Essential Trace Elements in Commonly Consumed Varieties of Sri Lankan Cooked Rice and Its Dietary Significance: A Focus on Recommended Daily Allowances

doi:10.21203/rs.3.rs-7542645/v1

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Essential Trace Elements in Commonly Consumed Varieties of Sri Lankan Cooked Rice and Its Dietary Significance: A Focus on Recommended Daily Allowances

https://doi.org/10.21203/rs.3.rs-7542645/v1

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published 10 Dec, 2025

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Essential trace elements (ETEs) are indispensable micronutrients required in trace amounts for maintaining metal homeostasis and supporting critical physiological functions. Dietary intake is the principal source, with deficiencies linked to numerous chronic conditions. In Sri Lanka, rice (Oryza sativa L.) is the staple food and a primary source of ETEs. However, post-harvest and culinary processes significantly influence ETE bioavailability. This study assessed Zn, Se, Mn, and Cu concentrations in raw and cooked grains from 25 rice-composites representing widely consumed Sri Lankan rice, including Traditional (Suwandel, Kaluheenati, Pachchaperumal), Improved (White/Red Nadu, Samba, Kekulu), and Imported (Indian Basmati) varieties. Samples were stratified by pericarp color (red/white) and parboiling treatment. Standardized domestic cooking methods were applied, and lyophilized samples were digested and profiled using ICP-MS. Mean ± SD concentrations in raw grains (mg/kg dry weight) were: Zn 32.02 ± 6.82, Se 0.049 ± 0.016, Mn 13.71 ± 3.86, Cu 0.47 ± 0.83. Red pericarp and parboiled varieties exhibited significantly higher ETE levels (p < 0.05), with Traditional cultivars enriched in Se and Mn (p < 0.05). Cooking led to significant reductions (p < 0.001): Zn (17.42–60.26%), Se (20.98–59.35%), Mn (20.92–53.73%), Cu (4.53–65.36%). Based on average rice intake (682.5 g/day), cooked rice contributed: Zn 73.50–101.06%, Se 19.63–21.42%, Mn 123.44–157.73%, Cu 44.51% of RDA. Notably, the Se insufficiency was consistently low across all varieties. While Sri Lankan rice provides meaningful ETE contributions, dietary diversification remains essential to meet micronutrient adequacy, particularly for elements with inherently low gut-absorption efficiencies.

Essential Trace Elements

Essential Minerals

Sri Lankan rice

Cooked rice

RDA fulfillment

Essential Trace Elements (ETEs) represent a critical group of elements including zinc (Zn), selenium (Se), manganese (Mn) and copper (Cu) that are essential for sustaining various physiological functions in the human body. Required only in trace quantities, ETE concentrations in the human body are carefully regulated, with deficiencies causing wide-range of nutritional and health complications, vary from multi-organ failures to mortality. Nutritional and public health authorities have established quantities deemed sufficient to fulfill nutritional requirements of 97–98% of health individuals across age and gender demographics, referred to as Recommended Daily Allowances (RDAs), which serve as guidelines to prevent ETE deficiencies [1, 2]. Regional and country-specific RDA adaptations are available to address sub-population requirements [3, 4]. Conversely, excessive intakes can disrupt metal homeostasis causing toxicosis, with guidelines such as Tolerable Upper Intake Limits (ULs) safeguarding against overexposures [2].

Diet represents the predominant mode of ETE intake into the human body. ETEs, are common constituents of various animal and plant products [5], enabling a well-balanced diet to provide recommended intakes without additional supplementation. Low dietary diversity renders certain sub-populations vulnerable to ETE deficiencies [6], particularly those populace with low socio-economic standing limited to caloric-rich daily diets, having few staple food items with poor nutritional quality. Factors including food accessibility, bioavailability, chemical composition, presence of contaminants and metal absorption inhibitors (phytates), food matrix interactions and consumption rates determine the dietary contributions toward RDAs [7].

Approximately 50% of Sri Lankan adults and children suffer from coexisting micronutrient deficiencies [8]. Published materials address ETE nutritional deficiency prevalence in Sri Lankans, with majority addressing Zn deficiency [9–12] and to a lesser extent Cu and Se [10, 13] in vulnerable populations including women and children. Low Zn and Se body stores are highlighted in Chronic Kidney Disease (CKD) [14, 15] and Chronic Kidney Disease of Unknown Aetiology (CKDu), affecting rural agricultural populations in Sri Lanka [13, 16]. Rural sub-populations, especially in CKDu-endemic areas, show lower kidney accumulation of both Zn and Se compared to non-endemic urban populations [17]. Dietary insufficiency represents a common factor in most ETE deficiency studies, reflecting rural households’ limited access to diverse food groups resulting in low dietary diversity [18]. National dietary patterns show insufficient intake of fruits, vegetables, animal products [19], and dairy [20], with observed shifts towards increased sugar, salt and alcohol consumption exceeding recommended amounts [8, 20].

Rice (Oryza sativa L.) serves as a major dietary ETE source [21] and represents Sri Lanka’s dietary staple with per capita consumption at 110–120 kg annually (Department of Agriculture - Sri Lanka, 2023; Department of Census and Statistics - Sri Lanka, 2022). Rice is consumed at least twice daily [24, 25], fulfilling ~ 45% of daily caloric requirements [22, 26]. Daily raw rice consumption by a Sri Lankan adult averages ~ 300 g, ranking among the world’s highest consumption rates [27, 28]. Two major cultivation categories exist in Sri Lanka; Traditional/heirloom varieties representing historic indigenous varieties, and Improved varieties consisting of hybridized cultivars [29, 30]. Lower growth cycles, higher yields and harvest potential make Improved varieties preferable to farmers.

