Assessment of Essential Elements and Potentially Toxic Elements (PTEs) in Organic and Conventional Flaxseeds: Implications for Dietary Exposure and Food Safety

Abstract

Flax (Linum usitatissimum L.) is valued for its fibers and nutrient-rich seeds, which are increasingly consumed for their health benefits. However, flaxseeds can also accumulate potentially toxic elements (PTEs), raising concerns about safety. This study quantified 11 essential elements (e.g., Ca, Fe, Mg, and Zn) and 9 PTEs (e.g., Al, Cd, Pb, and Ni) in commercial flaxseed samples using inductively coupled plasma–optical emission spectrometry. Two intake scenarios (15 g/day and 30 g/day) were analyzed to estimate dietary exposure, with health risks assessed through the target hazard quotient (THQ) and hazard index (HI). The results showed that organic flaxseeds had higher levels of certain elements (e.g., Cu, K, and Pb), while Al and Ni were more abundant in conventional samples. Cadmium levels in both remained below the EU regulatory limit. The highest estimated daily intakes were for K, Mg, and Ca, highlighting the seeds’ nutritional value. However, HI values suggested that Al and Pb could pose health risks. These findings emphasize flaxseeds’ dual nature as both beneficial and potentially harmful, particularly given the lack of specific regulatory limits and limited data on elemental composition. Continued monitoring and risk assessment are recommended to safeguard public health.

1. Introduction

Flax (Linum usitatissimum L.) is an herbaceous plant that can grow up to a height of one meter and is primarily cultivated for two commercial products: fibers and seeds. The plant’s fibers are highly valued in the textile industry due to their exceptional tensile strength, which is largely attributed to their high cellulose content [1,2]. Flaxseeds, on the other hand, have gained prominence in the food industry because they are rich in bioactive compounds, including alpha-linolenic acid (ALA), proteins, and lignans [3,4,5]. Additionally, flaxseeds are notable sources of essential elements such as calcium (Ca), copper (Cu), iron (Fe), potassium (K), magnesium (Mg), manganese (Mn), sodium (Na), phosphorus (P), and zinc (Zn) [6]. They can be consumed whole or in flour form and are included in both human and animal diets [3,7,8].

According to the Food and Agriculture Organization (FAO), in 2022, the largest producers of raw or retted flax fibers were countries within the European Union—specifically France, Belgium, and Belarus—followed by China and Russia [9]. In contrast, the leading producers of flaxseeds in 2022 were Russia (1.77 million tons), Kazakhstan (0.85 million tons), and Canada (0.47 million tons). The global market value for flaxseeds is projected to reach USD 0.89 billion in 2024 and could rise to USD 1.54 billion by 2029 [10].

Seeds such as chia and flax are increasingly recognized for their nutritional value, particularly as sources of vitamins and essential elements [6,11]. However, these agricultural products are also susceptible to contamination via multiple environmental pathways. Contaminants, especially heavy metals and other trace elements, may be introduced through the soil, irrigation with contaminated water, or atmospheric deposition [12,13,14]. For example, the application of phosphate fertilizers, waste deposition, metallurgical industry practices, mining operations, and the combustion of fossil fuels are the primary anthropogenic sources of Cd in the soil [15]. Numerous studies have documented the adverse effects of heavy metals on crop quality and food safety [16,17,18,19]. Moreover, many plant species, including food crops, have the capacity to bioaccumulate these elements, leading to potential transfer through the food chain and eventual human exposure [20,21]. In the case of fiber-rich plants, this can pose a problem, given that such bioaccumulation can occur in various plant structures, including the stem, root, and leaves [20].

Although the European Union has established regulatory limits for various contaminants in food products, only cadmium (Cd) currently has a defined maximum limit for flaxseeds [22]. Previous research has demonstrated the presence of multiple metals in seed samples, including chia and flax [11,23,24]. Among these studies, only one quantifies the metal concentration in flaxseeds and is limited to only 10 metals [23]. A recent study analyzing 20 chemical elements in chia seeds highlighted the potential health risks associated with chronic exposure to some of these elements [11]. The ingestion of food contaminated with potentially toxic metals remains a significant public health concern [25].

