An unexpected pathway for heavy metal toxicity

Cadmium is a toxic heavy metal found in many agricultural soils. Until now there has been little concern about excessive levels of cadmium getting into the food we eat. A recent paper from scientists at Zhejiang University in China, published in the peer-reviewed journal PLOS ONE, may change that thinking.

Farmers in many countries, including Canada, plant legumes such as clover in with their food crops. The legumes fix nitrogen from the air, reducing or eliminating the need for nitrogen fertilizer, and help solubilize phosphorous, another plant nutrient. The newly published research shows that the legumes also help the plants take up cadmium from the soil and that an increased concentration of cadmium can be found in the edible part of the plant.

Increased levels of cadmium attributable to the interplanting with legumes were found in both corn and tomatoes as well as in leafy foodstuffs such as cabbage. The amount of the increase in cadmium concentration varied with the legume chosen for interplanting. Tomatoes interplanted with clover exceeded the maximum permissible concentration standard for cadmium of the National Standard Agency in China. Canada does not have a limit for cadmium in tomatoes as it is not considered a likely contaminant. A joint FAO/WHO expert committee has estimated a provisional tolerable weekly intake of cadmium for an adult human to be from 0.4 to 0.5 mg. The Chinese study has found double this amount of cadmium in one kilogram of tomatoes grown in interplanting with legumes. Similar amounts of cadmium have also been found in cabbage and pakchoi. In the edible part of corn the amount of cadmium approached 0.14mg/kg, three times the provisional tolerable weekly intake for an adult..

The authors of the study state that the “results do not support growing legumes in soils with elevated Cd levels even though these Cd levels are near to or below the pollution threshold as the co-cultivated legumes will biologically increase the Cd level in the adjacent crop. This new revelation suggests that the future design of cropping systems involving legumes should be carefully considered to avoid food contamination by Cd although legumes and crops vary in responding to a specific legume -crop system.”

In parts of Canada industrial sewage sludge is used as a low-cost replacement for fertilizer on many agricultural fields. The results of this study suggest that governments should review the possibility that intercropping food crops with legumes  on land that has been spread with sewage sludge containing elevated levels of cadmium may be further increasing cadmium levels in the human food chain. No doubt this study will soon prompt researchers to look at the levels of cadmium in food products purchased from Canadian supermarkets.

The complete study is available at http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0042944

A Health Canada discussion of the toxicity of cadmium is available at http://www.hc-sc.gc.ca/ewh-semt/pubs/water-eau/cadmium/index-eng.php#Toxic

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