Sri Lankan rice is recognized as a nutritionally rich functional food containing bioactive compounds, minerals, and ETEs, utilized in traditional medicine since pre-historic times [31]. While remaining the major source of dietary ETEs to Sri Lankans [32], rice shows comparatively lower ETE accumulation versus other cereals [33, 34]. Critically important is the availability of these biomolecules in table-ready cooked rice, as many are lost from the grains during the raw-to-cook transformation. Studies report significant ETE losses during grain processing, including post-harvest modifications like parboiling and domestic cooking procedures involving washing, steaming, and boiling [35–37]. Many ETEs are compartmentalized within the rice bran, often removed during milling, polishing and washing [32, 37, 38].

Most published studies address ETE quantities in Sri Lankan rice, only in the raw state [24, 32, 39–41], with comprehensive lack of data regarding process induced transitions and viability. Integration of rice consumption patterns to evaluate ETE intake relevant to RDAs has not been conducted. Insights into complete nutritional assessment of ETEs in Sri Lankan rice could address important gaps in food fortification, implementation of food security, and public health policy, particularly for addressing ‘hidden hunger’ – micronutrient deficiencies [42].

The present study aimed to; (1) investigate Zn, Se, Mn and Cu levels in commonly consumed Sri Lankan rice varieties in raw and cooked stages, (2) quantify concentration changes during cooking, (3) estimate cooked rice contribution to RDA fulfillment by integrating daily consumption patterns, and (4) compare RDA fulfillment across rice varieties, discriminating between pericarp colors and parboiling stats to assess implications for varietal selection and dietary recommendations.

2.1. Rice sampling and processing

A total of fifty – four (54) raw, husked rice grains representing ten widely consumed Sri Lankan rice varieties [43–47] including; Traditional category (TRV): Kalu – heenati, Pachchaperumal, Suwandel; Improved category (IMV): Red/White Nadu, Red/White Samba, Red/White Kekulu; Imported category (IMP) Indian Basmati were purchased from bulk retail vendors at Dedicated Economic Centers (DECs) (Fig. 1). The rice varieties investigated included: These markets are government-established initiatives promoting farmer-to-consumer retail of agricultural goods from various geographical locations [48, 49].

Rice samples were collected in high-density-poly-propylene (HDPP) bags, sealed and stored at room temperature until processing. After evaluating basic grain morphological parameters (length, width, elongation ratio, shape, pericarp colors) and retail identifiers, samples were pooled in to twenty-five analytical composites for further analysis.

Raw rice grains underwent a standardized domestic cooking procedure [50], reflecting the most commonly utilized full-water-absorption cooking method in Sri Lankan households. Briefly, 2.5 cups raw rice were thoroughly washed and rinsed thrice using 0.75–1.00 L water respectively. Rinsed water was completely drained through a sieve before cooking in a domestic electrical rice cooker with 1:2.25 v/v rice-to-water ratio. After cooking completion, rice was cooled to room temperature before obtaining homogenized samples. Cooking water aliquots (Colombo Municipal Council water supply) were collected in HDPP containers, filtered, and stored at 4°C after stabilization with elemental grade HNO₃ (Suprapure®, Sigma-Aldrich Trace Select®, USA).

2.2. Sample preparation and digestion (in-vitro)

Raw, washed and cooked rice grains were lyophilized (-40°C: Lanconco FreezeZone, USA) until constant weight, homogenized, powdered, and stored at -20°C. Digestion protocols followed established standard laboratory practices for elemental analysis [17]. Approximately 0.2 g lyophilized grain powders (analytical balance 0.001 g, Satorius, Germany) were digested in-vitro using acid mixtures of HNO₃ (Suprapure®, Sigma-Aldrich, Germany), HCl, (Sigma-Aldrich Trace Select®, USA) and high-purity H₂O₂ (High purity, 33% wt., Sigma-Aldrich, Germany) in a high-pressure microwave digester (CEM/MARS-6, XP-1500, USA) ramped and maintained at 200°C for 40 minutes. Digests were filtered (Whatman® Grade 1, 11 µm, UK), volumerized to 50 mL with deionized water (Milli Q 18 Ω, Millipore, USA), transferred to metal-free HDPP containers (10% HNO₃ overnight bath, washed with deionized water), and refrigerated at 4°C until profiling.

2.3. Elemental Analysis

Elemental analysis was conducted using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) (Agilent 7900-ASX-500, USA), after removing total dissolved solids (single-use membrane filters, 0.45 µm, Thermofisher Scientific, USA). The grain sample digests were profiled alongside cooking water samples, method blanks, internal calibration standards and digests of external quality control materials, following standard laboratory practices [17]. Matrix-matched Certified Reference Material (CRM) (IRMM-804 Rice Flour, Institute for Reference Materials and Measurements, European Commission) was processed with samples as external quality control. ICP-MS calibration used standard calibration of 0–1000 ppb rare earth elements (2A°, Agilent, USA) under No gas and He modes respectively.

Results were expressed as milligram per kilogram of lyophilized rice grain powder on dry weight basis (mg kg^− 1 dw) for evaluating elemental transitions between raw and cooked stages, while results were converted to milligrams or micrograms per 100 g raw and cooked rice on wet weight basis (mg 100 g^− 1, µg 100g^− 1 ww) for consumption-based intake estimations. Cooking water contributions to elemental concentrations were subtracted from cooked grain values.

