How Does Arsenic Get into our Food?
Arsenic occurs naturally in the environment, in minerals such as realgar and arsenopyrite and from there enters the soil, water and even the air. The historical use of arsenic-containing pesticides, long before modern environmental safety regulations were in place, is also an important source of arsenic in the environment. Arsenic, like other metals and metalloids, stays in the soil for long periods of time, where it can either be taken up by plants or washed down into the groundwater, and present a risk to human health. However, the most important factor determining whether arsenic in the soil gets into the plant-based food crops we eat every day, is the genetic makeup of the plant itself.
Arsenic can take numerous chemical forms that are broadly classed into inorganic or organic. Here the term 'organic' is used in its scientific sense to indicate arsenic binding to the elements associated with organic chemistry - carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur. Inorganic forms are more acutely toxic, but both forms are toxic to humans. Almost half of the arsenic in U.S. rice, for example, is in the inorganic or more toxic form, but the arsenic in rice from Bangladesh is almost all (80%) inorganic. Interestingly, different varieties of rice from all over the world grown on the same soil accumulated different amounts and different species of arsenic. The key to resolving arsenic accumulation in food plants lies in understanding the genes that control uptake from the soil and storage within the plant's edible portions.
Why Does Rice Accumulate Arsenic?
Rice has been described as a natural arsenic accumulator, but the reasons why it does so make it particularly unusual in comparison with other cereal plants. Under normal circumstances rice plants actively take up large amounts of silicon from the soil, unlike its close relatives wheat, barley and oats. They use silicon to strengthen their stems and the husks that protect the grain against pest attack. Scientists have imaged large 'silica bodies' in the leaves of rice plants. Arsenic and silicon are chemically very similar under the soil conditions found in flooded rice paddies, and as a result arsenic literally fits into the silicon transporters, and is integrated into the plant as it grows, finding its way into the grain--the part of the rice plant we eat. Scientists are working to unravel the complex chemistry of arsenic as it moves from the soil into the plant and the genes that control its movement. Our challenge is to understand the metabolism of arsenic in plants, in particular how it finds its way into the grain, and find ways to prevent cells from taking up arsenic.