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Dietary reference values for thiamin

2016; Wiley; Volume: 14; Issue: 12 Linguagem: Inglês

10.2903/j.efsa.2016.4653

1831-4732

Dominique Turck, Jean‐Louis Bresson, Barbara Burlingame, Tara Dean, Susan J. Fairweather‐Tait, Marina Heinonen, Karen Ildico Hirsch‐Ernst, Inge Mangelsdorf, Harry J McArdle, Androniki Naska, Grażyna Nowicka, Kristina Pentieva, Yolanda Sanz, Alfonso Siani, Anders Sjödin, Martin Stern, Daniel Tomé, Henk Van Loveren, Marco Vinceti, Peter Willatts, Christel Lamberg‐Allardt, Hildegard Przyrembel, Inge Tetens, Jelena Gudelj Rakić, Sofia Ioannidou, Agnès de Sesmaisons‐Lecarré, Annette Cecilia Forss, Monika Neuhäuser‐Berthold,

Biochemical Acid Research Studies

EFSA JournalVolume 14, Issue 12 e04653 Scientific OpinionOpen Access Dietary reference values for thiamin EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA), EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA)Search for more papers by this authorDominique Turck, Dominique TurckSearch for more papers by this authorJean-Louis Bresson, Jean-Louis BressonSearch for more papers by this authorBarbara Burlingame, Barbara BurlingameSearch for more papers by this authorTara Dean, Tara DeanSearch for more papers by this authorSusan Fairweather-Tait, Susan Fairweather-TaitSearch for more papers by this authorMarina Heinonen, Marina HeinonenSearch for more papers by this authorKaren Ildico Hirsch-Ernst, Karen Ildico Hirsch-ErnstSearch for more papers by this authorInge Mangelsdorf, Inge MangelsdorfSearch for more papers by this authorHarry J McArdle, Harry J McArdleSearch for more papers by this authorAndroniki Naska, Androniki NaskaSearch for more papers by this authorGrażyna Nowicka, Grażyna NowickaSearch for more papers by this authorKristina Pentieva, Kristina PentievaSearch for more papers by this authorYolanda Sanz, Yolanda SanzSearch for more papers by this authorAlfonso Siani, Alfonso SianiSearch for more papers by this authorAnders Sjödin, Anders SjödinSearch for more papers by this authorMartin Stern, Martin SternSearch for more papers by this authorDaniel Tomé, Daniel ToméSearch for more papers by this authorHenk Van Loveren, Henk Van LoverenSearch for more papers by this authorMarco Vinceti, Marco VincetiSearch for more papers by this authorPeter Willatts, Peter WillattsSearch for more papers by this authorChristel Lamberg-Allardt, Christel Lamberg-AllardtSearch for more papers by this authorHildegard Przyrembel, Hildegard PrzyrembelSearch for more papers by this authorInge Tetens, Inge TetensSearch for more papers by this authorJelena Gudelj Rakic, Jelena Gudelj RakicSearch for more papers by this authorSofia Ioannidou, Sofia IoannidouSearch for more papers by this authorAgnès de Sesmaisons-Lecarré, Agnès de Sesmaisons-LecarréSearch for more papers by this authorAnnette Cecilia Forss, Annette Cecilia ForssSearch for more papers by this authorMonika Neuhäuser-Berthold, Monika Neuhäuser-BertholdSearch for more papers by this author EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA), EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA)Search for more papers by this authorDominique Turck, Dominique TurckSearch for more papers by this authorJean-Louis Bresson, Jean-Louis BressonSearch for more papers by this authorBarbara Burlingame, Barbara BurlingameSearch for more papers by this authorTara Dean, Tara DeanSearch for more papers by this authorSusan Fairweather-Tait, Susan Fairweather-TaitSearch for more papers by this authorMarina Heinonen, Marina HeinonenSearch for more papers by this authorKaren Ildico Hirsch-Ernst, Karen Ildico Hirsch-ErnstSearch for more papers by this authorInge Mangelsdorf, Inge MangelsdorfSearch for more papers by this authorHarry J McArdle, Harry J McArdleSearch for more papers by this authorAndroniki Naska, Androniki NaskaSearch for more papers by this authorGrażyna Nowicka, Grażyna NowickaSearch for more papers by this authorKristina Pentieva, Kristina PentievaSearch for more papers by this authorYolanda Sanz, Yolanda SanzSearch for more papers by this authorAlfonso Siani, Alfonso SianiSearch for more papers by this authorAnders Sjödin, Anders SjödinSearch for more papers