2.4. Rice Consumption Rates and RDA Assessment

Sri Lankan adult rice consumption was considered at recommended portion sizes [51]. Both internationally established RDA values (Meyers et al., 2006; Standing Committee on the Scientific Evaluation of Dietary Reference Intakes, Subcommittee of Interpretation, Uses of Dietary Reference Intakes, Subcommittee on Upper Reference Levels of Nutrients, & Panel on Micronutrients., 2002; Subcommittee on the Tenth Edition of the Recommended Dietary Allowances, Food and Nutrition Board, Commission on Life Sciences, National Research Council, 1989) and Sri Lankan RDA values (RDA_SL) [3] for each element were used to evaluate percentage fulfillment.

2.5. Data and Statistical Analysis

Rice sampling location map was constructed using QGIS software 2.18 for Windows© (Microsoft Corporation). Data representation used GraphPad Prism 10.0 for Windows© (Microsoft Corporation) while statistical analysis employed IBM SPSS 25.0 for Windows© (Microsoft Corporation). After evaluating data distribution, both parametric (student t-test, one-way analysis of variance − ₁ANOVA, paired student t-test, Pearson correlation- r) and non-parametric (Mann-Whitney-U, Kruskal-Wallis-H, Wilcoxon rank sum Test-W, Spearman’s Rho correlation - σ) statistical tools were applied accordingly. Data were not transformed and outliers were not removed. Statistical significance was set at p ≤ 0.05, with p ≤ 0.001 considered highly significant.

3.1. Basic Characteristics of Rice Varieties

Percentage distributions of red and white pericarp varieties were; 48% and 54% respectively, which consisted of 8% of IMP non – parboiled grains, 16% of TRV parboiled grains, and 76% IMV rice grains. Within the IMV rice category, 24% consisted of parboiled rice grains of Nadu; 20% consisted of parboiled Samba rice; and 32% of non – parboiled Kekulu rice.

3.2. ICP –MS Analysis and Quality Control Validation

Limits of Detection (LODs) for Zn, Se, Cu and Mn were; 0.991, 0.006, 1.069 and 1.007 ppb respectively, with instrumental Limit of Quantification at 1 ppt. Measured CRM (IRMM – 804) levels with total uncertainty (Δ_k2) were; Zn 23.1 (Δ_k2 1.2), Se 0.038 (Δ_k2 0.04), Cu 2.74 (Δ_k2 0.31) and Mn 34.2 (Δ_k2 0.5) mg kg^− 1, corresponding to recoveries of 96.51% 92.70%, 94.21% and 97.82% respectively, confirming the validity and reliability of the results. Regression coefficients (R²) for eight-point internal calibration standards (0.5–1000 ppb) ranged 0.98–1.0 for all elements.

3.3. ETE Concentrations in Raw Rice and Global Comparisons

Mean ETE levels in raw rice grains (mg kg^− 1 dw) were: Zn 32.020 ± 6.820, Se 0.049 ± 0.016, Cu 0.467 ± 0.827 and Mn 13.705 ± 3.858. A portion of 100 g of raw rice therefore contained; Zn: 3.012 ± 0.652 mg kg^− 1 ww, Se: 4.614 ± 1.511 µg kg^− 1 ww, Cu: 0.164 ± 0.078 mg kg^− 1 ww and Mn: 1.290 ± 0.367 mg kg^− 1 ww respectively.

Sri Lankan rice Zn levels exceeded USA averages [54] (Table 1). Cu and Man levels fell within USDA ranges, while Se levels were approximately three-fold lower than the averages.

Previous studies reported the Zn levels in Sri Lankan rice ranged 2.22–34.78 mg kg^− 1 [24, 32, 40, 55], consistent with current findings. Comparisons with Asian countries; Chinese rice (2.62–23.9 mg kg^− 1), Pakistani rice (2.60–13.40 mg kg^− 1), and Bangladeshi rice (2.54–22.91 mg kg^− 1) showed comparatively lower Zn levels [56–58], while Indian varieties showed markedly higher averages (117 ± 24 mg kg^− 1)[59].

Current Se levels align with previous Sri Lankan findings (0.0002–0.261 mg kg^− 1) [13, 24, 32, 40, 55] and South African rice (0.013–0.089 mg kg^− 1) [60], exceed Chinese non-fortified rice (0.008–0.0726 mg kg^− 1) [61] but remain below the global averages (0.002–1.57 mg kg^− 1) [62]. Soil Se content, pH, and fertilizer use affect soil-plant transfer and grain accumulation [61, 63] could be attributed to the observed differences.

Previously published Mn levels in Sri Lankan rice ranged 1.798–41.0 mg kg^− 1 [24, 32, 39, 55]. Bangladeshi rice Mn (Mean: 3.45 mg kg^− 1, range: 0.08–11.19 mg kg^− 1) showed lower levels than current study [56], while Pakistani rice (mean:13.89 mg kg^− 1, range: 2.70–30.50 mg kg^− 1) [57] demonstrated similar levels. These variations may be attributed to soil chemistry, climatic conditions, agronomic practices, and rice genotypes.

Cu levels in Bangladeshi (1.79 mg kg^− 1), Indian (4.6 mg kg^− 1), and Pakistani (36.07 mg kg^− 1) rice exceeded Sri Lankan levels [56, 57, 59].

Table 1
Comparison of ETE levels in Sri Lankan rice with international standards
Sri Lankan rice ^a								USDA data ^b
Pericarp color	Parboiling treatment	Variety	Grain stage	ETE quantity per 100 g portion (mean)				Pericarp color	Grain length	Parboiling treatment	Grain stage	ETE quantity per 100 g portion (mean)
Pericarp color	Parboiling treatment	Variety	Grain stage	Zn (mg)	Se (µg)	Cu (mg)	Mn (mg)	Pericarp color	Grain length	Parboiling treatment	Grain stage	Zn (mg)	Se (µg)	Cu (mg)	Mn (mg)
White	Parboiled	TRV (Suwandel)	Raw	3.49	5.61	0.18	1.99	White	Short	None	Raw	1.1	N/A	0.21	1.04
		TRV (Suwandel)	Cooked	1.49	2.45	0.07	0.80		Short	None	Cooked	0.4	N/A	0.072	0.357
		IMV: (Nadu, Samba)	Raw	3.02	4.17	0.14	1.14		Medium	None	Raw	1.16	N/A	0.11	1.1
		IMV: (Nadu, Samba)	Cooked	1.24	1.58	0.06	0.35		Medium	None	Cooked	0.42	N/A	0.038	0.377
	None	IMV (Kekulu)	Raw	2.54	2.96	0.12	0.89		Long	None	Raw	1.09	15.1	0.22	1.09
		IMV (Kekulu)	Cooked	0.71	0.71	0.04	0.27			None	Cooked	0.49	7.5	0.069	0.472
		IMP (Indian Basmati)	Raw	1.61	3.22	0.11	1.11			Parboiled	Raw	1.02	19.9	0.284	1.04
		IMP (Indian Basmati)	Cooked	0.69	0.86	0.04	0.39			Parboiled	Cooked	0.37	9.3	0.07	0.354
Red	Parboiled	TRV (Pachchaperumal, Kalu heenati)	Raw	3.35	7.48	0.25	1.89	Brown	Medium	None	Raw	2.02	N/A	0.277	3.74
		TRV (Pachchaperumal, Kalu heenati)	Cooked	1.39	3.23	0.13	0.84		Medium	None	Cooked	0.62	N/A	0.081	1.1
		IMV (Nadu, Samba)	Raw	3.71	5.30	0.21	1.48		Long	None	Raw	2.13	17.1	0.302	2.85
		IMV (Nadu, Samba)	Cooked	1.39	2.08	0.11	0.63				Raw	2.13	17.1	0.302	2.85
	None	IMV (Kekulu)	Raw	2.94	4.37	0.14	1.15				Cooked	0.71	5.8	0.106	0.974
	None	IMV (Kekulu)	Cooked	0.90	1.07	0.05	0.38				Cooked	0.71	5.8	0.106	0.974
^a data from the current study ^b [54]

3.4. ETE Distribution by Rice Characteristics

3.4.1. Pericarp Color Effects

Red pericarp varieties contained significantly higher levels of all ETEs compared to white varieties (t_Zn=2.915, t_Se=3.560, t_Mn=2.650, t_Cu=2.259, p < 0.05) (Fig. 2). USDA data supports these findings (Table 1) where brown rice Zn, Se, Cu, Mn levels (2.02–2.13 mg 100g^− 1, 17.1 µg 100 g^− 1, 0.277–0.302 mg 100 g^− 1, 2.85–3.74 mg 100 g^− 1) exceeded the levels in white rice (1.10–1.16 mg 100 g^− 1, 15.1–19.9 µg 100 g^− 1, 0.11–0.28 mg 100 g^− 1, 1.04–1.10 mg 100 g^− 1) [54]. Red varieties are typically consumed as bran-intact rice due to minimal polishing, with ETEs compartmentalized in rice bran [32]. Genetic predispositions enable red/pigmented varieties to accumulate more ETEs than white rice [64].

3.4.2. Parboiling Treatment Effects

Parboiled varieties contained significantly higher ETE levels (t_Zn=3.877, t_Se=3.292, t_Mn=3.313, t_Cu=2.078, p < 0.05) compared to non-parboiled rice. Parboiling minimizes grain breakage during milling while mobilizing biomolecules from husk and bran, fixing them within the endosperm [65–67] which may help retain ETEs in rice grain while withstanding the influence of milling and polishing. USDA data shows parboiled white rice Se levels (19.9 µg 100 g^− 1) exceeded non-parboiled white rice (15.1 mg 100 g^− 1), though Zn, Cu, Mn differences were negligible [54] (Table 1). While Chandrasiri et al. [39] observed higher toxic element accumulation in parboiled rice, no significant ETE differences were noted. In this study, Nadu and Samba rice types within the IMV category showed higher ETE levels, likely due to parboiling. This aligns with previous findings where parboiled red Nadu and red Samba (Red-Samba Kekulu) had slightly higher Zn, Mn, Cu, and Se than non-parboiled red Kekulu rice, while no such differences were seen in white pericarp varieties [55]. The water used in the parboiling process could be an exogenous factor contributing towards the elemental bioaccumulation in rice grains.

3.4.3. Rice Category Variations

Except Cu, Zn, Se, and Mn showed significant inter-categorical variation between TRV, IMV, and IMP (₁ANOVA, p < 0.01) (Fig. 4). Se (t_Se=4.741, p < 0.001) and Mn (t_Mn=5.303, p < 0.001) levels were significantly higher in TRV compared to IMV. The Zn, Se and Cu levels in TRV were approximately two times that of the levels resulting for IMP (t_Zn = 9.048, t_Se=4.404, t_Cu=2.907; p < 0.05). The levels of Zn in both IMV were approximately two times higher than in IMP (t_Zn=3.889; p < 0.05) while the others did not show a marked difference.

Recent studies confirm the varietal superiority of Traditional rice with higher Zn (20.8–30.4 mg kg^− 1) and Mn (10.7–27.8 mg kg^− 1) levels compared to Improved varieties (Zn: 21.0–26.3 mg kg^− 1, Mn: 11.9–17.1 mg kg^− 1 [41]. Traditional varieties have longer growth cycles (6-6.5 months) compared to Improved varieties (3–3.5 months) that may affect mineral profiles alongside their inherent genotypic differences [64].

Within IMV category, Zn and Mn varied significantly across rice types (₁ANOVA, p < 0.05), with Nadu rice showing higher levels than Kekulu rice (Fig. 5). Elements decreased in order: Nadu > Samba > Kekulu, except Se which was higher in Samba. These differences primarily reflect parboiling treatment in Nadu and Samba types.

3.5. Inter – elemental Correlations

All ETEs showed significant positive inter-elemental correlations: Zn-Se (r = 0.518, p < 0.05), Zn-Mn (r = 0.590, p < 0.05), Se-Mn (r = 0.808, p < 0.05), Se-Cu (r = 0.530, p < 0.05), Mn-Cu (r = 0.589, p < 0.05). These correlations reflect the possibility of shared biochemical pathways for uptake, transportation, and bioaccumulation. Zn-Se bio-fortification studies demonstrate synergistic relationships improving grain yield and nutritional quality [68, 69]. Zn-Mn transporters share similar origins, with AhNRAMP1 protein facilitating simultaneous transport [70]. Soil Se complexation with manganese oxide represents one pathway for plant Se uptake [71] while soil nitrogen content directly influences simultaneous uptake of Zn, Se, and Cu [72, 73], which may underline the broader pattern positive elemental concentrations found in the current study.

Table 2
Inter – elemental correlation matrix with significance levels
	Zn	Se	Mn
Zn
Se	.518^*
Mn	.590^*	.808^*
Cu	0.357	.530^*	.589^*
** Pearson correlation (r) is significant at p < 0.05 (2-tailed).

3.6. Cooking – Induced ETE Transitions

Significant reductions occurred in all ETE concentrations during raw-to-cook transformation (W, p < 0.001) (Fig. 6). Mean ± SD cooked rice levels (mg kg^− 1 dw) were: Zn 21.141 ± 6.117, Se 0.031 ± 0.015, Cu 1.290 ± 0.876, Mn 9.062 ± 3.774 respectively. The percentage losses of ETEs during the cooking raged: Se 20.98–59.35%, Mn 20.92–53.73%, Zn 17.42–60.26% and Cu 4.53–65.36%.

The full-water-absorption method utilized in Sri Lanka involves cooking rice with approximately twice the water volume, with complete water absorption except the amount lost as steam. The alternative excess-water-removal methods which are popular in Europe, Southeast Asia, and in the Indian subcontinent involve 1:6–12 v/v rice: water ratios with water discarded post-cooking. Washing raw rice varies globally. In Sri Lanka, rice is rinsed 3–4 times until water runs clear which help eliminate residual dirt and contaminants.

Studies using excess-water cooking methods report 10–49% ETE reductions [37, 74], with minimum Cu losses consistent with current findings. However, significant variation in current study necessitates careful consideration of cooking methods. Washing alone can remove 8.25% Zn and 28.4% Mn from white rice [75]. Due to limited research on Sri Lankan rice, a comprehensive evaluation was not feasible.

Red pericarp varieties showed higher Zn losses (36.02%) versus white varieties (29.79%), while white rice exhibited higher Se (47.59%), Mn (39.59%), and Cu (32.20%) losses compared to red rice (Se: 30.60%, Mn: 26.32%, Cu: 16.36%). Non-parboiled rice experienced significantly higher losses than parboiled rice (Zn, Se: U, p < 0.001; Mn, Cu: U, p < 0.05), reflecting enhanced retention potential through element mobilization and fixation during parboiling.

Se losses varied significantly across rice categories (H, p < 0.05). IMP category showed lowest Zn loss (25.01%) but higher losses for other ETEs (Se: 53.17%, Mn: 37.24%, Cu: 33.88%). TRV category demonstrated superior retention (%losses: Zn 28.61%, Se 25.65%, Mn 26.31%, Cu 12.85%) compared to IMV (%losses: Zn 36.03%, Se 39.92%, Mn 35.64%, Cu 18.68%), with significant differences for Se and Mn (U, p < 0.05).

Within IMV category, Kekulu showed higher losses than Nadu and Samba, which were significant for Zn and Se (U, p < 0.05). Samba rice retained the highest Cu amounts (lowest loss: 10.58%) compared to Nadu (12.30%) and Kekulu (29.58%). These results highlight the role of parboiling treatment in reducing nutrient loss. Additionally, the cooking time (per standard portion of 100 g of raw rice) negatively correlated with ETE losses, significantly for Se (σ= -0.513, p < 0.05) and Mn (σ= -0.408, p < 0.05). Parboiled varieties required lengthier cooking times than non – parboiled rice elucidating the observed negative correlation.

3.7. Cooked rice ETE Content and Consumption – Based Intake

A 100 g portion of cooked rice of the total Sri Lankan rice cohort contained: Zn 1.116 ± 0.350 mg, Se 1.726 ± 1.165 µg, Mn 0.480 ± 0.215 mg, Cu 0.072 ± 0.043 mg. Compared to USA cooked rice on average (Zn: 0.37–0.71 mg 100 g^− 1, Se: 5.8–9.3 mg 100 g^− 1, Mn: 0.354–1.1 mg 100g^− 1, Cu: 0.03–0.106 mg 100g^− 1), Sri Lankan rice showed higher Zn but lower Se levels [54]. A Spanish rice study revealed 7.76 ± 4.42 mg of Zn, 0.035 ± 0.009 mg of Se, 4.41 ± 1.50 mg of Mn and 1.50 ± 2.30 mg of Cu for a 100g portion of cooked rice [76] which were considerably higher than the values in the current cohort.

