Trace elements in native and improved paddy rice from different climatic regions of Sri Lanka: implications for public health

Paddy rice (Oryza sativa L.) is the second most important staple food for over half of the world’s population and also cultivated throughout all countries (Muthayya et al. 2014). 30 % of the dietary energy supply and 20 % dietary protein intake in Asia are provided by rice (WHO/FAO 2002). However, it is well-known that rice invariably contains significant amounts of trace elements that accumulate through the environmental food chain (Watanabe et al. 1996). Although trace elements in rice are found usually in low quantities, due to application of agrochemicals and irrigation of paddy fields with contaminated water, contents could considerably elevated. For instance, significantly higher amounts of Cd, Pb and As were reported in rice grains from contaminated areas and even from some non-polluted regions (Meharg et al. 2009, 2013; Watanabe et al. 1996). Exposure to such toxic trace elements through food chains could cause an adverse impact on human health leading to certain chronic diseases. Hence serious concerns over heavy metal accumulation in the soil-rice plant system have been addressed in recent years (Gupta and Gupta 1998; Fu et al. 2008; Zhu et al. 2008). In certain regions of Asia, elevated levels of toxic trace elements such as Cd and As in rice were reported. For instance elevated Cd in rice were reported in Bangladesh and Sri Lanka (Meharg et al. 2013) and also in few other Asian countries (Watanabe et al. 1996) while arsenic contaminated rice was reported in Bangladesh (Das et al. 2004; Abedin et al. 2002; Meharg and Rahman 2003).

Trace elements such as Se, Mo, Cr, Mn, Fe, Co, Cu, Zn are well-known as micronutrients that help in the proper functioning of human biological systems, while non-essential elements such as Pb, As, Cd, Hg do not have any clear physiological functions (Underwood 1979; Dissanayake and Chandrajith 1999). From among essential elements, Se plays a great role in human organisms by contributing large number of biological functions mainly enzymatic roles as an antioxidant and as a catalyst for thyroid hormone production. (Rayman 2000; Beckett and Arthur 2005; Sun et al. 2014). Selenium also plays a dual role as an essential micro-nutrient and as a toxic element with a narrow range between the edges of the supply spectrum (Fordyce et al. 2000; Tan et al. 2002). In Asia, most of the human Se requirement is supplied via cereals such as rice but supplementation is considerably low causing serious health problems (Williams et al. 2009). Therefore assessing Se levels in rice is extremely important since the majority of Se requirement is supplied from rice.

Similar to other counties in Asia, rice is the staple food in Sri Lanka and is deeply embedded with its economy, traditions and culture. Rice is grown in the entire island which is characterized with variable climate and geography. It is estimated that nearly 34 % (0.77 million ha) of the total cultivated areas in Sri Lanka have been used for rice cultivations from which over 95 % of the domestic requirement is fulfilled (DOASL 2015). Both rain-fed and irrigated rice paddies are common in Sri Lanka which is characterized by three distinct climatic zones known as the wet, intermediate and dry zones (Fig. 1). The wet zone receives over 2500 mm annual rainfall while the dry zone receives about 1000 mm of annual rain. The intermediate zone with about 1500 mm annual rain is located in between wet and dry zones (Domrös 1979). Monsoonal rainfalls from north-east (NE) and south-west (SW) of the island control the spatial variability of the climate of Sri Lanka. The wet zone rice cultivation is mainly rain fed, but due to scarcity of water, cascading system of reservoirs and canal irrigations (Panabokke et al. 2002; Mahatantila et al. 2008) are used for the paddy cultivation in the intermediate and dry zone regions.

In recent years, wide attention was paid to the quality of drinking water and food in Sri Lanka due to the emergence of geographically distributed health issues. As a tropical island, Sri Lanka is strongly interlinked between geographically distributed endemic health issues and environmental factors (Dissanayake and Chandrajith 2007, 1999). Among such health incidences, Chronic Kidney Disease of uncertain aetiology (CKDu) that is prevalent in the dry zone areas of Sri Lanka has received wide attention (Athuraliya et al. 2011; Chandrajith et al. 2011). The histopathological investigations on CKDu suggested an involvement of a possible environmental nephrotoxin (Athuraliya et al. 2011; Nanayakkara et al. 2012). Recent studies indicated high levels of toxic elements such as Cd (Meharg et al. 2013) and As (Jayasumana et al. 2013) in rice that was attributed to the aetiology of endemic CKDu in Sri Lanka. Jayatilake et al. (2013) inferred a possible link between deficiency of selenium and incidence of CKDu. Rice grains can be contaminated with toxic elements resulting from agrogenic processes such as extensive use of fertilizers and pesticides that are common in contemporary rice field agriculture in Sri Lanka. In tropical Sri Lanka, different new improved varieties of genotype Indica rice are widely cultivated and are more popular among farmers due to more economic benefits with higher yield, drought and flood tolerance and disease resistance. It was reported that over 2000 rice varieties were cultivated in Sri Lanka, but only a few are now widespread (Paroda 1999; Kennedy and Burlingame 2003). At present, several white and red types of improved rice types are widely cultivated in Sri Lanka (DOASL 2015). About 16 native varieties of rice are also grown in isolated paddy fields by mostly adapting organic agriculture practices. Therefore, the main objective of this study was to assess the levels of toxic trace elements in rice produced in Sri Lanka giving particular emphasis to Se, As and Cd since these elements were considered as important causative agents of CKDu. The toxic trace element levels were compared between different rice types (white and red) and their geographic locations in respect to the climatic zones.

