarize the observations across populations that demonstrated the overall impact of chronic lifelong exposure to Cd and Pb on life prognosis and risks of death from CVD and cancer.

#### 5.2.1. Cadmium and Mortality in the U.S.

Temporal trend analysis indicated a 29% reduction in environmental Cd exposure among a representative cohort of men in the U.S. over an 18-year follow-up period (1988–2006) during which the mean ECd fell from 0.58 to 0.41 μg/g creatinine [302]. A reduction in environmental Cd exposure in women over the same 18-year period was statistically insignificant.

Cd exposure was associated with heart disease in cross-sectional studies [303–305]. Cd exposure was also an independent risk factor for ischemic stroke in another cross-sectional study (*n* = 2540), with a mean and a 75th percentile ECd of 0.42 and 0.68 μg/g creatinine, respectively [306]. These associations may account for the increased mortality from CVD seen in various follow-up studies of NHANES participants. In a follow-up study of participants in NHANES 1988–2006, [Cd]b was linked to an increase in death from CVD, especially in women [302]. ECd of ≥0.37 to ≥0.65 μg/g creatinine was linked to increased risk of death from heart disease among participants in NHANES 1999–2008 [307,308].

A 4.29-fold increase in death from malignant disease was seen among participants from NHANES (1988–1994) who had ECd > 0.48 μg/g creatinine [309]. In men only, a 2-fold increase in ECd was respectively associated with 28%, 55%, 21%, and 36% increases in death from all causes, cancer, CVD, and coronary heart disease, after adjustment for potential confounders, including cigarette smoking [309]. ECd of ≥0.37 to ≥0.65 μg/g creatinine was linked to increased risk of breast cancer among women participating in NHANES 1999–2008 [307,308].

In other follow-up studies of NHANES 1988–1994 participants, a two-fold increase in ECd was associated with 26% and 21% increases in cancer mortality in men and women, respectively [310]. The mortality from lung cancer in men was increased by 3.22-fold, while the mortality from liver-related nonmalignant disease was increased by 3.42-fold in participants who had ECd of ≥0.58 to ≥0.65 μg/g creatinine [310,311].

Among the ≥65 years of age participants of NHANES 1999–2004, [Cd]b levels > 0.6 μg/<sup>L</sup> were associated with a 3.83-fold increase in the risk of the mortality from Alzheimer's disease [312]. In another publication based on the data from the same NHANES cycle showed that elevated urinary Cd levels were associated with a 58% increase in the risk of death from Alzheimer's disease in the 60−85 years age group [313].

#### 5.2.2. Cadmium and Mortality in Sweden and Australia

In a Swedish cohort study, the overall mortality was increased by 2.06-fold in women who had [Cd]b ≥ 0.69 μg/<sup>L</sup> compared with those with [Cd]b ≤ 0.18 μg/<sup>L</sup> [314]. In this Swedish study on women, the median, 25th and 75th percentile levels of [Cd]b were 0.28, 0.18 and 0.51 μg/L, respectively [314]. In a follow-up study of women in Western Australia (*n* = 1359), there was 2.7-fold higher [Cd]u in those with atherosclerotic vascular disease. This [Cd]u was associated with a 36% increase in the risk of dying from heart failure and a 17% increase in the risk of having a heart failure event [299]. Hence, a reduction in survival was observed even though the kidney burden of Cd among the study women was low: the median, 25th and 75th percentile levels of [Cd]u were 0.18, 0.09 and 0.32 μg/L, respectively [315].

#### 5.2.3. Cadmium and Mortality in Japan

A dose–response relationship between mortality risk and elevated body burden of Cd was seen in men who were residents of nonpolluted areas of Japan: the mortality risk from all causes was increased by 35% and 64% in men who had ECd 1.96–3.22 and ≥3.23 μg/g creatinine, respectively [316]. In women from the same nonpolluted areas, a 49% increase in mortality from all causes was associated with ECd ≥ 4.66 μg/g creatinine. A 6% increase risk of death from cancer at any site was also seen only in women. There was a 13% increase in mortality from pancreatic cancer for every 1 μg/g creatinine increment of ECd [317].