Recommended cooked rice consumption for healthy Sri Lankan adults ranges 8–13 daily servings of 0.5 cups (~ 65g), equivalent to 520–845 g day^− 1 (average 682.5 g day^− 1) [51]. The amount intakes of ETEs from rice consumption is listed in Table 3.

Table 3
ETEs intakes through cooked rice consumption
Rice consumption	Median intake (day^− 1)
Rice consumption	Zn (mg)	Se (µg)	Mn (mg)	Cu (mg)
Min	6.160 (3.049)	8.974 (6.059)	2.163 (1.646)	0.305 (0.317)
Average	8.085 (4.001)	11.779 (7.952)	2.839 (2.160)	0.401 (0.416)
Maximum recommended portion	10.010 (4.954)	14.583 (9.846)	3.515 (2.674)	0.496 (0.515)

3.8. RDA Fulfillment Assessment

A comprehensive depiction of the percentage fulfillment of ETEs, through average consumption of cooked rice across various rice grain characteristics, is included in Fig. 7.

3.8.1. Zinc

Rice consumption provided median (IQR) RDA (males 11 mg day^− 1, females 8 mg day^− 1) fulfillment: 73.50% (36.37%) for males and 101.06% (50.02%) for females. Against RDA_SL (males: 14 mg day^− 1, females: 11 mg day^− 1), the median fulfillment was 57.75% (28.58%) for males and 73.50% (36.37%) for females. These results indicate rice as a reliable Zn source, particularly due to high consumption rates.

Red rice provided 13.02% dietary advantage over white rice, while parboiled rice offered 24.60% advantage over non-parboiled varieties (U, p < 0.001). TRV category provided 9.49% advantage over IMV and 38.74% over IMP categories respectively. Nadu rice offered 8.20% advantage over Samba and 23.12% over Kekulu varieties (U, p < 0.05) (Fig. 7).

Common Sri Lankan foods like beef liver, goat chops, shrimp, and cashew nuts provide 5–6 mg Zn per serving (~ 15 g nuts, ~ 30 g meats) [3]. However, total Zn intake should not exceed the Tolerable Upper Limit (UL) of 25 mg day^− 1, as excess Zn may impair Cu absorption which leads to adverse effects such as; bone weakening, anemia, and immune dysfunction [77, 78]. Zn bioavailability is also reduced by dietary phytates, with 2.56–8.7 g found per 100 g rice bran [79–81]. These compounds form insoluble complexes, limiting Zn absorption; a factor considered in setting RDA_SL-Zn, given the estimated ~ 900 mg day^− 1 phytate intake in typical semi-refined Sri Lankan diets [3].

Zn deficiency, often caused by poor dietary intake, malabsorption, or excessive urinary loss, is linked to numerous health issues including growth delays, skin disorders (dermatitis, lesions, alopecia), gastrointestinal conditions (loss of appetite, diarrhea, Crohn’s disease, ulcerative colitis), respiratory infections (e.g., pneumonia), and impaired immune function [82–84]. Globally, 17–20% of the population is affected with Zn deficiencies, with the highest prevalence in Africa and Asia [83, 85]. Sri Lanka, in particular, shows a high rate of Zn deficiency within South Asia [83]. This stresses that even with moderate accumulation of Zn in rice and with higher consumption, Sri Lankan rice may not be able to provide the daily requirement of Zn to general population.

3.8.2. Selenium

Rice consumption provided median (IQR) RDA (males: 55 µg day^− 1 and females: 60 µg day^− 1) fulfillment: 19.63% (36.37%) for males and 21.42% (14.46%) for females. Currently there is no established country-specific adaptation of RDA_SL-Se, but an Adequate Intake (AI) of 70 µg day^− 1 has been recommended for both genders. At an average rice consumption, the median (IQR) AI_SL-Se fulfillment was 16.83% (11.36%). These findings reveal inadequate Se provision through rice consumption alone.

Red rice provided 34.28% Se advantage over white rice, while parboiled rice offered 37.47% advantage over non-parboiled varieties (U, p < 0.001). TRV category provided 43.69% advantage over IMV (U, p < 0.001) and 56.28% over IMP. Nadu and Samba provided similar Se fulfillment, both double that of Kekulu rice (U, p < 0.05) (Fig. 7).

Supplementing with Se-rich Sri Lankan foods including Queenfish (~ 736 µg 100 g^− 1), baby shrimp (~ 735 µg 100 g^− 1), tuna (~ 65.92 µg 100 g^− 1), beet greens (~ 65.92 µg 100g^− 1), and lentils (~ 69.26 µg 100g^− 1) [3] can enhance dietary Se. A complete Sri Lankan meal provides 48–70 µg of Se [86], though cooking method variations significantly affect availability. Significant proportions of Sri Lankan females may experience Se deficiencies, with prevalence reaching 40% in certain sub-populations [13]. Dietary Se deficiency affects cardiovascular health (Keshan disease) [87], induces inflammatory arthritic events (Kashin-Beck disease) [88], cardiovascular disease[89], and male infertility [90].

