Global warming and arsenic in rice

In a series of posts we are going to look at the impact of global warming on food production and the potential for an increase in toxic compounds in our normal diet. First off is rice and higher levels of arsenic found in the rice grain when exposed to higher temperatures during cultivation.

Rice is the world’s most important foodstuff providing nutrients and energy to more than one half of the world’s population. Unfortunately, rice can also contain arsenic, which can cause multiple health conditions and diet-related cancers. In an earlier post we described possible chronic health effects of natural levels of arsenic in food and water.

Here we will cover two issues – the influence of higher global temperatures on arsenic levels in rice and types of arsenic compounds formed in soil under different environmental conditions.

Temperature dependence of arsenic accumulation

Arsenic occurs naturally in soil at different levels across the world. When farmers grow crops like rice under flooded conditions, arsenic is drawn out of the soil and into the water. As rice plants extract water through their roots to its leaves, arsenic follows as it mimics other molecules that rice plants preferentially draw out of the soil.

Now researchers at the University of Washington have found that warmer temperatures, at levels expected under most climate change projections, can lead to higher concentrations of arsenic in rice grains at ranges where they begin to have further health concerns. Arsenic concentrations in the grain more than tripled between the low- and high-temperature treatments.

However, the researchers didn’t have the means to check the type of arsenic compounds found.

Some forms of arsenic are more toxic than others.

It is important to know that not all arsenic is the same as arsenic exists in several different forms. Fish and seafood usually contain high levels of arsenic, but most of this is arsenobetaine, an organic form with little toxicity. It is the inorganic arsenic that can be found in water and rice and a range of other food commodities that has been of particular concern.

However, arsenic speciation is not easy to perform, which has created some confusion. Inorganic and methylated oxyarsenic species have been a focus of research, but thioarsenates, in which sulfur takes the place of oxygen, have largely been ignored.

Now University of Bayreuth researchers, together with scientists from Italy and China, have for the first time systematically investigated under which conditions, and to what extent, sulphur-containing arsenic compounds are formed in rice-growing soils. It turns out that the amounts of thioarsenates formed are linked to the pH-values of the soils and other environmental parameters.

Formation of thioarsenates in soil, their uptake in rice plants and their potential risks to human health urgently require further research as at least one organic sulphur-containing arsenic compound discovered in rice fields is already known to be carcinogenic.

A bad situation potentially made even worse

Arsenic is one of WHO’s 10 chemicals of major public health concern and in particular for the millions of people who rely on rice as their staple food. Young children are also at risk if rice-based products make up a large part of their diet.

Global warming has the potential to make a bad situation even worse. With an increase in global temperatures higher levels of arsenic in rice will follow and the composition of the arsenic compounds may change, for better or worse.

So please be careful in contributing to global warming.

Arsenic – not a dead issue

Arsenic in rice a current issue (Photo: tamaki – Flickr)

If you thought that arsenic was just a way of murdering people in fiction books from the last century, think again. Just now arsenic in rice is a hot topic in the USA, with the Food and Drug Administration (FDA) busy analysing levels that can be found in products available on the American market. But the European Food Safety Authority (EFSA) was ahead of the game. At the request of the European Commission, the Authority’s CONTAM Panel reviewed the safety of arsenic in food and published its opinion in September 2009 raising some considerable concerns.

Toxic effects of chronic arsenic consumption

There is no doubt that deliberate poisoning or attempted suicide using arsenic in high doses will cause damage to nervous tissue and subsequent death. Such acute effects are well-documented in the scientific literature, but not normally associated with levels found in food.

Our interest is in chronic effects from natural arsenic levels in food and water. A range of toxic effects have been reported from long term ingestion of lower levels of arsenic including skin lesions, cancer in the lung, urinary tract and skin, cardiovascular disease, abnormal glucose metabolism, and diabetes. Skin lesions are particularly common in Bangladesh, where aid efforts to provide fresh water to the population through deep water bores lead to arsenic contamination. The CONTAM Panel also identified emerging evidence of negative impacts on foetal and infant development, particularly reduced birth weight seen in Bangladesh and northern Chile.

Food with high arsenic levels

To calculate population exposure to arsenic, EFSA collected analytical results of levels found in food and reported by European Union Member States as part of their routine monitoring. The CONTAM Panel evaluated the total arsenic concentrations reported for a number of food commodities.

Hijiki seaweed had high arsenic levels (Photo: kattebelletje – Flickr)

The highest arsenic levels were found in fish and seafood, food products or supplements based on algae, especially hijiki, and cereal and cereal products. Particularly high concentrations of arsenic were found in rice grains and rice-based products, and bran and germ.

Depending on the time and temperature used during food processing or final food preparation, changes in total arsenic concentration and arsenic species may occur. The arsenic content in cooking water seems to be of special importance because it determines whether the arsenic concentrations in the prepared food may be higher or lower compared to the raw product. As an example, cooking rice in excess water will reduce the arsenic levels in the rice consumed after discarding the remaining water.

Inorganic arsenic is the toxic form

But what about the arsenic species mentioned above? It is important to know that not all arsenic is the same since arsenic can be bound to a number of other compounds. Fish and seafood usually contain high levels of arsenic, but most of this is arsenobetaine, an organic form with little toxicity. It is the inorganic arsenic that can be found in water and rice and a range of other food commodities that is of particular concern. Soluble inorganic arsenic is rapidly and nearly completely absorbed after ingestion, widely distributed to almost all organs and readily crosses the placental barrier.

And here we have a real problem in that arsenic analysis at speciation level is very difficult. The CONTAM Panel had to make assumptions about the proportion of inorganic arsenic in different food commodities based on the total arsenic levels reported to allow calculation of specific exposure to the toxic species.

Some consumers might be at risk of arsenic poisoning

It is recommended that arsenic intake be reduced (Photo: Mirror – Flickr)

The EFSA opinion concluded that estimated dietary exposure to inorganic arsenic for average and high level consumers in Europe were within the range of toxic effects and that the possibility of a risk to some consumers could not be excluded. Consumer groups with higher exposure levels included high consumers of rice, such as certain ethnic groups, and high consumers of algae-based products. It was thus recommended that dietary exposure to inorganic arsenic should be reduced.

The CONTAM Panel pointed out that in order to refine risk assessment of inorganic arsenic there was a need to produce speciation data for different food commodities to refine the dietary exposure assessment and to get access to more detailed dose-response data for toxicity to better quantify possible health effects.

The EFSA opinion was published in 2009 and not much has happened since at a European level in coming to grips with the issue. There has been some further focus on rice-based dishes for children, but not much else. Maybe the American rice survey will prompt some worldwide action. It is clear that the issue is of vastly more importance in some South East Asian and South American countries.

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This blog post is based on the EFSA Scientific Opinion on Arsenic in Food

Further reading: