1 Introduction

The consumption of food supplements is a common phenomenon worldwide. They contain a variety of nutrients, botanicals, and with attractive advertisements, these products are becoming more and more popular among people looking for solutions to improve or prevent health problems. According to EU legislation, food supplements are indicated “to correct nutritional deficiencies, maintain an adequate intake of certain nutrients, or to support specific physiological functions” (Directive 2002/46/EC), while they cannot reveal pharmacological activity and be used for the treatment or prevention of diseases. However, some food supplements contain ingredients in doses that may exert the pharmacological effect, although less pronounced.

High amounts of caffeine and its botanicals (e.g. green tea or pu-erh, yerba mate, guarana, green coffee) are often found in food supplements, advertised as weight loss products or stimulants for concentration improvement. However, only the impact of caffeine on increased alertness and attention was positively evaluated by EFSA, while no such recommendation was presented for its effect on the reduction of body weight. As far as the safety of caffeine is concerned, its consumption by healthy adults up to 400 mg/day is considered as harmless, while single dose of 100 mg may cause sleep disturbances (EFSA NDA 2015). In the EU, food supplements are regulated as foods, so their quality and content of the declared ingredients are only randomly controlled. Thus, analytical verification of the caffeine content in food supplements seems to be an important strategy for gaining insight into their safety and efficacy.

Iodine is a trace element of great importance for human health, especially in terms of its impact on thyroid function. Due to the regulatory role of thyroid hormones in many physiological processes in the human body, iodine may affect metabolic rate, which translates into its potential significance as a weight loss support. Iodine can be found in different marine organisms, including fish, or seaweeds, among which fucus is the most popular and one of the richest sources of the element, in terms of its use in phytotherapy (Catarino et al. 2018). With its exceptionally high iodine content (13 − 73 mg/100 g dry weight), fucus is a common ingredient in food supplements recommended for thyroid disorders, or for weight loss support (Catarino et al. 2018). However, due to such significant variations in the iodine content, fucus-containing food supplements should be consumed with great care, especially by people with confirmed thyroid disorders, where any uncontrolled intake of iodine can result in health consequences. In addition, those products should always have information on the label indicating the amount of iodine. Thus, knowledge on the level of iodine in food supplements is an important element for choosing the product, so that the consumption will not disturb the health balance.

The purpose of this study was to assess the quality and safety of food supplements, in different forms, namely tablets, capsules (Table S1), but also herbal blends (Table S2) to prepare infusions, with a declared and undeclared caffeine content, respectively, available in pharmacies, grocery stores and health food stores. The supplements use were intended for weight loss (FS1–4), or concentration improvement (FS5–10). For this study, products with both declared (n = 10) and undeclared (n = 9) caffeine content were chosen. A medicinal product was also included for analysis to verify the applied methods. For products in tablet or capsule forms, we used ultrasound-assisted extraction with water, previously optimized by our group (Wołonkiewicz et al. 2019). Herbal blends, following the manufacturer’s instructions, were poured over with boiling water and left to infuse. The previously validated HPLC method was used to determine the caffeine content (Wołonkiewicz et al. 2019), while for iodine the approved method of Sandell-Kolthoff was applied (Dobrowolska-Iwanek et al. 2022). The details of the experimental procedures are included in Supplementary Material.

2 Results and discussion

The caffeine content in the tested products ranged from 91.8% of (FS10) to 138.9% (FS1) of the declared caffeine content (Table 1). The differences were statistically significant only for FS2, FS9 and FS1, with p-values < 0.05, < 0.01 and < 0.001, respectively. Interestingly, up to 7 of the 10 tested preparations contained more caffeine than the manufacturer declared, with a statistically significant difference only for FS1 and FS9 (p < 0.01). In FS1, the presence of caffeine was undisclosed, since the product label indicated only green coffee extract and green tea leaf extract. On the other hand, in FS9, in addition to the caffeine itself, the label also indicated guarana extract, standardized to 10% caffeine. Some similar studies on caffeine-containing food supplements were performed mainly in the USA and Brazil, and their results are inconclusive. Andrews et al. (2007) showed in 25 of 28 tested food supplements a caffeine level of ± 16% of the declared content. In contrast, 86% of the 35 food supplements tested on the US market indicated less than 90% of the declared caffeine content (Haller et al. 2004), which is also consistent with our previous study (Wołonkiewicz et al. 2019). And a Brazilian study on 109 food supplements indicated that the amount of caffeine in almost 27% of the products exceeded the declared content by 20% (da Justa Neves et al. 2017).

