Slimming pill with a mercury taste: Study tested 47 dietary supplements and found that 1 in 4 had a risk index above the threshold

Weight loss supplements are one of the most "googled" segments of the supplement market: their compositions are often multi-ingredient (up to a dozen items), the dosages are varied, and the raw materials are often of plant origin - from algae to the peels of exotic fruits. Such products may "arrive" with impurities from soil and water, including heavy metals. Polish researchers checked how real the mercury load from popular weight loss supplements is and how it compares with the EU maximum permissible level for dietary supplements (100 mcg / kg). At the same time, they calculated EDI / EWI (estimated daily / weekly dose), % TWI (share of tolerable weekly intake) and THQ - a dimensionless indicator of the likelihood of adverse effects with chronic consumption.

Background of the study

The market for "weight loss" dietary supplements is structured in such a way that a dozen plant ingredients often come together in one capsule - from microalgae (spirulina, chlorella) to mulberry extracts, green coffee and garcinia. Plant raw materials have a downside: they can accumulate impurities from water and soil, including heavy metals. This is especially true for microalgae: according to reviews, commercial products based on spirulina/chlorella often contain trace amounts of mercury, cadmium, lead and arsenic, and the level depends heavily on the place of cultivation and purification technology. Therefore, even "natural" supplements do not guarantee "zero" metal content - production control of each batch is important.

In the EU, mercury in food products is regulated as "total mercury", and for food additives, the maximum permissible level is 0.10 mg/kg (100 μg/kg). This is a single "label threshold", but when assessing the health risk, the real dose entering the body is taken into account, taking into account the daily portion. The European Food Safety Authority (EFSA) recommends focusing on the tolerable weekly intake (TWI): for methylmercury - 1.3 μg/kg body weight per week, for inorganic mercury - 4 μg/kg/week. It is the ratio of the dose received with the product to the TWI that shows how "significant" the contribution of dietary supplements is compared to other sources (primarily fish and seafood).

In practical studies, integral indicators such as EDI/EWI (estimated daily/weekly dose) and THQ (Target Hazard Quotient) - a dimensionless chronic risk index - are often calculated together with the metal concentration. It is convenient to read it like this: if THQ < 1, the probability of adverse effects with long-term consumption is low; if THQ ≥ 1, the alarm signal does not mean "exceeding the norm", but suggests that at the doses and duration taken, the risk cannot be excluded, and it is worth looking at the source of raw materials, dosages and cumulative contribution from other products in the diet. This is especially important for dietary supplements with algae: different brands and batches can differ in mercury by an order of magnitude, although all formally "fit" into the general EU limit.

Finally, the context for the consumer: the main contributor to methylmercury exposure in Europeans usually comes from fish and seafood (especially predatory species), while the contribution of dietary supplements is often minimal - but it can become noticeable with long-term daily intake of products with "sensitive" ingredients without quality control. Hence the simple rules: choose brands with transparent certificates of analysis, do not exceed the recommended doses and remember that "natural" ≠ "safe by default".

What and how they did it

The team collected 47 supplements from the Polish market (pharmacies and online, 2023-2024): tablets (n=30) and powder capsules (n=17). The main "tricks" of the formulas are spirulina, chlorella, white mulberry, "green barley", garcinia cambogia, green coffee, l-carnitine, African mango, etc. Mercury was measured using an AMA-254 atomic absorption analyzer (amalgamation method). Then, the mercury content was compared with the EU standard and EDI/EWI, %TWI (for inorganic and methylmercury, and their sum) and THQ were calculated.

Key figures

• Mercury range: 0.12 to 46.27 μg/kg; median 2.44 μg/kg; mean about 5.8 μg/kg (in tables: AM 5.80±8.47 μg/kg). All samples are below the EU standard of 100 μg/kg. Variation coefficient is 146%, the spread is large.
• By ingredients (average): chlorella 21.58, white mulberry 10.98, spirulina 6.13, “young barley” 5.09, garcinia 3.99, green coffee 2.10, African mango 1.57, l-carnitine 1.07 mcg/kg. No statistically significant differences were found between the groups, but it was the microalgae (chlorella/spirulina) that yielded the highest average and range (CV 91-108%).
• By form: for “powders in capsules” the average mercury content is 7.15 μg/kg, for tablets 5.03 μg/kg ( p > 0.05).
• Consumer load: average EDI for all samples is about 0.011 µg per day, but between the minimum and maximum the spread is ~1800×. %TWI (share of EFSA tolerance) fluctuated between 0.0009-1.23% - that is, far from the threshold.
• Risk index (THQ): 36 samples <1 (unlikely to cause adverse effects with chronic consumption), but 11 of 47 THQ ≥1 (range 1-17.31). Individual supplements in this “red zone” included spirulina (4 samples), chlorella (2), white mulberry extract (2), and one each of African mango, garcinia, and green tea.

How to read it in plain language

All tested did not exceed the EU limit, and the estimated share of the weekly "permissible" mercury intake is usually a fraction of a percent. But THQ is not a "law", but a risk flag: if the index is ≥1, then with the adopted dosage and duration scheme, the probability of adverse effects in the population cannot be ruled out. That is why the authors put a "yellow signal" for some samples (approximately every fourth). The supplement market is very heterogeneous: even within one "category" (say, chlorella), different batches and brands can differ by an order of magnitude.

What does this mean for the buyer - practical conclusions

• Don't equate "natural" with "safe." Plant-based raw materials often "pull" impurities from soil and water; algae are especially sensitive to this. Check the manufacturer and look for control protocols.
• Form and dosage matter. On average, the “powder in a capsule” showed slightly higher mercury levels than tablets (though not statistically significant). Do not exceed recommended dosages, and avoid “marathons” without breaks.
• Look at the composition. If the formula contains microalgae (chlorella/spirulina) or white mulberry in the foreground, the risk of encountering higher mercury variability is higher - choose brands with transparent reporting.
• Be aware of the “expectancy effect.” Systematic reviews show modest effectiveness of “weight loss” supplements; the feeling of “I’ve already done something” can weaken self-control and interfere with real changes in diet and exercise.

For those in the know (regulators and laboratories)

• Regulation: The EU limit for dietary supplements is 100 µg/kg for mercury; no sample exceeded it. However, the risk indices (THQ) for some products are ≥1, which indicates the heterogeneity of the market and the importance of batch-specific control.
• Where the "tails" are concentrated: the box plots show that some manufacturers have outliers (up to 40-50 µg/kg), although the medians are low. This is an argument for regular screening of raw materials and finished products.
• Risk communication: Even with low %TWI, the consumer sees “natural” and “weight loss” and may take the supplement on their own for months; it is useful to label not only “benefits” but also duration limitations, referring to THQ/EDI.

Limitations of the study

This is a cross-section of 47 products from one national market; larger batches and geographies would have changed the picture. The author's design looks only at mercury (no other metals were measured), and there are few head-to-head comparisons across brands. Finally, THQ is a model assessment; it is not sufficient for making clinical conclusions about a particular person's health.

What to check next

• A wide range of pollutants (cadmium, lead, arsenic) in the same categories of dietary supplements.
• Batch monitoring of microalgae and plant materials, where the spread is maximum.
• Agreed methods for calculating THQ/EDI in the instructions so that the buyer sees not only “herb - benefit”, but also “how much and for how long”.

First source: Brodziak-Dopierała B. et al. Analysis of Mercury Concentration in Dietary Supplements Supporting Weight Loss and Health Risk Assessment. Nutrients 2025;17(11):1799. https://doi.org/10.3390/nu17111799