3.8.3. Manganese

Rice consumption provided median (IQR) RDA-Mn (males: 2.3 mg day^− 1 and females: 1.8 mg day^− 1) fulfillment: 123.44% (93.90%) for males and 157.73% (119.99%) for females. Against AI_SL- Mn (3 mg day^− 1, both genders), fulfillment was 94.64% (72.00%). Although, Sri Lankan rice provide significant amounts of Mn, the lower gastrointestinal absorption (< 10%) may substantially reduce its bioavailability [91]. Gender disparities in Mn absorption occur, with males absorbing less than females due to iron status competition for shared transporters [92]. Dietary Mn also influence gastrointestinal absorption or biliary excretion [93].

Red varieties provided 24.12% advantage over white varieties, parboiled rice offered 25.64% advantage over non-parboiled (U, p < 0.05). TRV provided 31.83% advantage over IMV (U, p < 0.001) and 33.64% over IMP. Nadu rice offered maximum dietary advantage of Mn advantage over Samba (13.40%) and Kekulu (27.26%, U, p < 0.05) (Fig. 7).

Sri Lankan diet, rich in plant foods maximizes the daily Mn intake. For instance, the peel of Ash plantain contain ~ 53.52 mg 100g^− 1 [3], which is commonly used in rural cooking. Tea, the leading Sri Lankan beverage, contains 0.4–1.3 mg Mn per cup [93–95], with black tea containing up to 1094 mg kg^− 1 [96]. Chronic high-dose exposures cause nervous system toxicity including neurodegenerative disorders (Manganism, Idiopathic Parkinson's disease, Amyotrophic Lateral Sclerosis, Alzheimer's), muscle/joint pain, headaches, fatigue, and depression [97, 98]

3.8.4. Copper

Rice consumption provided median (IQR) RDA-Cu (0.9 mg day^− 1 both genders) fulfillment: 44.51% (46.18%). Against RDA_SL-Cu (males: 1.6 mg day^− 1 females: 1.3 mg day^− 1), the fulfillment was 30.82% (31.97%) for females and 25.04% (25.98%) for males respectively.

Red rice provided 33.51% dietary advantage over white rice (U, p < 0.05), while parboiled rice offered 37.19% advantage over non-parboiled rice (U, p < 0.05). TRV provided 32.40% advantage over IMV (U, p < 0.05) and 46.82% over IMP respectively. Nadu rice provided ~ 2.5-fold Cu compared to Kekulu rice, corresponding to 43.11% additional RDA contribution (Fig. 7.).

These results reveal that the intake of Cu through consumption of Sri Lankan rice may not be sufficient to meet optimum recommended levels. Cu deficiency causes iron-depleted anemia, leukopenia [99], skeletal problems (osteoporosis), connective tissue development hindrance [100, 101], cardiac problems (hypertrophy, ischemic heart disease, heart failure) [102, 103], impaired immune function, and myeloneuropathy (Human Swayback disease) [104].

Supplementing Sri Lankan rice with Cu rich foods can help achieve the daily dietary requirement. Sri Lankan Cu-rich foods include lagoon crab (Kalapu Kakuluwa: 123 mg 100g^− 1), beef liver (~ 3.63 mg 100 g^− 1), baby shrimp, eels, and cashews (2.33–3.54 mg 100 g^− 1) [3]. High Cu absorption in humans (12–71%) [105] enables dietary adjustment as a reasonable strategy in place of synthetic/ pharmaceutical supplementation.

3.8.5. Overall ranking of ETE nutritional adequacy, varieties with nutritional superiority and factors affecting ETE bio-availabilities

Overall ETE contribution is ranked: Mn (123.44–157.73%) > Zn (73.50–101.06%) > Cu (44.51%) > Se (19.63–21.42%), highlighting inadequate Se provision for preventing dietary deficiency. Red pericarp, parboiled Traditional varieties consistently provided 9.49–56.28% dietary advantages of ETEs over white, non-parboiled, Improved/Imported alternatives.

Parboiled Improved Nadu rice demonstrated maximum dietary benefit among commercially available options, offering superior ETE provision compared to Samba and Kekulu varieties across all elements studied.

While quantitative assessment provides valuable insights, bioavailability varies significantly across populations and individuals due to genetic factors, gut health, concurrent nutrient intake, and physiological status. Phytate content in rice bran can significantly reduce Zn, Fe, and Mn absorption [79]. Iron-selenium antagonistic interactions and zinc-copper competition for absorption pathways require careful consideration in dietary planning [106]. The observed Se inadequacy presents particular concern given its role in thyroid function and antioxidant defense. Geographic variations in soil Se content across Sri Lankan regions may contribute to observed deficiencies [13]. Targeted bio-fortification programs using Se-enriched fertilizers could address this gap, with studies demonstrating potential for increasing grain Se content up to 1.81 mg kg^− 1 [63].

Traditional varieties’ nutritional superiority, combined with cultural preferences and perceived health benefits, supports their promotion in national nutrition programs. However, lower yields and longer growing cycles may present economic challenges requiring policy support for farmers transitioning to Traditional variety cultivation.

3.9. Strengths and Limitations of the study

This study presents the first comprehensive investigation into the losses of essential trace elements (ETEs) in Sri Lankan raw rice subjected to commonly employed domestic cooking processes, alongside an assessment of the element-specific nutritional adequacy of cooked rice based on average adult consumption rates. The sampling methodology was designed to reflect consumer practices, thereby simulating real-world market-to-table scenarios in food composition analysis. By selecting the most widely consumed rice varieties, encompassing multiple pericarp colors and parboiling treatments, we quantitatively determined the daily intake of ETEs through rice consumption. Additionally, this study evaluates the practical implications of commercially available rice options, particularly within the improved rice category that dominates the Sri Lankan diet. The findings further identify complementary food sources necessary to address potential inadequacies in ETE intake when rice constitutes the primary dietary staple.

