Caffeine and synephrine are substances normally included in weight loss products due to claims that they promote a thermogenic effect, that is, to stimulate the metabolism and increase caloric expenditure [1-4].

Caffeine is an alkaloid found in varying amounts in the grains, leaves, and fruits of more than 60 plants, and coffee is the world’s leading source of caffeine in the diet [5,6]. This substance contributes to positive effects on sports performance, as it increases attention and alertness, delays the feeling of tiredness, increases time-trial performance, can increase performance in anaerobic exercises, increases the rate of glycogen synthesis, and has no impact on hydration status [3]. This substance is effective in trained athletes when consumed in low to moderate doses (~3-6 mg/kg) because when consumed in higher doses, it does not result in an ergogenic effect (>9 mg/kg) [3]. Given these effects, caffeine is often used in dietary supplements for weight loss.

Synephrine is a derivative of phenylethylamine and a sympathomimetic agent that can be used to stimulate specific adrenergic receptors (β₃, but not β₁, β₂, or α₁ ), which stimulate fat metabolism without any side effects typically associated with other compounds that boost adrenergic receptors [7]. Its wider interest became relevant when companies started substituting Citrus aurantium for ephedra in their formulations, as the US Food and Drug Administration had banned the use of ephedrine in dietary supplements [8]. Synephrine is naturally present in bitter orange (Citrus aurantium) and other citrus species [9]. Citrus aurantium is a known fruit that is commonly used as a herbal medicine to treat digestive problems in Asia, and as a mild stimulant, it contributes to appetite suppression and increased metabolic rate and lipolysis [8]. It has been shown in clinical trials that the ingestion of p-synephrine raises fat oxidation during exercise [7,10]. This component is identified as being responsible for action in weight loss products containing bitter orange or Citrus aurantium.

Synephrine is widely used in combination with caffeine and other ingredients in products designed to support weight management [2,6,11]. Given their use in Dietary Supplements (DS), several health and food authorities have issued recommendations on the levels deemed safe for human use. Table 1 summarizes several reference values for synephrine and caffeine consumption.

The legislation for dietary supplements varies among countries, but within the European Union, harmonization across member states ensures less disparity. The European Union legislation was transposed to Portuguese legislation, and the manufacturer or person responsible for placing food supplements on the market must notify the Directorate General for Food and Veterinary (Portuguese DGAV) when starting to market a food supplement [12]. The labels must include all the ingredients and state that the product is a dietary supplement. Moreover, that organization is responsible for recording and following up on adverse reactions [12]. In the past few years, the problems associated with overweight and obesity have been getting worse, and the market for DS for weight loss has been increasing and is expected to continue increasing, which means more users are exposed to the risks associated with DS intake [13]. This paper aimed to report the amount of caffeine and synephrine found in dietary supplements available on websites that are commonly used by Portuguese consumers and that could be shipped to Portugal.

In this observational study, dietary supplements were searched on internet platforms selling sports supplements. The product labels’ description of the ingredients was the only basis for the supplement analysis. For the selection of DS within this research, inclusion and exclusion criteria were established. For the inclusion criteria, only dietary supplements that contained both active ingredients, synephrine (either pure or in the form of Citrus aurantium extract) and caffeine, regardless of quantity and use, were included. As for the exclusion criteria, dietary supplements were not included if only one of the components was on the label, or if the formula and ingredient amounts were not fully described. The search for dietary supplements was conducted by the first author, and inclusion and exclusion criteria were evaluated by the last author. This type of study and methodology was previously described by our group in an earlier publication [14].

The information contained on the labels of DS was taken from the websites of the company’s Prozis (www.prozis.pt), Eunutrition (eunutrition.com), Myprotein (pt.myprotein.com/), NutriTienda websites (www.nutritienda.com/en), Amazon (www.amazon.com), Fitnis (www.fitnis.pt) and PlanetaHuerto (www.planetahuerto.pt). These websites were chosen because they are major players in selling supplements to the Portuguese market and could be shipped to Portugal. The ingredients disclosed on the label were thoroughly analyzed and grouped. The search for supplements was conducted between October 2020 and November 2020. After the investigation, twenty supplements were found that met the inclusion criteria. Statistical analysis and graphics were performed using GraphPad Prism 10 software (CA, USA).

