Our research revealed a high prevalence of monoterpene exposure among a sample that represents the adult population of the United States. We also found a significant positive correlation between serum levels of three specific monoterpenes and ALT. Furthermore, our findings suggest a possible synergistic effect between these three monoterpenes with regards to ALT. This is the first study to report on the internal monoterpene exposure in general adult population, and our preliminary data provides the first evidence of a potential link between monoterpene exposure and liver health. The significance of our study lies in the reliable and comprehensive data obtained from the NHANES repository, as well as our inclusion of a representative cross-section of American adults aged 18 years and above.
The results of this research indicate that exposure to the monoterpenes α-pinene, β-pinene, and limonene cannot be avoided during daily activities, with detection rates of 74.1%, 74.3%, and 100%, respectively.α-pinene and β-pinene are bicyclic monoterpene dominate emissions from conifer trees. The location of a double bond in their molecular structures is what distinguishes the two 28. Limonene, a monocyclic monoterpene, exists in two isomeric forms, d-limonene and l-limonene. D-limonene is obtained as a by-product in the extraction of citrus juices, and it is also found in caraway oil. L-limonene, on the other hand, is present in pine needles and cones 29. Monoterpenes are volatile compounds found in essential oils and are commonly used as fragrances and food flavors or additives. Following uptake, monoterpenes are small molecules that can be absorbed by passive diffusion and undergo hydroxylation and epoxidation in phase I reactions. This leads to the production of hydroxylated metabolites that may undergo oxidation or conjugation in phase II reactions. The metabolites are mostly excreted through urine 9. Exposure to these compounds can occur through inhalation, ingestion, and dermal contact 2. Several previous occupational studies have investigated the exposure and health impacts of inhaling monoterpenes among workers 6,7. Thus far, there has been little research conducted on biomonitoring the exposure of the general population to monoterpenes. The only available study conducted in Germany examined the relationship between indoor air pollution from three monoterpenes (α-pinene, Δ3-carene, and limonene) and the internal exposure of children in 45 daycare centers. Despite the indoor air levels of monoterpenes were below the German health hazard guide value, urine analyses of 222 children revealed exposure to all three monoterpenes. The study found a weak association between urinary and indoor air levels of α-pinene and Δ3-carene, suggesting that other sources, such as diet and consumer products, significantly impact internal exposure 8. It was also observed that limonene has higher exposure levels than the other two monoterpenes in our current study. Among the monoterpenes, limonene is one of the most prominent in foods, as well as in cosmetic and household products 30. Although monoterpenes are appraised as relevant indoor air pollutants 31, these findings emphasize the importance and need for further research on sources of monoterpenes beyond respiratory exposure, especially in the general population.
Our results reveal that male participants and current smokers had higher levels of limonene and Σmonoterpene, whereas individuals with a higher BMI and metabolic syndrome showed elevated levels of α-pinene and β-pinene. Additionally, Mexican-Americans exhibited higher levels of β-pinene, limonene. Tobacco smoking is a significant source of exposure to monoterpenes. Furthermore, tobacco plants that have been genetically modified with monoterpene synthases from lemon exhibit higher levels of monoterpenes compared to the wild type 32. Monoterpenes are also commonly added to e-cigarette fluids as flavorings 33. It is also believed that demographic characteristics can affect a person's exposure to many organic compounds, such as monoterpenes 34.
Studies have investigated the impact of monoterpenes on liver function at high doses in animals 11. In male Wistar rats, a study found that 25 and 75 mg/kg of limonene by gavage for 30 and 45 days induced pathological hepatocellular changes such as steatosis, necrosis, or fibrosis, but no significant changes in liver enzymes were observed 35. Another study administered essential oil containing 27.5% α-pinene at doses of 50, 100, and 200mg/kg to male Wistar rats for 21 days, and found increased levels of aspartate aminotransferase and ALT in a dose-response relationship 12. However, a different study in adult Wistar rats found that administration of an essential oil containing α-pinene, 1,8-cineole, linalool, and limonene at 250 mL kg− 1 body weight via intraperitoneal injection for 15 days significantly inhibited the elevation of hepatic biomarkers in plasma induced by carbon tetrachloride-mediated hepatotoxicity 13.