In a region of Japan with Cd pollution, all-cause mortality increased by 1.57-and 2.40-fold in men and women with proteinuria and glycosuria, attributable to their elevated Cd exposure [318]. An increase in deaths from ischemic heart disease and incidences of diabetes and kidney disease was observed [302]. A 1.49-fold increase in deaths from cancer in any site was observed, especially in women with evidence of Cd-related kidney pathologies [319]. The increase in the risk of dying from a specific cancer type was 3.85, 7.71 and 10.1 for cancer of the uterus, kidney and kidney plus urinary tract [319]. The median [Cd]u in women and men with proteinuria and glycosuria was 8.3 μg/<sup>L</sup> and 10 μg/L, respectively. Paradoxically, in men, the risk of lung cancer and the risk of dying from cancer were reduced by 47% and 21%, respectively [319].

#### 5.2.4. Lead and Mortality in the U.S., Korea and China

In a follow-up study of a subset of participants in NHANES 1988–1994, there was a 48% increase in cancer mortality risk in those with [Pb]b ≥ 5 μg/dL [320]. In another follow-up of participants in the same cycle, [Pb]b 1.0–6.7 μg/dL was associated with 1.37-, 1.70- and 2.08-fold increases in death from all causes, CVD, and ischemic heart disease [321]. In the NHANES 1999–2010 follow-up study (*n* = 5316), exposure to low levels of Pb, reflected by [Pb]u > 1.26 μg/dL, may increase the risk of deaths from all causes and cancer by 1.79- and 6.60-fold, respectively [196]. A 44% increase in CVD mortality was observed for every 10-fold increase in hematocrit-corrected [Pb]b in a follow-up of participants, aged ≥40 years, in NHANES 1999–2010 (*n* = 18,602) [322].

In a cohort study of lead-exposed workers of South Korea (*n* = 81,067), [Pb]b 10–20 μg/dL was associated with 36% and 93% increases in the risk of death from all causes in men and women, respectively [323]. The mortality risk from bronchial and lung cancer rose by 10.45- and 12.68-fold in female workers with [Pb]b of 10–20 μg/dL. In male workers, the same [Pb]b range of 10–20 μg/dL was associated with hospital admission for ischemic heart disease, cerebrovascular disease, angina pectoris and cerebral infarction [324].

A 25% increase in deaths from all causes was recorded in a follow-up study of a Chinese population (*n* = 2832) with the median Pb intake level of 101.9 μg/day [16]. Compared with Pb intake levels in Quartile 1 (67 μg/day), Pb intake levels in Quartile 3 (111.4 μg/day) and Quartile 4 (147 μg/day) were associated with 1.52- and 3-fold increases in cancer mortality, respectively [16].

Table 2 summarizes exposure levels reflected by blood concentrations, urinary excretion, and dietary intake estimates of Cd and Pb that have been associated with nephrotoxicity, enhanced risks for CKD, and mortality in various populations that include the U.S., Sweden, Australia, Japan, China, Thailand, Belgium, and Korea.


**Table 2.** Toxic exposure levels of cadmium and lead observed in various populations.

[x]u = urinary concentration of x; [x]b = blood concentration of x; CKD = chronic kidney disease; ESKD = end stage kidney disease; ECd = excretion rate of Cd; β2MG = β2-microglobulin; <sup>E</sup>β2MG = excretion rate of β2MG; NAG = N-acetyl-β-D-glucosaminidase. CKD is defined as estimated glomerular filtration rate < 60 mL/min/1.73 m2. <sup>E</sup>β2MG ≥ 300 μg/g creatinine was the conventional cutoff value to define an adverse effect of excessive intake of Cd [41].