In a later stage, the databases PubMed, Science Direct, and Google Scholar were used to search scientific articles for bibliographic support, with the keywords “Caffeine and synephrine, toxicology”, “Citrus aurantium and caffeine, adverse cardiovascular”; “dietary supplements, body weight”.

The ingredients and respective quantities mentioned on the label, prepared by the producers and/or distributors of 20 DSs containing synephrine and caffeine, are described in Table 2. The table describes different groups: synephrine or bitter orange extract (Citrus aurantium), caffeine, vegetable extracts with caffeine, other plant extracts, amino acids, micronutrients, and finally, others. It’s important to mention that the amounts referred to are for the recommended daily dose. The energy and macronutrient value of these supplements is not available as they do not provide relevant calories, thus, the caloric value is not listed on the label.

Of the 20 supplements containing caffeine and synephrine or Citrus aurantium, none of the supplements contained pure synephrine in their composition, but rather in the form of bitter orange extract (Citrus aurantium). Supplements contain standardized extracts of Citrus aurantium for their synephrine content, which is normally standardized at 6%. Regarding the amount of bitter orange extract and considering the daily recommended dose, the minimum value found was 10 mg, corresponding to supplements number 2 and 18, and the maximum value was 840 mg, corresponding to number 4. Therefore, 6 supplements provide less than 100 mg, 6 provide between 100 mg and 200 mg, and 8 supplements provide more than 200 mg of Citrus aurantium per daily dose.

Regarding the amount of caffeine and considering the daily recommended dose, the minimum value found was 100 mg, which corresponds to DS number 7 and 9, while the maximum value found was 450 mg, corresponding to number 4. Also, 6 DS have more than 200 mg of caffeine. In addition to caffeine, about 95% of supplements contained other combinations of plant extracts that contained caffeine, namely green tea, green tea extract, guarana extract, and kola nut. This addition makes it impossible to estimate the exact amount of caffeine present in those DS.

With a total of 20 DS found, the median was calculated, as well as the 95% confidence interval for caffeine, bitter orange extract, and the estimated value of synephrine, and those values are depicted in Figure 1. The synephrine levels were estimated considering that bitter orange extracts are normally standardized at 6%, except in some formulations that mention other percentages (10% or even 95%). For synephrine, the median is 11 mg with a 95% confidence interval of 6 – 20 mg. Thus, the median for caffeine is 200 mg with a 95% confidence interval of 176 – 250 mg. We calculated the caffeine amount based on the amount of caffeine anhydrous reported on the label and could not estimate the amount provided by plant sources. As for the bitter orange extract, the is median is 153 mg with a 95% confidence interval of 40 – 334 mg (Figure 1).

Apart from the primary constituents, several other plant extract constituents have been disclosed, such as polyphenols, cayenne, cocoa, and black pepper extracts. Moreover, DS contain amino acids, micronutrients, and others that can be considered vestigial. Furthermore, the nutritional information on the label or the description on the websites sometimes included the following warnings: “Food supplements should not be used as a substitute for a varied and balanced diet, as well as a healthy lifestyle”, “Keep out of the reach of children”, “Do not exceed the recommended daily dose”, “This product is not intended to diagnose, treat, cure or prevent any disease”, “The consumption of this product is contraindicated for people with physical or mental problems, those taking any medication, under the age of 18, pregnant women and those under medical supervision”, “Avoid this product if you are allergic to aspirin or have high blood pressure, arrhythmia, glaucoma, seizures, ulcers, difficulty urinating, an enlarged prostate, diabetes, thyroid or eating disorders”.