Despite the increasing nutritional and commercial relevance of flaxseeds, limited data are available on their content of essential and toxic elements. There is a clear knowledge gap regarding both the nutritional contribution of essential metals and the toxicological risks associated with potentially toxic elements (PTEs) in flaxseeds. Thus, the aims of the present study were to (1) quantify the concentrations of 20 chemical elements in commercially available flaxseed samples; (2) assess nutritional value and toxicological risks based on dietary exposure; and (3) conduct a risk characterization for elements with established reference doses (RfDs).

2. Materials and Methods

2.1. Materials

A total of 52 flaxseed samples, commercially obtained in the European Union and other European countries, were analyzed in this study. Regarding the origin of the seeds, 37 were classified as conventional and 15 as organic. The seeds were obtained from supermarkets, herbalists, and online stores.

2.2. Sample Preparation Procedure

In triplicate, 5 g samples of flaxseeds were weighed into a pre-weighed porcelain capsule (Staatlich, Berlin, Germany) and dried in an oven (Nabertherm, Lilienthal, Germany) at 80 °C for 24 h. After dehydration, the samples were weighed again and transferred to a muffle furnace (Nabertherm, Lilienthal, Germany) at a temperature of 450 °C. The temperature was gradually increased during the first 24 h, followed by an additional 48 h, totaling 72 h in the muffle furnace. After cooling, the ash content was weighed. The ash samples were treated with 65% nitric acid (HNO₃) (Sigma-Aldrich, Taufkirchen, Germany) for 24 h and 48 h after the start of the procedure in the muffle furnace. Finally, the ash samples were diluted in 1.5% nitric acid (HNO₃) (Sigma-Aldrich, Taufkirchen, Germany) in 25 mL of distilled water.

2.3. Determination of Essential and Toxic Elements

The quantification of the elements, including essential and potentially toxic ones, was performed using inductively coupled plasma–optical emission spectrometry (ICP-OES) with a Thermo Scientific iCAP PRO ICP-OES autosampler (Waltham, MA, USA). Quality control was ensured by utilizing certified reference materials. The instrumental limits of detection and quantification were estimated based on the instrumental response of the equipment. Specifically, these were determined through the analysis of 15 blanks under reproducibility conditions [26]. The operational parameters of the ICP-OES are detailed in Supplementary File Table S1, while the recovery percentages (RPs), detection limits (LODs), quantification limits (LOQs), and wavelengths corresponding to each analyzed element, as part of the quality control, are provided in Supplementary File Table S2.

Although comprehensive studies assessing the concentration of numerous elements in flaxseeds are limited, inductively coupled plasma–optical emission spectrometry (ICP-OES) has proven to be a rapid, sensitive, and reliable method for determining metals such as Cd and Pb specifically in oilseeds, with the potential for quantifying other metals [11,27]. In this regard, as in our study, using ICP-OES to determine the concentration of PTEs allows for understanding the levels of these elements in this group of foods.

2.4. Dietary Exposure Assessment

Exposure assessment consists of calculating the estimated daily intake (EDI) (Equation (1)). The calculation of EDI is a flexible model that allows for variation in its parameters to estimate the daily intake.

$$EDI={\displaystyle \frac{C_{Metal}\times DI\times EF\times ED\times {10}^{-3}}{BW\times AT}}$$

(1)

where C_metal is the average concentration of each metal in flaxseeds (mg/kg); DI is the daily intake of flaxseeds (15 or 30 g flaxseeds/day); EF is exposure frequency (365 days/year); ED is the average duration of exposure to flaxseeds (25 years); BW is the average body weight (70 kg); and AT is the average exposure time (EF × ED). To assess the nutritional and toxicological implications of the estimated dietary intake (EDI) of the studied elements via flaxseeds, the nutritional reference intake (NRI), the tolerable daily intake (TDI), the tolerable weekly intake (TWI), and the tolerable upper intake level (UL) served as reference values (Supplementary File Table S3) [28,29,30,31,32,33,34,35,36,37,38].

The calculation of the EDI’s contribution to the nutritional and toxicological reference values was performed and measured as a percentage (Equation (2)).

$$Contribution \left(\%\right)=\left({\displaystyle \frac{EDI}{NRI \mathrm{or} TDI \mathrm{or} TWI \mathrm{or} UL}}\right)\times 100$$

(2)

where EDI means the estimated daily intake; NRI means the nutritional reference intake; TDI means the tolerable daily intake; TWI means the tolerable weekly intake; and UL means the tolerable upper intake level.

2.5. Risk Characterization

Finally, risk characterization was calculated for all elements with a pre-established reference dose (RfD) (

Table 1. Reference doses of the essential elements and potentially toxic elements (PTEs) [39].