Table 1 Analyzed amount of caffeine (mean ± SD) in food supplements with its declared content
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In the next step, we focused on food supplements with herbal blends, where caffeine-containing botanicals were at least one of the ingredients, and without a declaration of caffeine content. Such products are available not only in the pharmacy, but also in grocery stores. Thus, we wanted to verify their safety and potential effectiveness. The caffeine content in the herbal blends varied from 2.6 ± 0.3 (FS11) to 21.8 ± 2.8 (FS19) mg/portion of the products (Table 2). Interestingly, FS19 with the highest caffeine content was packed as loose herbs, while the rest of the FSs from this group were infusion sachets. This may suggest that the degree of fragmentation can strongly influence the extraction process of caffeine from the herbal blend. Moreover, in the FSs with the highest caffeine content, namely FS19, FS15, and FS13, the percentage share of caffeine-containing botanicals was > 50% (60, 56, and 51%, respectively), relative to the other components. This was also observed for the lowest caffeine content FS11, with the lowest percentage share (3%) of botanicals containing caffeine. However, the amount of caffeine in FS17, with 41% caffeine-containing botanicals, was much lower than in FS14 (15% of botanicals in its composition). In a similar study, the amount of caffeine in tea leaves, determined by UV and NIR spectroscopy, was in the range of 2.28 − 19.53 mg /g dw of leaves (Dankowska et al. 2018), which is in line with our observations. In contrast, another study described the HPLC analysis of caffeine in different food supplements for weight loss, with a caffeine content in herbal blends of 65.71–89.64 mg/single dose recommended by the manufacturer, which is a few times higher compared to our study (Croitoru et al. 2017). The difference may result from the ultrasonic extraction method applied by the authors or the details of the experimental procedures.

Table 2 Analyzed undeclared caffeine content (mean ± SD ) in food supplements
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Additionally, the caffeine content in all tested FSs did not exceed 12% of the maximal daily doses suggested by ESFA (Table 2). This indicates that the tested products were safe for consumption in their recommended daily portion, even with simultaneous consumption of caffeine from other sources (coffee, tea, soft drinks, energy drinks, etc.).

Our evaluation also included the determination of the iodine content in 4 herbal blends (FS12, 15, 18 and 19), in which fucus was one of the ingredients (Table 3). The analyzed amount of iodine ranged from 10.6 ± 0.4 to 52.5 ± 3.7 µg per portion of the examined products. Like for caffeine, the highest content of iodine showed FS19 in the form of loose herbs, which may also underline the relationship between the efficacy of extraction and the degree of fragmentation of particular ingredients in the blend. Furthermore, the iodine content for the suggested daily intake of FS19 comprised 70% of the recommended food allowances (RDA), which is concerning as the product’s label did not provide that information. However, when referred to tolerable upper level intake (UL, 600 µg/day for adults according to EFSA), the risk of adverse health effects was rather low.

Table 3 Analyzed iodine content (mean ± SD ) in food supplements containing fucus
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It is worth noting that only 2 examined products actually declared the iodine content and the reference to RDA. However, those 2 products showed a 3-fold lower iodine level than declared. Such a difference cannot be explained by the method effect of analysis but rather suggests a low quality of the tested products. To our knowledge, so far no similar studies on the iodine content in food supplements have been conducted. Our results, although preliminary with a small sample size, are indicating problems regarding the quality and safety of food supplements and therefore, justify a continued analysis.