Several international safety authorities in different European Union (EU) Member States have carried out risk assessments or issued warnings for synephrine intended for weight loss and sports performance improvement, such as its combination with caffeine [6]. Of the organizations analyzed, it is worth noting that they present very different daily limits. For instance, the German Federal Institute for Risk Assessment (BfR) deemed a safe maximum daily dose of 6,7 mg of synephrine in food supplements [15]. In this study, only seven supplements respected this recommendation, while approximately 65% of supplements exceeded the limits imposed by this organization and may endanger the health of those who consume these products, according to that organization. Based on the Medsafe organization in New Zealand, the recommended dose is 30 mg. Of the analyzed supplements, 95% fell within the recommended dose except one, number 4. In 2010, Health Canada established a maximum limit of 30 mg/day of synephrine, reasoning that supplements containing both components, synephrine and caffeine, were not allowed [16]. However, in 2011, the same safety authority, Health Canada, mentioned that a maximum dose of 50 mg was recommended when synephrine is taken alone. Still, when combined with caffeine, the maximum amount allowed was 40 mg of synephrine or less in combination with a maximum of 320 mg of caffeine in healthy adults [17]. For the recommendation of 2011 by Health Canada, all supplements comply, not exceeding the maximum of 50 mg of synephrine, however, when the two ingredients are combined one supplement recommends taking 450 mg of caffeine and 50mg of synephrine (number 4). Of note, the real caffeine amount in several supplements is certainly higher, as several contain plant extracts that also contain caffeine. This addition may increase the total amount of caffeine intake, rendering it impossible for the consumer to know the exact amount of caffeine included in the supplement. Regarding caffeine consumption, the European Food Safety Authority (EFSA) recommends a maximum intake of up to 400 mg for healthy adults but only up to 200 mg for pregnant and breastfeeding women [6].

In 2014, the institution ANSES concluded that the effects of p-synephrine alone are observed from around 20 mg, as with 50 mg there is an evident effect on heart rate and systolic and diastolic blood pressure, and that caffeine can potentiate the cardiovascular effects of synephrine [18]. Thus, ANSES recommends avoiding the combination of p-synephrine with caffeine [18]. This agency also recognizes that using mixtures containing p-synephrine during physical exercise may modify blood pressure tolerance and increase acute cardiovascular risk [18]. It is worth mentioning that four supplements had p-synephrine levels higher than 20 mg. Considering the high amount of p-synephrine and its combination with caffeine in these four products without the need for a prescription, they can have consequences for patients with cardiovascular problems.

Several clinical trials were designed to assess the potential adverse effects and toxicity of synephrine and caffeine in supplements. A recent randomized, double-blind, and placebo-controlled trial was conducted involving 12 male adults (mean age 21 years) [19]. Citrus aurantium (600 mg, p-synephrine at 30% concentration [180 mg]) or placebo (600 mg of starch) was ingested 90 min before the evaluation of several cardiovascular parameters. No differences from placebo were found in diastolic blood pressure, pulse pressure, mean arterial pressure, and heart rate, leading the authors to conclude that p-synephrine has a safe cardiovascular profile under these conditions [19]. A review analyzed published case reports concerning adverse effects related to multi-ingredient dietary supplements (DS) containing bitter orange extract [20]. More than 30 clinical studies involving bitter orange extract and p-synephrine have been published, and despite many cardiovascular or other significant adverse effects associated with the consumption of bitter orange extract and p-synephrine, the authors ruled that at daily and individual doses as high as 104 mg p-synephrine there is no demonstration of adverse events [20]. Still, the authors mention that “in some cases products were not being consumed as recommended, and it was not always clear whether the subjects were using other unreported DS and/or drugs” [20, p.3]. Finally, the authors stress that the studies “do not demonstrate a direct relationship between bitter orange extract (p-synephrine) and the observed effect” [20, p.7]. More recently, a new review of reports on cases of adverse events related to synephrine intake was performed, and the authors concluded that DS containing synephrine might lead to serious adverse health events [21]. This study contradicts the earlier findings of Stohs and Ray, who in their review of case reports, argued for the safety of synephrine [20]. De Jong and co-authors reviewed 30 case reports describing a total of 35 patients who reported adverse events following the use of synephrine-containing supplements [21]. Patients presented chest pain, palpitations, syncope, and dizziness, but of more concern were the diagnoses of ischemic heart disease, cardiac arrhythmias, and cerebrovascular disease. The authors concluded that there was “an association between the use of pre-workout supplements containing synephrine and adverse events, mainly related to the cardiovascular system” [21, p. 1]. They also noticed that several patients diagnosed with diseases of the circulatory system used a supplement containing synephrine and caffeine, which might indicate that when combined the adverse effects might be higher than when used alone.

