Fu and Xi 2019 — Heavy metals and human metabolism
Fu and Xi review the metabolic mechanisms by which five major heavy metals — arsenic, lead, nickel, cadmium, and mercury — disrupt human physiology, drawing on occupational exposure literature, drinking water contamination studies, and experimental toxicology. The paper is oriented toward mechanisms rather than food-occurrence data; its value to this wiki is as a synthesis reference for the health and toxicology section, providing a peer-reviewed narrative of dose-response relationships and target-organ effects for each of the five metals in scope.
Key mechanisms covered
Arsenic. Fu and Xi summarize arsenic’s primary metabolic route: inorganic arsenic (arsenite, As(III); arsenate, As(V)) is methylated in the liver via arsenite methyltransferase (AS3MT) to monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA), a detoxification pathway whose efficiency varies with individual genetic polymorphisms in AS3MT and with nutritional status (folate, B12). Incomplete methylation leaves As(III) and MMA(III) — the most genotoxic species — in circulation. The review notes that chronic arsenic exposure at concentrations below WHO’s 10 µg/L guideline remains associated with increased risk of skin, bladder, and lung cancers in epidemiological studies, complicating a clean dose-response floor.
Lead. Lead substitutes for calcium in biological systems: it enters bone (where it can be mobilized during pregnancy, lactation, or osteoporosis), inhibits δ-aminolevulinic acid dehydratase (a key enzyme in heme biosynthesis), and crosses both the blood-brain barrier and placental barrier with particular developmental toxicity in children. The review cites the CDC’s current position that no blood lead level in children has been demonstrated to be without adverse effect, and notes that the primary human exposure route has shifted from gasoline/paint (historical) to diet, dust, and water in the post-leaded-gasoline era.
Cadmium. Cadmium is nephrotoxic at the proximal tubule; the kidney cortex accumulates Cd-metallothionein complexes that, once the tubule’s reabsorptive capacity is exceeded, cause β2-microglobulin and retinol-binding protein to spill into urine — the standard biomarkers for subclinical cadmium nephropathy. The review notes an estimated half-life of cadmium in the kidney of 10 to 30 years, making cumulative dietary exposure the clinically relevant metric rather than acute intake events. It identifies rice (particularly in paddy regions with contaminated irrigation water) and tobacco as the dominant dietary/inhalation sources in general populations.
Mercury. The paper distinguishes inorganic mercury (Hg²⁺), which is corrosive and nephrotoxic, from methylmercury (MeHg), which is the form bioaccumulated in fish and the primary human health concern from dietary exposure. MeHg crosses the blood-brain barrier and placental barrier; its neurodevelopmental effects — reduced IQ, motor dysfunction, language delays — have been documented at exposures achievable through regular fish consumption by pregnant women. The Minamata disease episode is cited as the anchor case, but the review’s thrust is at the lower end of the dose-response curve relevant to population-level fish consumption advisories.
Nickel. The review identifies nickel dermatitis (allergic contact sensitization) as the best-established human effect, with dietary nickel intake producing flares in sensitized individuals. Nickel sulfate is classified as a Group 1 human carcinogen by IARC based on occupational inhalation evidence; the relevance of dietary nickel to cancer risk in non-occupationally-exposed populations is described as uncertain. The review notes that food — whole grains, legumes, nuts, chocolate — is the dominant exposure route for most people outside of occupational settings.
Methods
This is a narrative review, not a systematic review or meta-analysis. The authors searched PubMed and relevant regulatory databases through late 2019. No explicit PRISMA protocol or quality weighting of included studies is described. The review is useful as a secondary source for mechanisms and exposure context but should not be treated as a systematic synthesis of dose-response data; for that, EFSA scientific opinions (for Cd, Pb, iAs) and JECFA monographs are the A-tier references.
Limitations
The abstract and opening framing emphasize drinking water as the “main source of human exposure” to heavy metals, which is accurate for arsenic in specific high-exposure geographies (South and Southeast Asia) but is not accurate for the broader population where dietary exposure — particularly food — dominates for Pb, Cd, and MeHg. This framing should be held against the source when using specific claims; the mechanistic content is solid but the exposure-route emphasis reflects the authors’ research focus (occupational and water-pollution toxicology at China Medical University) rather than the balanced dietary-exposure picture.
Implications
Health and toxicology: provides a useful peer-reviewed mechanism narrative for index and the per-metal toxicology sections of lead, cadmium, arsenic, mercury, and nickel.
Courses: can be cited for mechanism-of-action teaching content; instructors should note the water-focus caveat and supplement with dietary-exposure data.
Certification: not directly applicable to threshold-setting; the review does not report occurrence data or dose-response floors at food-relevant concentrations.