Essential Elements	Reference Dose (RfD) (mg/kg/day)	Potentially Toxic Elements	Reference Dose (RfD) (mg/kg/day)
Ca	-	Al	4 × 10−4
Cr	3 × 10−3	B	0.2
Co	3 × 10−4	Ba	0.07
Cu	4 × 10−2	Cd	1 × 10−3
Fe	0.7	Li	-
K	-	Ni	2 × 10−2
Mg	-	Pb	1 × 10−7
Mn	0.14	Sr	0.6
Mo	5 × 10−3	V	5.04 × 10−3
Na	-
Zn	0.3

). This was done by calculating the ratio between the EDI and the RfD for each element, yielding the target hazard quotient (THQ). Subsequently, the sum of the THQs for these elements results in the hazard index (HI), where values exceeding 1 indicate a potential health risk under the consumption scenario defined by EDI.

Equations (3) and (4) represent the calculations for THQ and HI, respectively.

$$THQ={\displaystyle \frac{EDI}{RfD}}$$

(3)

$$HI=\sum THQ$$

(4)

where THQ means the target hazard quotient, EDI means the estimated daily intake, RfD means the reference dose, and HI means the hazard index.

2.6. Statistical Analysis

Mean comparison tests (t-test) and distribution comparison tests (Mann–Whitney test) were performed to analyze organic and conventional flaxseed groups. The choice of tests was based on data distribution, as determined by the Kolmogorov–Smirnov normality test [40]. GraphPad Prism 8.2.1 was used for all statistical analyses and graph construction.

3. Results

The mean and range of concentrations of metals in flaxseed samples are presented in

Table 2. Concentrations of elements (mg/kg) in organic and conventional flaxseed.

	Element	Organic Seeds (mg/kg)			Conventional Seeds (mg/kg)
		Mean	Minimum	Maximum	Mean	Minimum	Maximum
Essential Elements	Ca	2866.142	2040.609	3890.695	3199.202	1698.206	6173.952
	Co	3.963	0.048	56.165	5.509	0.063	56.031
	Cr	0.113	0.047	0.168	0.249	0.030	2.168
	Cu	11.407	6.993	15.929	11.035	5.666	20.100
	Fe	35.296	5.581	51.619	41.098	5.957	71.521
	K	7685.005	4844.334	9326.960	7348.621	2999.681	11,155.318
	Mg	6733.650	4539.060	8592.595	6140.583	2657.309	10,571.919
	Mn	15.745	0.249	34.084	16.428	0.175	54.657
	Mo	3.204	0.070	44.722	4.621	0.032	44.933
	Na	1182.777 *	667.913	2020.104	898.004	177.760	1640.311
	Zn	37.739	21.837	44.034	38.714	22.980	49.230
Potentially Toxic Elements	Al	6.015 *	0.226	23.654	8.940	0.326	21.469
	B	11.469	4.562	46.923	10.099	1.943	48.941
	Ba	1.210	0.638	2.373	1.158	0.225	2.228
	Cd	0.116	0.043	0.290	0.127	0.033	0.578
	Li	1.835	0.250	3.961	1.951	0.385	5.233
	Ni	0.652 *	0.219	2.104	0.976	0.105	2.667
	Pb	0.368	0.015	1.654	0.286	0.015	2.762
	Sr	6.452	1.768	13.449	6.394	1.615	34.778
	V	0.033	0.025	0.042	0.033	0.020	0.057

* Denotes a statistically significant difference (p < 0.05) in metal concentration in organic seeds compared to their conventional counterparts.

. The elements were grouped according to seed type (organic and conventional) and categorized as essential elements and potentially toxic elements (PTEs). Among the essential elements (C, Co, Cr, Cu, Fe, K, Mg, Mn, Mo, Na, and Zn), only 4 out of 11 (Cu, K, Mg, and Na–36.4%) showed higher concentrations in the organic group compared to the conventional samples. On the other hand, more than half (55.6%) of the PTEs (B, Ba, Pb, Sr, and V) classified as organic exhibited higher levels when compared to their conventional counterparts.

When grouped by seed type (Figure 1 for essential elements and Figure 2 for potentially toxic elements), t-tests and Mann–Whitney tests revealed that metal concentrations varied between organic and conventional seeds for certain elements. Al and Ni exhibited higher levels in conventional flaxseeds compared to organic flaxseeds (p = 0.0312 and p = 0.0142, respectively). Moreover, Na showed a decrease in concentration in the conventional samples compared to the organic group (p = 0.0072).