by this authorMartin Stern, Martin SternSearch for more papers by this authorDaniel Tomé, Daniel ToméSearch for more papers by this authorHenk Van Loveren, Henk Van LoverenSearch for more papers by this authorMarco Vinceti, Marco VincetiSearch for more papers by this authorPeter Willatts, Peter WillattsSearch for more papers by this authorChristel Lamberg-Allardt, Christel Lamberg-AllardtSearch for more papers by this authorHildegard Przyrembel, Hildegard PrzyrembelSearch for more papers by this authorInge Tetens, Inge TetensSearch for more papers by this authorJelena Gudelj Rakic, Jelena Gudelj RakicSearch for more papers by this authorSofia Ioannidou, Sofia IoannidouSearch for more papers by this authorAgnès de Sesmaisons-Lecarré, Agnès de Sesmaisons-LecarréSearch for more papers by this authorAnnette Cecilia Forss, Annette Cecilia ForssSearch for more papers by this authorMonika Neuhäuser-Berthold, Monika Neuhäuser-BertholdSearch for more papers by this author First published: 19 December 2016 https://doi.org/10.2903/j.efsa.2016.4653Citations: 8 Correspondence: nda@efsa.europa.eu Requestor: European Commission Question number: EFSA-Q-2011-01225 Panel members: Jean-Louis Bresson, Barbara Burlingame, Tara Dean, Susan Fairweather-Tait, Marina Heinonen, Karen-Ildico Hirsch-Ernst, Inge Mangelsdorf, Harry J McArdle, Androniki Naska, Monika Neuhäuser-Berthold, Grażyna Nowicka, Kristina Pentieva, Yolanda Sanz, Alfonso Siani, Anders Sjödin, Martin Stern, Daniel Tomé, Dominique Turck, Henk Van Loveren, Marco Vinceti and Peter Willatts. Acknowledgements: The Panel wishes to thank EFSA member of staff Fanny Héraud for the support provided to this scientific opinion. Adopted: 22 November 2016 This publication is linked to the following EFSA Supporting Publications article: http://onlinelibrary.wiley.com/doi/10.2903/sp.efsa.2016.EN-1138/full AboutSectionsPDF ToolsExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract Following a request from the European Commission, the EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA) derived dietary reference values (DRVs) for thiamin (vitamin B1). The Panel considers that data from depletion–repletion studies in adults on the amount of dietary thiamin intake associated with the erythrocyte transketolase activity coefficient (αETK) < 1.15, generally considered to reflect an adequate thiamin status, or with the restoration of normal (baseline) erythrocyte transketolase activity, without a sharp increase in urinary thiamin excretion, can be used to estimate thiamin requirement. In the absence of new scientific evidence, the Panel endorses the average requirement (AR) of 0.072 mg/MJ (0.3 mg/1,000 kcal) for all adults proposed by the Scientific Committee for Food (SCF) in 1993 on the basis of one depletion–repletion study, in which both αETK and urinary thiamin excretion were measured. Results from other depletion–repletion studies are in agreement with this value. The Panel agrees on the coefficient of variation of 20% used by the SCF to cover uncertainties related to distribution of thiamin requirements in the general population, and endorses the population reference intake (PRI) of 0.1 mg/MJ (0.4 mg/1,000 kcal) set by the SCF for all adults. The same AR and PRI as for adults, expressed in mg/MJ, are proposed for infants aged 7–11 months, children aged 1 to < 18 years, and during pregnancy and lactation, under the assumption that the relationship between thiamin requirement and energy requirement is the same in all population groups. Summary Following a request from the European Commission, the EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA) was asked to deliver a Scientific Opinion on Dietary Reference Values for the European population, including thiamin (vitamin B1). Thiamin is a water-soluble vitamin composed of a thiazole and a pyrimidine ring linked by a methylene group. In human tissues, thiamin occurs mostly in phosphorylated forms as thiamin monophosphate (TMP), thiamin diphosphate (TDP, called also thiamin pyrophosphate), thiamin triphosphate (TTP), as well as its non-phosphorylated form ('free thiamin'). Free thiamin functions as the precursor for TDP, which acts as a coenzyme for