As Contamination in Food. Distribution of As in Food in Various Parts of the World

As discussed earlier, drinking water is the predominant source for As intake by humans and animals. However, exposure to As through food is also of major concern. This is because, in spite of having access to safe drinking water, people may get exposed to As through food. The daily intake of As adds up when people consume both contaminated food and water. Sometimes it could go higher than the safe levels for iAs consumption (10 ppb kg^-1 body weight), causing serious health effects. Knowledge about As in food will help in analyzing the risk of contamination through food, and thus facilitating the development of strategies to avoid As-exposure related health problems.

Arsenite and arsenate are the most toxic species of As found in food. Tetramethylarsonium ion (TMA⁺), MMAA, and DMAA are commonly observed too. Organic arsenicals such as arsenocholine (AC), arseno- betaine (AB), trimethylarsine oxide (TMAO), and arsenosugars are practically harmless. However, in human beings, iAs is biotransformed into carcinogenic forms DMA and methylarsonate (MA) and excreted through urine.

As in Rice. Rice is the staple food of people in southeast and south Asia. When compared to cereals such as wheat and barley, rice accumulates 10 folds higher As in its grains. iAs is subsequently found in food products made with rice. As accumulation by rice can be attributed to the innate tendency to uptake silicon in the form of silicic acid, required for optimal growth. Owing to the chemical similarity of Silicon and As, which are present as neutral molecules (arsenous acid - As(OH)₃ and silicic acid - Si(OH)₄) in soil, transport of As(III) occurs.

When these are consumed by children and infants, health may be deleteriously affected, as children are more vulnerable to iAs when compared to adults. Davis et al. found that intake of rice containing iAs by women during pregnancy lead to its occurrence in the toenails of infants. Currently, mechanisms of As accumulation is widely studied to develop techniques for decreasing its load in rice.

Accumulation of As in rice is primarily due to irrigation of soil with contaminated groundwater. Rice is extensively irrigated with groundwater (not surface water, which is lower in As (1-2 pgL^-1) during dry seasons. In Bangladesh, soil irrigated with groundwater had an average concentration of 8.5 mg kg^-1 of As whereas irrigation with surface water accounted to 5.7 mg kg^-1. Annually, 1000 kg (approx.) of As is added to soil by irrigation with groundwater across the farmlands of the country, where level of iAs in groundwater is about 1 ppb.

Under highly flooded conditions, the iron-reducing bacteria promote reduction of iron-oxyhydroxides and thereby increase the As bioavailability for rice crops [55]. However, according to a heavy metal accumulation study conducted on rice by Soujit et al. out of 505 varieties, 70 rice varieties did not show any As uptake. These varieties include Bhim-sal, Kharah, Kali-Jira, Kamini, and Hati-dhan grown on a conservation farm.

Deficit irrigation of rice with groundwater contaminated with As results in 17.6-25% lower accumulation of As in polished rice grains. This technique takes advantage of the water stress tolerance of rice, which does not affect the yield. Reduction of As(V) to As(III) can be decreased by maintaining the soil aeration through proper water management. As mentioned earlier, As(III) has higher bioavailability, solubility, and toxicity.

Figure 4. Sources of As in the human diet

As can also enter food during cooking. Cooking rice in As-contaminated water results in As retention in rice leading to ~50% increase in concentration. Since rice grains have a tendency to bind to iAs, the level of As increases after cooking. As can also readily leach from potteries and ceramic cookware into the food cooked in them. The first report of such leaching was given by Ciftci and Henden in 2016 based on studies in Turkey. Pottery is commonly used for processing and storage of food in Turkey. The weekly intake of iAs in such cases rises by 34-79%, above the benchmark dose lower limit. This correlates with a 0.5% increase in the incidence of lung cancer (whenever As intake exceeds 3.0 pgkg^-1, i.e. 3ppb ofbody weight day^-1). Figure 4 shows the sources of As and the various routes through which it reaches the human diet.

Table 5. Distribution of As in food in various parts of the world

As in Seafood. The health benefits of seafood are well known. However, As in seafood is present in substantially higher concentrations when compared to cereals, vegetables, and meat. Organoarsenicals such as arsenobetaine, arsenosugars, and arsenolipids are found in seafood currently. iAs is also found; however, it gets biotransformed into arsenosugars by algae that take it up from seawater. The level of iAs is approximately 1% of the total As content in seafood. However, this could even reach 3-5% depending on the country and water-specific situations. The concentration of different species of As in various types of food across the world is mentioned in Table 5.