Table 3. Estimated daily intakes (EDIs) in both dietary consumption scenarios.

Group	Element	15 g Flaxseeds/Day		30 g Flaxseeds/Day
		Organic Seeds	Conventional Seeds	Organic Seeds	Conventional Seeds
Essential Elements	Ca	6.14 × 10−01	6.86 × 10−01	1.23 × 10+00	1.37 × 10+00
	Cr	8.49 × 10−04	1.18 × 10−03	1.70 × 10−03	2.36 × 10−03
	Co	2.41 × 10−05	5.33 × 10−05	4.82 × 10−05	1.07 × 10−04
	Cu	2.44 × 10−03	2.36 × 10−03	4.89 × 10−03	4.73 × 10−03
	Fe	7.56 × 10−03	8.81 × 10−03	1.51 × 10−02	1.76 × 10−02
	K	1.65 × 10+00	1.57 × 10+00	3.29 × 10+00	3.15 × 10+00
	Mg	1.44 × 10+00	1.32 × 10+00	2.89 × 10+00	2.63 × 10+00
	Mn	3.37 × 10−03	3.52 × 10−03	6.75 × 10−03	7.04 × 10−03
	Mo	6.87 × 10−04	9.90 × 10−04	1.37 × 10−03	1.98 × 10−03
	Na	2.53 × 10−01	1.92 × 10−01	5.07 × 10−01	3.85 × 10−01
	Zn	8.09 × 10−03	8.30 × 10−03	1.62 × 10−02	1.66 × 10−02
Potentially Toxic Elements	Al	1.29 × 10−03	1.92 × 10−03	2.58 × 10−03	3.83 × 10−03
	B	2.46 × 10−03	2.16 × 10−03	4.92 × 10−03	4.33 × 10−03
	Ba	2.59 × 10−04	2.48 × 10−04	5.19 × 10−04	4.96 × 10−04
	Cd	2.49 × 10−05	2.72 × 10−05	4.99 × 10−05	5.45 × 10−05
	Li	3.93 × 10−04	4.18 × 10−04	7.86 × 10−04	8.36 × 10−04
	Ni	1.40 × 10−04	2.09 × 10−04	2.79 × 10−04	4.18 × 10−04
	Pb	7.89 × 10−05	6.12 × 10−05	1.58 × 10−04	1.22 × 10−04
	Sr	1.38 × 10−03	1.37 × 10−03	2.77 × 10−03	2.74 × 10−03
	V	7.14 × 10−06	6.97 × 10−06	1.43 × 10−05	1.39 × 10−05

presents the estimated daily intake (EDI) of each element based on daily flaxseed consumption of either 15 g/day or 30 g/day. The highest EDI values were observed for K, Mg, and Ca, respectively. Hazard index (Figure 3) values above 1 were observed for Al and Pb in both exposure scenarios (15 and 30 g/day) for both organic and conventional seed groups. In the 15 g/day exposure scenario, Al yielded an HI of 3.22 and 4.79, while Pb yielded an HI of 789.15 and 612.01 (organic and conventional seeds, respectively). Similarly, in the exposure scenario with flaxseed consumption of 30 g/day, Al yielded an HI of 6.45 and 9.58, whereas Pb presented HI values of 1578.3 and 1224.02 (organic and conventional seeds, respectively).

4. Discussion

This study assessed the concentrations of a range of essential elements and potentially toxic elements (PTEs) in flaxseeds, a food increasingly consumed as part of health-conscious diets. In accordance with Regulation (EU) 2023/915—which sets maximum permissible levels for contaminants in food within the European Union—Cd is the only regulated element for linseeds (flaxseeds), with a limit of 0.50 mg/kg. Our findings indicate that Cd concentrations in both organic (0.116 mg/kg) and conventional (0.127 mg/kg) flaxseed samples were well below this regulatory threshold. Traditionally, Cd is recognized for its toxic, carcinogenic, and stimulating effects, in addition to its association with other diseases such as emphysema, asthma, bronchitis, and hypertension, particularly in cases of chronic intoxication [41].