enzymes involved in carbohydrate and branched-chain amino acid metabolism, and in energy-yielding reactions. Thiamin deficiency leads to disorders that include several forms of beriberi, with mostly neurological and cardiovascular manifestations. Thiamin in food exists mainly in phosphorylated forms in animal products, and in free form in foods of plant origin. Upon ingestion, thiamin phosphate esters are hydrolysed in the intestinal lumen by phosphatases. Free thiamin is taken up through the mucosal membrane by a specific saturable transport system. In healthy subjects, thiamin absorption is above 95% at usual intakes. Alcohol and anti-thiamin factors (such as some phenolic compounds, sulfites and thiaminases) can reduce thiamin bioavailability. Thiamin in blood is mainly found in erythrocytes (> 80% of total thiamin in the blood) in the form of TDP and TTP, while low amounts of the vitamin are present in plasma, as free thiamin, TMP and protein-bound TDP. Thiamin in the body is mostly located in the skeletal muscles, heart, brain, liver and kidneys. Urine is the main route of thiamin excretion, mainly in the form of free thiamin and thiamin metabolites. The Panel notes that 24-h urinary thiamin excretion is related to thiamin intake, particularly to short-term intakes, in thiamin-replete individuals. However, the thiamin intake cannot reliably be estimated from the urinary excretion of the vitamin. The determination of 24-h urinary thiamin excretion is not a reliable marker of thiamin body stores and cannot, on its own, be used as a biomarker of the thiamin status of individuals. Still, in experimental studies where 24-h urinary thiamin excretion is assessed in response to various intakes of the vitamin, a sharp increase in thiamin excretion is considered to be indicative of the saturation of the thiamin body stores. Measurement of the erythrocyte transketolase activity (ETKA), a TDP-requiring enzyme, is a functional test of thiamin status. The erythrocyte transketolase activity coefficient (αETK, also called 'TDP effect') represents the degree to which ETKA rises in response to addition of TDP. This test can discriminate low ETKA due to thiamin deficiency from a lack of the apoenzyme. A value of αETK < 1.15 is generally considered to reflect an adequate thiamin status. The concentrations of total thiamin (free thiamin and its phosphate esters) in whole blood, serum and erythrocytes have also been investigated as biomarkers of thiamin status. Erythrocyte TDP concentration was found to have similar performance as the erythrocyte transketolase activation assay for assessment of thiamin status. The Panel notes, however, the lack of established cut-offs for these biomarkers. The Panel considers that data from depletion–repletion studies in adults on the amount of dietary thiamin intake associated with αETK < 1.15 or with the restoration of normal (baseline) ETKA, without a sharp increase in urinary thiamin excretion, can be used to estimate thiamin requirement. In the absence of new scientific evidence, the Panel endorses the average requirement (AR) of 0.072 mg/MJ (0.3 mg/1,000 kcal) for all adults set by the Scientific Committee for Food (SCF) in 1993 on the basis of one depletion–repletion study in seven healthy males, in which both αETK and urinary excretion of thiamin were measured. Results from other depletion–repletion studies are in agreement with this value. The Panel notes that the AR was based on data on a small number of men, and agrees on the coefficient of variation of 20% used by the SCF to cover uncertainties related to distribution of thiamin requirements in the general population. The Panel endorses the population reference intake (PRI) of 0.1 mg/MJ (0.4 mg/1,000 kcal) set by the SCF for all adults. No new evidence has become available that the relationship between thiamin requirement and energy requirement differs between men and women, or between younger and older adults. The Panel proposes the same AR and PRI as for adults, expressed in mg/MJ, for infants aged 7–11 months, children aged 1 to < 18 years old, and during pregnancy and lactation, under the assumption that the relationship between thiamin requirement and energy