Several studies in humans assessed the safety of the combination of synephrine and caffeine [22-24]. In a double-blind clinical trial 16 subjects (13 male and 3 female, with a mean age of 20 years) were given several treatments, including either 325 mg of caffeine or 337 mg of caffeine plus 46 mg of p-synephrine [23]. After 2 hours of being quietly seated, these treatments significantly increased mean systolic blood pressure [23]. Ten physically active males (mean age 25 years) were included in a double-blind trial and tested the metabolic and cardiovascular effects of a complex containing 100 mg of Citrus Aurantium + 100 mg of Caffeine [22]. This dose was sufficient to promote glucose sparing at rest and modest increases in SNS activity, though the role of each component was not disclosed [22]. Haller and collaborators conducted a three-arm randomized placebo-controlled crossover clinical trial involving 10 healthy men and women aged between 18-45 years [24]. Subjects ingested one dose of DS with 21 mg synephrine and 304 mg caffeine under resting conditions and 1 hour before moderately intense exercise, with a placebo exercise control [24]. Blood pressure and plasma glucose increased post-exercise after supplementation use, which might be problematic for patients with cardiovascular problems and diabetes [24]. Overall, the studies above emphasize the need for more quality-controlled and randomized trials, as well as double-blind and placebo-controlled studies, regarding the safety of DS that contain caffeine and p-synephrine.

This study has several limitations. First, our analysis is based on the information described on the labels provided by the producers, which might not be fully accurate. A recent study conducted in the United States found that 40% of the 57 sports supplements analyzed didn’t even have detectable quantities of ingredients that were listed on their labels [25]. In this report, dietary supplements were reconstituted in methanol and analyzed by liquid chromatography quadrupole time-of-flight mass spectrometry. Moreover, 89% of these products had labels that inaccurately indicated the amounts of those ingredients, and 12% contained at least one ingredient prohibited by the U.S. Food and Drug Administration (FDA) [25]. It should also be noted that dietary supplements can be contaminated or artificially fortified with doping agents, leading to athletes failing a doping test, despite inadvertent administration of doping agents that were not included in the label [26]. Another study in Italy analyzed weight loss supplements by Liquid Chromatography-tandem Mass Spectrometry (LC–MS/MS) to detect and quantify simultaneously different categories of active molecules [27]. The amount of caffeine reported on the label by the producer did not always correspond to what was detected (about ±30% variation) [27]. The same was found for synephrine, and there was even one product that contained synephrine that was not declared on the label [27]. These previous studies highlight that fraud can be frequent in the supplements market. We estimated the content of synephrine contained in the supplements based on the information contained in the labels, and we did not perform any analytical measurements. Importantly, it is impossible to estimate based solely on the label the full amount of caffeine provided by some DS that are made of mixtures of several botanical sources of caffeine, rendering the quantity of caffeine in these supplements higher than estimated. Also, the 20 DS described herein do not reflect consumer preferences, and the dose administered by the user might not correspond to the one recommended by the product, leading to lower or higher amounts of substances to which consumers are being exposed to daily.

Several supplements have amounts of synephrine and caffeine that exceed those recommended by health safety agencies, which can put consumers’ health at risk. In addition, not only can the combination of p-synephrine and caffeine components cause adverse cardiovascular effects, but other compounds can potentiate these problems or even other medications taken at the same time for a disease. Also, individuals with known cardiovascular diseases who consume this type of supplement and do not have this information may be at greater health risk. Unfortunately, there is a lack of literature and studies on this topic, as well as a lack of scientific evidence regarding adverse effects.