Intestinal absorption and metabolism of hydrocarbons

In recent years there have been significant advancements in the understanding of mineral oil hydrocarbons (MOH) in foods and their potential risk to health. However, important gaps in knowledge remain, such as the lack of validated and standardized analytical methods for relevant food matrices and gaps in assessing the risk for consumers’ health.

A workshop was organized by the European Branch of the International Life Science Institute to identify knowledge gaps in analytical methods, assessment of exposure, hazard characterisation, and risk assessment of MOH. This work captures the outcome of the workshop and builds upon it by combining the perspectives of the participants with an updated review of the literature to provide a roadmap for future management of the topic.

Most participants to the workshop agreed that the key issue underlying many of the knowledge gaps in the field of MOH risk analysis and management is the lack of standardized, validated analytical methods able to assure good inter-laboratory reproducibility and to enable understanding of MOH occurrence in foods. It has been demonstrated that method EN 16995 used for MOH determination in vegetable oils and fats is not reliable below 10 mg/kg of food. There is also a need for confirmatory methods that provide a detailed characterization of the unresolved complex mixture observed from one-dimensional chromatographic methods. This is required to enable adequate substance identification and quantification for input into risk assessment. A major gap in the exposure estimation is the limited number of surveys covering a wide range of foods and enough samples to detect major sources of contamination other than packaging in paperboard. Data on concentration of MOH fractions in human body needed to determine internal exposure estimates is scarce. Data relating concentration in tissues with personal data, lifestyle, food intake and the use of cosmetics are needed to clarify the complex system of distribution of MOSH in the body and to possibly establish relationship between external and internal exposure. Additional toxicological studies to better characterize the hazards of relevant MOH are required for a better human health risk assessment.

Previous genome-wide association analyses for obesity related genes demonstrated the association of BDNF gene variant rs6265 and MC4R gene variant rs17782313 with body mass index (BMI). However, the associated metabolite pathways are still behind the curtain. The aim of the current study is to investigate the associations of metabolic changes in obesity with MC4R gene variant rs17782313 and BDNF variant rs6265. Gas chromatography-mass spectrometry based untargeted metabolomics approach was used and 42 identified serum metabolites were selected for statistical analyses. Significant association of seven metabolites with MC4R gene variant rs17782313 based on obesity and thirty metabolites with obesity dependent BDNF variant rs6265 using additive model (adjusted p < 0.05) was observed. This study highlights the importance of alteration of fatty acid biosynthesis, probably due to high consumption of fats may cause to develop obesity. But obesity is a complex disorder and the full clarification of this complex machinery is still distant. To understand the obesity in a better way, more studies are required to identify remaining metabolites and also mechanism of these metabolic entities.

Key issues around the evaluation of risks to humans from mineral oils in food and feedstuffs are discussed. MOHs (MOAH and MOSH) occur in food due to intentional use, contamination from environmental sources and during transport/processing, or through migration from food contact materials. Problems in setting and enforcing human health guidelines for MOH include uncertainty around MOH toxicity and the specialist expertise needed for analysis of complex food matrices.

Currently, the method of choice for measuring mineral oils is LC-GC-FID, however some complex food matrices also require additional analytical techniques to differentiate between some naturally occurring hydrocarbons and those from other sources, including of petrogenic origin. This requires the skills of an experienced analyst.

Significant toxicological gaps for MOHs prevent robust human health risk assessment and the derivation of guidance values. As food-grade mineral oils are virtually MOAH-free, the key issue explored here is the relevance to humans of liver (micro)granulomas observed in F344 rats following oral intake.

Available data suggest that despite the ubiquitous nature of MOH in the human diet, the prevalence of liver lipogranulomas in the population is low. These are not associated with inflammation and based on current evidence are not considered of human health significance.