requirement is the same in all population groups. Based on data from 13 dietary surveys in nine countries of the European Union, average thiamin intakes across countries ranged between 0.31 and 0.65 mg/day (0.11–0.21 mg/MJ) among infants (< 1 year old), between 0.58 and 0.98 mg/day (0.12–0.21 mg/MJ) among children aged 1 to < 3 years old, between 0.68 and 1.29 mg/day (0.10–0.21 mg/MJ) among children aged 3 to < 10 years old, between 0.93 and 1.92 mg/day (0.11–0.20 mg/MJ) among children aged 10 to < 18 years old and between 0.88 and 1.99 mg/day (0.11–0.24 mg/MJ) among adults (≥ 18 years old). Background as provided by the European Commission The scientific advice on nutrient intakes is important as the basis of Community action in the field of nutrition, for example, such advice has in the past been used as the basis of nutrition labelling. The Scientific Committee for Food (SCF) report on nutrient and energy intakes for the European Community dates from 1993. There is a need to review and if necessary to update these earlier recommendations to ensure that the Community action in the area of nutrition is underpinned by the latest scientific advice. In 1993, the SCF adopted an opinion on the nutrient and energy intakes for the European Community.1 The report provided reference intakes for energy, certain macronutrients and micronutrients, but it did not include certain substances of physiological importance, for example, dietary fibre. Since then new scientific data have become available for some of the nutrients, and scientific advisory bodies in many European Union (EU) Member States and in the United States have reported on recommended dietary intakes. For a number of nutrients, these newly established (national) recommendations differ from the reference intakes in the SCF (1993) report. Although there is considerable consensus between these newly derived (national) recommendations, differing opinions remain on some of the recommendations. Therefore, there is a need to review the existing EU Reference Intakes in the light of new scientific evidence, and taking into account the more recently reported national recommendations. There is also a need to include dietary components that were not covered in the SCF opinion of 1993, such as dietary fibre, and to consider whether it might be appropriate to establish reference intakes for other (essential) substances with a physiological effect. In this context, the European Food Safety Authority (EFSA) is requested to consider the existing population reference intakes (PRIs) for energy, micro- and macronutrients and certain other dietary components, to review and complete the SCF recommendations, in the light of new evidence, and in addition advise on a PRI for dietary fibre. For communication of nutrition and healthy eating messages to the public, it is generally more appropriate to express recommendations for the intake of individual nutrients or substances in food-based terms. In this context, EFSA is asked to provide assistance on the translation of nutrient-based recommendations for a healthy diet into food-based recommendations intended for the population as a whole. Terms of reference as provided by the European Commission In accordance with Article 29(1)(a) and Article 31 of Regulation (EC) No 178/20022, the Commission requests EFSA to review the existing advice of the SCF on PRIs for energy, nutrients and other substances with a nutritional or physiological effect in the context of a balanced diet which, when part of an overall healthy lifestyle, contribute to good health through optimal nutrition. In the first instance, the EFSA is asked to provide advice on energy, macronutrients and dietary fibre. Specifically, advice is requested on the following dietary components: carbohydrates, including sugars; fats, including saturated fatty acids, polyunsaturated fatty acids and monounsaturated fatty acids, trans fatty acids; protein; dietary fibre. Following on from the first part of the task, the EFSA is asked to advise on PRIs of micronutrients in the diet and, if considered appropriate, other essential substances with a nutritional or physiological effect in the context of a balanced diet which, when