Brodziak-Dopierała et al. 2023 — Mercury in oil-based dietary supplements on the Polish market
This full peer-reviewed study measures total mercury (tHg) in 36 oil-based dietary supplements purchased from Polish pharmacies, drugstores, and herbal shops, using atomic absorption spectrometry via the amalgamation technique (AMA 254, Altec; LOD 0.01 ng Hg). The 36 preparations span vegetable oils (n = 18, all cold-pressed and unrefined), cod liver oils (n = 12), and shark liver oils (n = 6). All samples fell below the EU maximum level for dietary supplements of 0.10 mg/kg (100 µg/kg). Counterintuitively, vegetable oil supplements contained significantly higher tHg concentrations than fish oil supplements. No preparation approached a health-relevant dose at standard recommended intakes.
This paper is distinct from Brodziak-Dopierała et al. 2024 (which covers a broader set of medicines and dietary supplements, n = 139, 2022–2023 purchase dates) and from Torović et al. 2024 (which covers fish oil supplements only in Serbia/Srpska, n = 42).
Key numbers
All concentrations tHg, µg/kg (= ppb), wet weight. Measurements by AAS amalgamation, AMA 254:
Overall (n = 36):
- Range: 0.023–0.427 µg/kg
- Mean ± SD: 0.165 ± 0.092 µg/kg
- Median: 0.165 µg/kg
- Coefficient of variation: 55.8%
By supplement type:
- Vegetable oils (n = 18): mean 0.218 µg/kg (range 0.131–0.427); highest individual = cedar nut oil (0.427 µg/kg); lowest = grape seed oil (0.131 µg/kg)
- Cod liver oils (n = 12): mean 0.106 µg/kg (range 0.030–0.207); highest = preparation No. 7 (0.207 µg/kg), No. 3 (0.205 µg/kg)
- Shark liver oils (n = 6): mean 0.065 µg/kg (range 0.023–0.087)
- Vegetable > fish oils difference statistically significant (p < 0.001)
Vegetable oil–specific (n = 18, individual preparations):
- Cedar nut oil: 0.427 µg/kg (highest in the study)
- Evening primrose oil: 0.320 µg/kg
- Safflower oil, soybean oil, wheat germ oil: > 0.200 µg/kg
- Grape seed oil: 0.131 µg/kg (lowest vegetable oil)
Dietary intake estimates at manufacturer-recommended doses:
- Maximum daily Hg intake (organic soybean oil, 25 mL/day): 0.0045 µg
- Maximum annual Hg intake (safflower oil): 1.64 µg
- Maximum TWI contribution among all preparations: 0.03% of EFSA TWI for methylmercury (91 µg/week for 70 kg adult)
- All preparations well below 0.10 mg/kg (100 µg/kg) EU maximum level for dietary supplements
Reference comparison from discussion:
- Augustsson et al. 2021: Hg in fish oil supplements 1.1 µg/kg; vegetable supplements 2.1 µg/kg — approximately 5–10x higher than the present Polish study
- Smutna et al. 2009: fish oil total Hg 0.013–2.03 µg/kg (upper range much wider)
- Foran et al. 2003 (US): fish oil supplements 6–12 µg/kg (notably higher, earlier generation products)
Methods
AAS amalgamation technique; AMA 254 spectrometer (Altec, Praha, Czech Republic); wavelength 253.65 nm; inlet pressure 200–250 kPa, oxygen carrier gas; drying 200 s, decomposition 250 s, measurement 90 s. LOD = 0.01 ng Hg per sample. No sample preparation required before AMA 254 analysis. Recovery from certified reference material INCT-MPH-2 (Mixed Polish Herbs): 0.0184 ± 0.0003 mg/kg, recovery 102.3%. Three 100 µL sub-samples per preparation; result = arithmetic mean of three measurements. Non-normal distribution confirmed by Shapiro-Wilk; non-parametric tests used (Mann-Whitney U and Kruskal-Wallis). Significance level p ≤ 0.05.
Speciation: This study measures total mercury (tHg) only. No speciation into methylmercury (MeHg) vs. inorganic Hg was performed. For vegetable oils, the predominant form is likely inorganic Hg; for fish-derived oils (cod liver, shark liver), MeHg may contribute but its proportion is not established by this study. The finding that vegetable oil supplements contain higher tHg than fish oils runs counter to the bioaccumulation narrative for MeHg, consistent with vegetable oils accumulating inorganic rather than methylmercury.
Implications
Certification: The finding that vegetable oil supplements carry higher tHg than fish oil supplements is a counterintuitive but important result for supplement product categories. Even so, all concentrations are more than 200-fold below the EU maximum level. This supports a low-priority risk classification for oil-based dietary supplements with respect to mercury.
Courses: Useful teaching case for the distinction between expected contamination narratives (fish = mercury bioaccumulation) and observed data (vegetable oil supplements measured higher tHg in this study), reinforcing the importance of direct measurement over assumption.
App: tHg concentrations for cod liver oil (mean 0.106 µg/kg) and shark liver oil (mean 0.065 µg/kg) provide reference data for fish-oil supplement ingredient profiles.
Microbiome: Not applicable.