#### **6. Summary and Conclusions**

Dietary assessment by the total diet study method shows that both Cd and Pb are present in virtually all foodstuffs. Foods which are frequently consumed in large quantities, such as cereals, rice, potatoes and vegetables, contribute the most to the total intake of these toxic metals. Seafood (shellfish), offal, spinach, lettuce and chocolate are Cd sources among high consumers of these foods. Beverage, chocolate syrup, raisins, fish, meats (offal included), preserved soybean, and fungus products are sources of Pb for high consumers of these products. Cd intake levels of 23.2, 29.6 and 36.9 μg/day were associated with 1.73-, 2.93- and 4.05-fold increments of CKD risk, compared with the 16.7 μg/day intake rate. A Cd intake level of 23.2 μg/day is 40% of the FAO/WHO current tolerable intake level. Pb intake levels of 111.4 and 147 μg/day were associated with 1.52- and 3-fold increases in cancer mortality, compared with the 67 μg/day intake rate. A Pb intake level of 111.4 μg/day exceeds the FDA interim safe intake rate of 12.5 μg/day.

Historically, the health risk assessment of Cd has relied on <sup>E</sup>β2MG. This practice follows the FAO/WHO guidelines in which <sup>E</sup>β2MG ≥ 300 μg/g creatinine were cutoff values to define the level of health concern (nephrotoxicity). However, multiple lines of evidence discussed in this review indicate

that the established cutoff value of <sup>E</sup>β2MG is not a sensitive indicator of tubular cell toxicity. KIM1 is the first identifiable marker of Cd-induced injury, and in our opinion, any elevation of ECd also signifies such injury. Estimated GFR is a function of intact nephron mass and is universally employed for diagnosis and staging of CKD. Health risk assessment of Cd should be based on the dose–response relationship between ECd and GFR.

The variable effect of low-level environmental exposure to Cd and Pb on GFR has caused some controversy. Consequently, governments worldwide have not established the necessary regulations to protect their populations. To improve comparability of guidelines among populations, normalization of [Cd]u to Ccr is proposed to nullify urine flow rate as a confounder, circumvent the effect of muscle mass on [cr]u, and facilitate the expression of relevant excretion rates as functions of intact nephron mass.

Risk assessment of Cd is conventionally based on the urinary Cd threshold limit of 5.24 μg/g creatinine, which was the mean ECd at which Eß2MG exceeded 300 μg/g creatinine. However, a [Cd]u level as low as 1 μg/<sup>L</sup> (ECd ~0.5 μg/g creatinine) is associated with a significant increase in the risk of CKD and mortality from cardiovascular disease and cancer. As Cd and Pb exposure is highly prevalent, even a small increase in disease risk can result in a large number of people affected by a disease that is preventable. Environmental exposure to low-level Pb ([Pb]b 1.0−6.7 μg/dL) is associated with mortality from cardiovascular disease and ischemic heart disease. [Pb]u levels > 1.26 μg/<sup>L</sup> are associated with increased mortality from cancer.

Given the continuing rise in the incidence of CKD worldwide and the escalating treatment costs associated with dialysis and/or kidney transplants needed for survival, developing strategies to prevent CKD is of global importance. Furthermore, Cd and Pb are associated with cardiovascular morbidity and reduced life expectancy, independently of CKD. Prevention of Cd- and Pb-related ailments and mortality requires minimization of their environmental contamination. Accordingly, public measures to reduce environmental pollution and the food-chain transfer of Cd and Pb are vital, as are risk reduction measures through setting a maximally permissible concentration of Cd and Pb in staple foods to the lowest achievable levels.

**Author Contributions:** S.S., G.C.G., D.A.V. and K.R.P. conceptualized the review. S.S. prepared an outline and an initial draft with G.C.G., and D.A.V. provided logical data interpretation. K.R.P. wrote the section on the assessment of nephrotoxicity. G.C.G., D.A.V. and K.R.P. reviewed and edited the draft manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** This work was partially supported with the resources of the Kidney Disease Research Centre, The University of Queensland Faculty of Medicine and Translational Research Institute. Additionally, this work was supported by the Stratton Veteran Affairs Medical Center, Albany, NY, USA, and was made possible by resources and facilities at that institution. Opinions expressed in this paper are those of the authors and do not represent the official position of the United States Department of Veterans' Affairs.

**Conflicts of Interest:** The authors have no potential conflict of interest to declare.