Although Pb is included in the regulation for several food categories, there is currently no specific limit established for flaxseeds or oilseeds more broadly. This regulatory gap is noteworthy given that Pb was detected in our samples and may pose health concerns depending on exposure levels. On the other hand, for certain metals like Al, their concentration in food merely serves as a basis for their potential toxicity, given that the exposure pathway and toxicodynamics of each metal within the organism bear a distinct relevance to the degree of toxicity [42].

Among the elements analyzed, Al exhibited a statistically significant difference between organic and conventional flaxseeds in our study. This finding is of particular concern, as Al has garnered increasing attention due to its potential neurotoxic effects. Several studies have suggested a link between elevated Al exposure and the progression of neurodegenerative diseases, including Alzheimer’s disease [43]. Although the concentration of Cd is below the legally established limits, it is important to emphasize that the average Cd concentration in European soils is higher than in other regions such as China and the United States [15], and this may influence its concentration in seeds.

When comparing our results with data from the literature, such as a recent study on sesame seeds, the concentrations of Fe, Zn, Cu, and Ni in our flaxseed samples were generally lower, except for manganese (Mn), which was found at comparable or slightly lower levels [44]. Similarly, Gu et al. [23] reported higher concentrations of most analyzed elements in flaxseeds, including Al, Cd, Cu, Fe, Ni, Mn, and Zn, while Pb was found at lower levels than those observed in our study. In their study, the concentration of Al (13.8 ± 3.4 mg/kg), Cd (0.49 ± 0.17 mg/kg), and Pb (0.0048 ± 0.003 mg/kg) was higher than those reported in our study for Al (6.02 and 8.94 mg/kg) and Cd (0.12 and 0.13 mg/kg), but lower for Pb (0.37 and 0.29 mg/kg), for organic and conventional groups, respectively.

Our results confirm that flaxseeds can serve as valuable dietary sources of essential elements such as K, Mg, and Na. The nutritional benefits of flaxseed consumption have been well documented, particularly for cardiovascular and metabolic health [45,46]. Daily intake of 15 to 40 g has been associated with improved lipid profiles, reduced blood pressure, and lower fasting glucose and insulin resistance indices [47]. According to our risk assessment, the maximum permissible intake value to avoid a hazard index above 1 for Al would be 3.5 g/day of flaxseeds, but there would still be a risk associated with Pb. It is important to emphasize that risk assessment employs a quantitative approach that depends on a series of variables and that Pb notably elevates the hazard index in many risk assessments conducted within these parameters.

Some metals, such as Cr, Ni, Cd, Pb, Zn, and Cu, can pose risks to plant life and human health [48]. Furthermore, excessive consumption of contaminated foods can trigger adverse health effects, including liver and kidney diseases from Cd [49] and neurotoxic damage from Pb [50]. Conversely, Ni, for example, is highly mobile in plants and does not accumulate in plant tissues, thus posing less harm to consumers [51]. The implications of consuming flaxseeds contaminated by metals are still rarely discussed in literature. In a review study evaluating 67 articles that investigated the benefits of flaxseed consumption, it was observed that none of them mentioned the risk of heavy metal contamination [52]. However, our findings also indicate that these potential health benefits may be accompanied by the risk of exposure to PTEs such as Al and Pb, which are known to exert toxic effects at elevated levels. Although the risk assessment employed in this study is based on predictive and deterministic exposure scenarios, it highlights the need for continued surveillance of chemical contaminants in emerging food products.

Finally, while flaxseeds represent a promising functional food, their safety profile must be closely monitored. Given their growing popularity, regular analytical assessments and the establishment of regulatory limits for a broader range of elements are essential to protect public health and inform evidence-based dietary recommendations.

5. Conclusions

The consumption of novel alternative foods and dietary supplements has increased in recent years, with oilseeds such as flaxseeds gaining popularity. This study evaluated the concentrations of 20 chemical elements in flaxseeds obtained from both organic and conventional sources. Among these, only Cd is currently regulated for this food category within the European Union. Notably, the Cd levels detected in our samples were within the established regulatory limits. The assessment of the hazard index revealed that Al and Pb may pose potential health risks.

These findings highlight a broader concern: the lack of established safety thresholds for potentially toxic elements in flaxseeds and similar emerging food products. Coupled with the limited availability of analytical studies in this area, this regulatory gap may obscure potential risks to human health.

Therefore, we underscore the urgent need for more comprehensive analytical investigations into the elemental composition of such foods. These efforts are essential to support evidence-based policymaking and to enable risk managers to safeguard public health by effectively addressing dietary exposure risks.