A rice cooker was employed as the cooking vessel during the standardization of the domestic cooking procedure; however, this may not fully represent the variety of cooking vessels used in Sri Lankan households, such as; clay pots, aluminum, or stainless steel cookware. For the nutritional adequacy assessment, the analysis was limited to average rice intake based on the recommended daily rice consumption for the national adult population. This approach may not precisely capture the variability in rice consumption patterns among Sri Lankan adults, which can be influenced by geographic location (particularly between agricultural and urban communities), food accessibility, gender differences, socioeconomic status, and health considerations. Additionally, seasonal variations in the elemental composition of rice were not accounted for, as rice samples were obtained from bulk market sources with limited traceability regarding the harvest period. Furthermore, element quantification was conducted on cooked rice, with the cooked grain fraction assumed to represent the bioavailable fraction for the purpose of evaluating nutritional adequacy.

3.10. Public Health Implications and Recommendations

The identified nutritional superiority of Traditional varieties supports their inclusion in national nutrition strategies, though Se deficiency remains a critical concern requiring comprehensive intervention approaches. Integrating optimal rice variety selection with diversified ETE-rich food consumption could significantly improve population nutritional status and reduce micronutrient deficiency prevalence in Sri Lanka.

Future research utilizing in-vitro gastro-digestion models is recommended to evaluate ETE bioavailability superiority of Traditional varieties prior to application in dietary strategies, bio-fortification programs, and nutritional recommendations. These findings provide essential data for evidence-based nutrition policy development and targeted interventions addressing hidden hunger in Sri Lankan populations.

This study comprehensively evaluated essential trace elements (ETEs) (Zn, Se, Mn, Cu) in commonly consumed Sri Lankan rice varieties and their contributions to recommended dietary allowances (RDAs) following typical cooking practices. Cooking induced significant ETE losses of 17.4–65.4%, with higher retention observed in red pericarp and parboiled Traditional varieties compared to white, non-parboiled, Improved, and Imported types. Cooked rice contributed most substantially to Mn (123–158%) and Zn (74–101%) RDAs, moderately to Cu (45%), but inadequately to Se (19–21%), highlighting a critical Se deficiency risk from rice consumption alone. Traditional red parboiled varieties, particularly Pachchaperulal, Kaluheenati, and Suwandel, demonstrated superior essential trace element retention and dietary contributions, with parboiled Improved Nadu rice providing the greatest overall ETE benefit among widely available Improved rice options. These findings confirm that Sri Lankan rice serves as a reliable dietary source of several ETEs; however, addressing Se inadequacy requires targeted interventions such as bio-fortification, dietary diversification, or supplementation. Considering variability in gastrointestinal absorption and individual metabolic rates, further in vitro bioavailability studies are warranted to validate the nutritional superiority of specific rice varieties prior to their inclusion in public health strategies and nutritional recommendations.

Funding:

This study was funded by University of Colombo – Sri Lanka research grants (No: AP/3/2/2020/SG/18)

Ethical Clearance:

Not Applicable

Data availability:

All relevant data generated in this study have been included within the results section of this manuscript. Further data will be available from the corresponding author on a reasonable request.

Conflicts of interest:

Authors have no financial or non – financial conflict of interests to disclose.

Author contributions:

Conceptualization (SG, JWG), Acquisition of funds (SG, JWG), Research mythology, experimental design (JWG, SG), Sampling & conduction of research activities (JWG), Results and data acquisition (JWG), Analysis of results, data and statistics (JWG, SG, ICP), Manuscript framework (JWG, SG, ICP, NDA, CW), Writing of the original draft (JWG, SG, ICP, NDA, CW), Review and revisions of the manuscript drafts (SG, ICP, NDA, CW), Principal supervision and mentorship (SG, ICP), Co-supervision & mentorship (NDA, CW).

Acknowledgements:

The technical expertise of National Aquatic Resources, Research and Development Agency (NARA) Sri Lanka, Sri Lanka Institute of Nanotechnology (SLINTEC) and Rice Research and Development Institute (RRDI)- Bathalegoda Sri Lanka.

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Essential Trace Elements in Commonly Consumed Varieties of Sri Lankan Cooked Rice and Its Dietary Significance: A Focus on Recommended Daily Allowances

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Journal Publication

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Abstract

Figures

1. Introduction

2. Methodology

2.1. Rice sampling and processing

2.2. Sample preparation and digestion (in-vitro)

2.3. Elemental Analysis

2.4. Rice Consumption Rates and RDA Assessment

2.5. Data and Statistical Analysis

3. Results and discussion

3.1. Basic Characteristics of Rice Varieties

3.2. ICP –MS Analysis and Quality Control Validation

3.3. ETE Concentrations in Raw Rice and Global Comparisons

3.4. ETE Distribution by Rice Characteristics

3.4.1. Pericarp Color Effects

3.4.2. Parboiling Treatment Effects

3.4.3. Rice Category Variations

3.5. Inter – elemental Correlations

3.6. Cooking – Induced ETE Transitions

3.7. Cooked rice ETE Content and Consumption – Based Intake

3.8. RDA Fulfillment Assessment

3.8.1. Zinc

3.8.2. Selenium

3.8.3. Manganese

3.8.4. Copper

3.8.5. Overall ranking of ETE nutritional adequacy, varieties with nutritional superiority and factors affecting ETE bio-availabilities

3.9. Strengths and Limitations of the study

3.10. Public Health Implications and Recommendations

4. Conclusions

Declarations

References

Additional Declarations

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