A total of 226 samples of new improved rice and 21 samples of native rice varieties were collected, covering most parts of the rice growing regions in Sri Lanka (Fig. 1). From new improved rice samples, 163 samples were white rice and 81 samples from wet zone, 70 from intermediate zone and 75 from dry zone were collected. However almost all traditional varieties were obtained from wet zone paddies. Samples of rice seeds were directly collected from paddy fields and stored in zip-lock bags. Seeds were then dried at 40 °C to remove moisture and de-husked to separate grains that were finely powdered using an agate mortar and pestle. Samples were dried for a few hours at 60 °C and were stored at 4 °C in screw-capped plastic containers. About 0.50 g of the subsamples were digested using Mars-6 microwave digester (CEM; Matthews, NC) equipped with EasyPrep Plus high pressure vessels. Digestion was performed with 10 mL HNO₃ (≥69.0 % TraceSelect; Fluka, Switzerland) and 1 mL of H₂O₂ (35 wt. %; Sigma-Aldrich, Germany) and diluted to 50 ml using ultra-pure water. Concentration of trace elements Li, B, Al, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Sr, Mo, Cd, Ba, Pb and Bi were determined using Thermo ICapQ (Thermo-Fisher Scientific Inc., Bremen, Germany) Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Multi-element ICP-MS standards (Sigma-Aldrich, Germany) were used for the instrumental calibration and ¹⁰³Rh was used as the internal calibration standard. All measurements were done at least in duplicates and analytical results were expressed on dry matter basis. The NIST 1568a rice flour reference (National Institute of Standard Technology; MD, USA) material was used to validate the analytical procedure. We have obtained 0.330 ± 0.004 mg/kg for Se against 0.38 ± 0.04 mg/kg; 0.260 ± 0.003 mg/kg for As against 0.29 ± 0.03 mg/kg and 0.0280 ± 0.003 mg/kg for Cd against 0.022 ± 0.002 mg/kg with NIST 1568a. For Cu, Zn and Mn recoveries were within ±10 % while slightly higher recoveries (±15 % RSD) were observed for Mn and Al with the NIST 1568a.

All statistical analyses were performed by SPSS ver. 16.0 in which concentrations less than the limit of detection (LOD) are allocated a value of half-LOD for subsequent statistical interpretations. Two way multivariate analysis of variance (MANOVA) was applied to evaluate the differences in elemental composition in rice between climatic zones and the rice type (white or red). In this case, the overall mean of the groups were compared by test statistics. In all cases probability value <0.05 (95 % level) was considered as significant. Non-gender specific Estimated Daily Intake (EDI) of trace elements through rice consumption were estimated as EDI = C _i  × Q, where C _i is the average concentration of element i and Q is the amount of average daily consumption of rice which is considered as 284 g/day (Fordyce et al. 2000).

Rice is one of the most widely consumed foods worldwide while in Sri Lanka it is consumed at a rate over 100 kg/person rice annually. Recently it has been reported that rice in Sri Lanka contains high levels of highly toxic elements such as cadmium and arsenic. Such elements were then attributed to the onset of chronic kidney diseases in certain geographically discrete areas of the country. The results obtained in this study increase our attention of trace metal contents in rice from different climatic regions and also difference between indigenous rice and improved rice varieties. A multivariate data analysis method was used to identify the difference in trace metal composition in improved rice on climatic condition hence the soil characteristics and on different varieties as white and red rice.

Based on the results of descriptive statistics and multivariate analyses, it can be concluded that the As content in rice does not significantly differ among climatic zones and rice types while Se differs among rice types. However Cd content in rice differs drastically according to rice type and climate zone. The elevated levels of toxic elements, Cd and As in rice suggests that paddy soils have been subject to a high input of anthropogenic contaminants that are most likely related to the agrogenic activities such as application of fertilizer used to improve production and quality since the geogenic input is not considerable in Sri Lanka. Interestingly, native rice varieties contained significantly higher content of Se which is an essential trace element that is considered as being deficient among Sri Lankan population. Since the Se intake among Sri Lankans seems to be low and critical while the main staple food does not supply adequate amounts, assessing the Glutathione peroxidase in blood in the population is highly warranted. It is also important to adopt regulations to control the agrogenic trace metal inputs.