Prebiotics reveal positive effects on the growth performance of pigs and poultry, and might influence intestinal microflora. This, in consequence, could alter recovery rates of digestibility markers. In the current study, we evaluated the suitability of chromium (III) oxide (Cr₂O₃) and the synthetic alkanes n-dotriacontane (C32) and n-hexatriacontane (C36) as external markers for digestibility estimation compared with the standard total collection method in calves supplemented with galacto-oligosaccharides. Eight male German Holstein calves (average age ± SD = 57 ± 8 days) were divided into 2 milk replacer feeding groups (group receiving galacto-oligosaccharides [A] and control group [B]). Each of 2 groups of 4 individually fed calves received a distinct milk replacer with added markers for 14 days. They were fed twice daily restrictively with milk replacer, concentrate and hay. After an adaptation period of 10 days, total faeces were collected. Faecal marker recoveries (FMR, means ± SD) for C32 were (72 ± 14)% for A and (80 ± 12)% for B. Faecal marker recoveries for C36 was (82 ± 15)% and (88 ± 13)% for groups A and B, respectively. The FMR for Cr₂O₃ was (102 ± 11)% and (100 ± 1)% for groups A and B, respectively. There were no significant differences between total collection organic matter digestibility and marker based organic matter digestibility when using Cr₂O₃ and C36. But, when utilizing C32 to calculate nutrient digestibilities, results differed from the total collection method for organic matter, crude protein and ether extract. The results indicate that Cr₂O₃ and C36 can be applied in digestibility studies with calves and give accurate estimates for OM and nutrient digestibilities without correction for FMR.

Female Fischer 344 rats were exposed to three MOSH mixtures: oils largely below and above C₂₅ (S-C25 and L-C25) and a 1:1 mixture of L-C25 with a wax; doses of 400, 1000 and 4000 mg/kg feed were administered during 120 days. MOSH were determined by on-line HPLC-GC-FID in liver, spleen, adipose tissue and the carcass. The composition of the hydrocarbons accumulated in the tissues was further analyzed by comprehensive two-dimensional GC (GC × GC). MOSH in the mass range of C_26–30 were more strongly accumulated than those between C_20–25, which does not support the present classification of MOSH differentiating at n-C₂₅ for risk assessment. Compared to the total of the MOSH, n-alkanes and n-alkyl monocyclic naphthenes were generally enriched in adipose tissue. In liver and spleen, n-alkanes up to C₂₅ were eliminated, but strongly accumulated at around C₃₀. Based on this profile, poor solubility and the melting points, it is hypothesized that crystallization protects these wax components against metabolism and elimination. In the liver, relative retention of n-alkanes decreased again beyond C₃₀, accentuated at high exposure, suggesting reduced absorption. Compared to the animal data, accumulation of n-alkanes from food sources, such as apples, into human tissues seems low, perhaps because of low absorption due to their presence in crystalline form. A series of dominant isoalkanes, accumulated in all tissues analyzed, was characterized, though without proposing a structure. Implications on present regulation of white mineral oil products are discussed.

Previously, we have shown that green tea extract (GTE) lowers the intestinal absorption of lipids and lipophilic compounds in rats. This study was conducted to investigate whether GTE inhibits the intestinal absorption and biliary secretion of benzo[a]pyrene (BaP), an extremely lipophilic potent carcinogen, present in foods as a contaminant. Male rats with lymph or bile duct cannula were infused at 3.0 ml/h for 8 h via a duodenal catheter with lipid emulsion containing ¹⁴C-BaP with or without GTE in PBS buffer. Lymph and bile were collected hourly for 8 h. The ¹⁴C-radioactivities in lymph, bile and intestine were determined and expressed as % dose infused. Results showed that GTE drastically lowered the lymphatic absorption of ¹⁴C-BaP (7.6±3.2% in GTE-infused vs. 14.4±2.7% dose/8 h in control rats), with a significantly higher amount of ¹⁴C-radioactivity present in the small intestinal lumen and cecum in rats infused with GTE. GTE also markedly increased the hourly rate (3.9±0.1% dose/h in GTE-infused vs. 3.0±0.1% dose/h in control rats) and the total biliary secretion of ¹⁴C-BaP (31.5±0.8% dose/8 h in GTE-infused vs. 24.3±0.4% dose/8 h in control rats). The findings provide first direct evidence that GTE has a profound inhibitory effect on the intestinal absorption of BaP and promotes the excretion of absorbed BaP via the biliary route. Further studies are warranted to investigate whether green tea could be recommended as a dietary means of ameliorating the toxicity and carcinogenic effect of BaP.