There have been reports of encouraging impacts on health attributed to monoterpenes. Certain monoterpenes have been found to possess established biological activities such as antimicrobial, analgesic, and anti-inflammatory properties 9. In recent times, the potential of monoterpenes to control energy metabolism, as well as their ability to exhibit antidiabetic, anti-obesity, and modulation activities on gut microbiota, have been documented 9. However, much of this evidence is based on animal studies rather than clinical studies by randomized controlled studies or large prospective cohorts. Furthermore, due to the presence of monoterpenes in daily diet, there is a lack of understanding regarding their concentration in food, their biotransformation by gut microbiota, and their bioavailability, which poses a challenge in estimating the human intake dosage. As a result, it is challenging to extrapolate the results for their use in human daily life 9.
The majority of investigations exploring the effects of monoterpene exposure on human health have centered predominantly on the outcomes of intoxication. Some monoterpenes have been found to have more toxic effects on the liver 14. In some individuals, administration of a single high dose of monoterpenes has been associated with the development of acute liver injury, as evidenced by marked elevations in liver enzymes, prolonged prothrombin time, and increased risk of hepatic failure 36. Some monoterpenes commonly found in cleaning products, air fresheners, and personal care products, such as limonene and pinene, have been associated with negative effects on human health. High concentrations of these monoterpenes can react with ozone and lead to respiratory symptoms and asthma when inhaled 37,38. In addition, the topical application of limonene can induce skin irritation due to its potential to cause contact dermatitis 29. Given the limited prior research on biomonitoring monoterpene exposure and exploring their association with liver function tests in the general population, our study observed a high prevalence of monoterpene exposure and a significant positive correlation between serum levels of specific monoterpenes (α-pinene, β-pinene, and limonene) and ALT in the general adult population in the United States. Additionally, our findings suggest a dose-response relationship, as higher Σmonoterpene quartiles were linked with notable elevations in ALT levels. The highest and third quartiles exhibited significant increases compared to the lowest quartile. Due to their metabolic processing mainly by liver enzymes, monoterpenes may produce reactive toxic metabolites that can interact with hepatic macromolecules. Furthermore, the metabolism of monoterpenes can accelerate the production of reactive oxygen intermediates, which may overwhelm the capacity of protective antioxidant systems 11. It is possible that higher levels of monoterpenes in the serum reflect greater metabolism in the liver, which in turn leads to higher ALT levels.
We observed a significant increasing trend in ALT levels when each of the monoterpene levels was above the 50th percentile when using the reference group of individuals with all three monoterpenes ≤ 50th percentile. Individuals with all three monoterpenes > 50th percentile had a substantially higher mean ALT value compared to those with all three monoterpenes ≤ 50th percentile. These findings suggest a possible synergistic effect between these three monoterpenes with regards to ALT. A study conducted in vitro has demonstrated that the combined use of α-pinene and β-pinene with paclitaxel had a synergistic antitumor effect against non-small-cell lung carcinoma, inducing morphological changes indicative of apoptosis 39. The synergistic effects observed between α-pinene, β-pinene, and limonene on liver function tests could be explained by their combined biological activities. Each of these monoterpenes has been shown to possess potential hepatotoxicity. By working together, they may have a more significant impact on liver function tests than when acting alone. However, more research is needed to fully understand the mechanisms behind these synergistic effects and their clinical relevance.
It is essential to acknowledge the limitations of this study when interpreting the findings. To begin with, the sample size was confined to serum monoterpenes and liver function tests exam from NHANES 2013–2014, which may have impeded the capacity to conduct a comprehensive analysis. Additionally, the cross-sectional design of this investigation renders it unsuitable for making any causal inferences. Thirdly, monoterpenes are often found in cleaning products, personal care products, and fragrances. Exposure to these products has been associated with liver toxicity. It is possible that individuals with higher serum levels of monoterpenes are also more likely to be exposed to other hepatotoxicants, which may explain the higher ALT levels 40. Fourthly, it is important to note that monoterpenes are rapidly metabolized into oxidized species, which are then excreted through urine. Therefore, serum levels of monoterpenes may not accurately reflect the extent of exposure in the body. However, it is worth mentioning that the NHANES survey does not include measurements of these metabolites. Fifthly, it is possible that individuals with pre-existing liver damage have higher ALT levels, and this damage also impairs the metabolism of monoterpenes, leading to their accumulation in the serum. Lastly, the study's participants were exclusively adult individuals from the United States, which restricts the generalizability of the conclusions to other age groups and geographical regions.