Cadmium is a naturally occurring heavy metal found throughout the Earth’s crust, often existing as a trace component in various ores alongside elements like zinc. It has no known beneficial biological function in the human body and is classified instead as a toxic environmental pollutant. While humans can be exposed through air and water, the primary pathway for non-smokers and the general population is through food consumption. This exposure is concerning because cadmium accumulates in the body over time, leading to long-term health risks.
Environmental Sources and Contamination Pathways
Cadmium enters the environment through both natural processes, like volcanic activity and rock weathering, and through human activities. Industrial and agricultural practices have significantly accelerated the global mobilization of cadmium. A major source of agricultural contamination is the widespread use of phosphate fertilizers, as the phosphate rock used naturally contains cadmium. Other industrial activities, such as mining, smelting, and burning coal, release cadmium into the atmosphere. These airborne compounds settle onto soil and water surfaces, where the metal is highly mobile and easily taken up by plant roots.
Dietary Sources of Cadmium Exposure
Once cadmium contaminates the soil, it is efficiently absorbed by plants and enters the food chain, making diet the main source of exposure for most people. The concentration of cadmium varies significantly depending on the food type, the specific part of the plant consumed, and the local soil conditions. Cereals and grains, particularly rice, are major contributors to dietary exposure worldwide because they are starchy staples consumed in large quantities. Rice plants grown in contaminated soil are known to accumulate high levels of cadmium. Leafy and root vegetables, such as spinach, lettuce, and potatoes, also tend to accumulate the metal.
Certain animal products bioaccumulate cadmium, meaning they concentrate the metal in specific tissues over their lifetime. Organ meats, such as the liver and kidney from cattle or other animals, contain especially high concentrations compared to muscle meat, as these organs are the body’s primary sites for processing and storing the metal. Aquatic filter feeders like crustaceans and mollusks (oysters, mussels, and crabs) are also concentrated sources of cadmium from contaminated water. Shellfish consumption is recognized as a dominant route of exposure in populations that consume them frequently. Tobacco is another significant source of exposure, as the plant absorbs the heavy metal from the soil before it is inhaled through smoking.
Health Impacts of Cadmium Accumulation
Cadmium exposure is dangerous due to its exceptionally long biological half-life, estimated between 10 and 30 years. The body retains the metal for decades, leading to a continuous increase in the total body burden over a person’s lifetime.
The kidney is the primary target organ for chronic cadmium toxicity, responsible for up to 50% of the total accumulated body burden. Cadmium absorbed from the diet travels first to the liver, where it binds to a protein called metallothionein. This complex is eventually filtered by the kidneys, where it is reabsorbed and then degraded inside the renal tubular cells, releasing free cadmium ions.
The accumulation of these free cadmium ions damages the proximal tubules, which are responsible for reabsorbing substances back into the blood. This damage, known as nephrotoxicity, impairs tubular function, leading to the excessive excretion of low-molecular-weight proteins, amino acids, and glucose into the urine. This condition is often detected as proteinuria and represents the earliest observable sign of chronic cadmium poisoning.
Long-term exposure is also strongly associated with skeletal damage, even in non-occupationally exposed populations. Cadmium-induced kidney damage causes hypercalciuria, where excessive calcium is lost in the urine. This loss contributes to bone demineralization, osteoporosis, and an increased risk of fractures. Severe chronic exposure has historically been linked to the painful “Itai-Itai” disease.
The International Agency for Research on Cancer (IARC) has classified cadmium and its compounds as carcinogenic to humans. Epidemiological studies suggest that dietary cadmium intake may be associated with an increased risk of certain cancers, including those of the breast and prostate. Chronic exposure is further linked to cardiovascular disease and hypertension.
Monitoring, Regulation, and Reducing Dietary Intake
International health organizations, such as the Joint Food and Agriculture Organization (FAO)/World Health Organization (WHO) Expert Committee on Food Additives (JECFA), establish standards to protect the public. These bodies set limits like the Provisional Tolerable Weekly Intake (PTWI) or Provisional Tolerable Monthly Intake (PTMI) for cadmium. For instance, the European Food Safety Authority (EFSA) established a Tolerable Weekly Intake (TWI) of 2.5 micrograms per kilogram of body weight. These reference values are set at a level where adverse health effects are not expected over a lifetime of exposure. However, some studies suggest that adverse effects, particularly kidney dysfunction, may occur even at exposure levels near or below these established guidelines.
Consumers can adopt practical strategies to reduce their overall dietary exposure to cadmium. The most effective step is to maintain a varied and balanced diet, avoiding over-reliance on a single food source known to accumulate the metal.
Avoiding tobacco inhalation is a significant way to lower the body’s total cadmium burden, as smoking is a major non-dietary source. For high-absorption foods like rice, proper preparation techniques, such as rinsing the grains before cooking, can help reduce the surface concentration of the metal. Ensuring adequate nutritional status of minerals like zinc and iron may also help decrease the body’s absorption of cadmium.