Crabmeat and Health Study: Do Brown Crabmeat Consumers Have a Higher Body Burden of Cadmium Compared to Non-crabmeat Consumers?

Crab meat is low in fat and is a good source of sustainable protein that also provides high levels of minerals (Se, Zn, I), vitamins (B12) and long chain omega-3 fatty acids. However, crab meat can also contain high levels of the heavy metal cadmium (Cd), both in the white meat but especially so in the brown meat (the hepatopancreas and liver). While European legislation sets a limit of 0.5mg Cd/kg wet weight for the white meat from crab, there currently is no limit set for the brown meat from crab due to the large range in concentrations found across member states.

Cd occurs in an inorganic form as an environmental contaminant, both through natural occurrence and from agricultural and industrial sources. The general population is exposed to Cd from various sources, including smoking, but in the non-smoking general population, food is the principal source. Cd is predominantly toxic to the kidney, but it also causes bone demineralisation and is statistically associated with increased risk of cancer in the lung, endometrium, bladder and breast. Moreover, blood Cd levels as low as 0.5 - 0.8 µg/l have been associated with chronic disease. A study of 8,989 US adults from the National Health and Nutrition Examination Survey (NHANES) found such levels to be associated with all-cause mortality, cardiovascular disease, and heart disease after around 5 years follow up and adjustments for a number of potential confounding factors (Tellez-Plaza et al. 2012).

Cd is thought to be absorbed within the body through the use of the same transport pathways as essential metals including zinc, calcium and iron in the intestine. After absorption, Cd is transported in the blood to the liver where it forms a complex with metallothionein, a low molecular weight family of proteins. In the kidney, the Cd-metallothionein complex is filtered in the glomeruli and then reabsorbed in the renal tubules. Although absorption of Cd from the diet in humans is comparatively low (3-5%), it is efficiently retained in the kidney and liver, with a long biological half-life (10 to 30 years). Cd excreted in the urine is a well-characterized exposure biomarker and can be used as a quantitative indicator of cumulative internal dose.

Accumulation of Cd increases with age (Olsson et al, 2002), with 50yrs being the age where urinary cadmium is maximal (WHO/FAO, 2004). Cd is primarily toxic to the kidney, especially to the proximal tubular cells where accumulation over time may cause a decrease in the glomerular filtration rate and eventually, renal failure. Cd causes bone demineralisation, either through direct bone damage or indirectly as a result of renal dysfunction. It is through long term accumulation of Cd that its toxic effects are mainly manifested. As Cd has a high affinity for the main intestinal iron transporter (Kim et al, 2007), the absorption of Cd is influenced by body iron status and there is a close inverse association between serum ferritin (a marker of iron stores) and blood Cd (Berglund et al, 1994). In this regard, studies on human populations have shown increasing blood Cd levels with decreasing serum ferritin in women at fertile age and during pregnancy due to their generally decreased iron status (Berglund et al., 1994).

There is concern that Cd from crab meat consumption can contribute greatly to the body's Cd burden however, no studies have investigated whether the Cd present in crabmeat increases the levels of Cd in the body and similarly, it is not known whether higher Cd intakes in regular crab meat eaters is associated with increased incidence of kidney damage. This study will examine some of these issues. There is evidence from some studies suggesting that selenium and perhaps zinc, which are both present at high levels in crab meat, may counteract the toxicity of Cd (Lindh et al, 1996; Skröder et al, 2015). Moreover, some evidence may suggest that the bioavailability of Cd from crab meat may be less compared to Cd in other foods (Lind et al, 1995). The study will investigate whether volunteers who regularly consume brown crabmeat have higher levels of Cd compared to volunteers who do not consume crabmeat. A frequency questionnaire will be designed to estimate participants' total brown crabmeat intake and to evaluate Cd exposure from crab consumption and this will be compared to the Cd body burden (assessed as urinary Cd and blood Cd levels) to determine whether an association between these variables exists and whether regular crab meat consumers have a higher body burden of Cd compared to non-crabmeat consumers.