part of an overall healthy lifestyle, contribute to good health through optimal nutrition. Finally, the EFSA is asked to provide guidance on the translation of nutrient-based dietary advice into guidance, intended for the European population as a whole, on the contribution of different foods or categories of foods to an overall diet that would help to maintain good health through optimal nutrition (food-based dietary guidelines). Assessment 1 Introduction In 1993, the SCF adopted an opinion on the nutrient and energy intakes for the European Community (SCF, 1993). The SCF set an average requirement (AR) and a PRI for thiamin, expressed in μg/MJ, which applied to all age and sex groups. PRIs expressed in mg/day were also derived, considering the average energy requirements of infants, children, adults, and pregnant and lactating women. A lower threshold intake (LTI) expressed in μg/MJ was set for all age and sex groups, and converted to mg/day for adults, again using the values for average daily energy requirements for men and women. 2 Definition/category 2.1 Chemistry Thiamin, also called vitamin B1 or aneurine, is a water-soluble vitamin. Thiamin is chemically defined as 3-[(4-amino-2-methyl-5-pyrimidinyl) methyl]-5-(2-hydroxyethyl)-4-methyl-1,3-thiazol-3-ium, with molecular formula C12H17N4OS and a molecular mass of 265.35 Da. Thiamin is composed of a thiazole and a pyrimidine ring linked by a methylene group. In human tissues, thiamin occurs mostly in phosphorylated forms as thiamin monophosphate (TMP), thiamin diphosphate (TDP, called also thiamin pyrophosphate) (Figure 1), thiamin triphosphate (TTP), as well as its non-phosphorylated form ('free thiamin'). Adenosine thiamin triphosphate (ATTP) is also found in some tissues (Gangolf et al., 2010) (Section 2.3.3). Figure 1Open in figure viewerPowerPoint The thiamin and thiamin diphosphate molecules There are different methods of measurement of thiamin content in foods and biological samples (urine, blood and other tissues), such as high-performance liquid chromatography (HPLC) followed by fluorescence or ultraviolet detection, fluorimetry and microbiological assay (Icke and Nicol, 1994; Lynch and Young, 2000; Talwar et al., 2000; Mickelsen and Yamamoto, 2006). Techniques based on fluorimetric detection involve the oxidation of thiamin into thiochrome (Fayol, 1997). These methods showed comparable performance in foods (Hollman et al., 1993). The analytical procedure may comprise a step of enzymatic hydrolysis of phosphorylated thiamin, allowing the quantification of total thiamin content. The amounts of the respective forms of thiamin (i.e. free thiamin and its phosphate esters) can be determined after separation by HPLC (Gangolf et al., 2010). 2.2 Function of the nutrient 2.2.1 Biochemical functions Free thiamin functions as the precursor for TDP, which acts as a coenzyme for enzymes involved in carbohydrate and branched-chain amino acid metabolism, and in energy-yielding reactions. TDP is needed for the activity of pyruvate dehydrogenase responsible for the conversion of pyruvate to acetyl-coenzyme A, α-ketoglutarate dehydrogenase converting α-ketoglutarate to succinyl-coenzyme A within the Krebs cycle, and branched-chain α-keto acid dehydrogenase involved in the oxidation of the α-keto acids from branched-chain amino acids. These enzyme complexes play a key role in processes related to mitochondrial energy metabolism. TDP is also the coenzyme for transketolase in the pentose phosphate pathway, which is essential for the generation of pentoses and nicotinamide adenine dinucleotide phosphate (NADPH) (Singleton and Martin, 2001; Combs, 2008; Lonsdale, 2012; Manzetti et al., 2014). TDP is required for the function of the brain and nervous system as acetyl-coenzyme A and α-ketoglutarate are involved in the production of the neurotransmitters acetylcholine and gamma-aminobutyric acid. TDP may be further phosphorylated to TTP. TTP is able to phosphorylate proteins and to activate large conductance anion channels as, e.g. a chloride channel in nerves (Bettendorff et al., 1993; Nghiem et al., 2000; Bettendorff and Wins, 2009). The precise physiological role of TTP has not yet been elucidated (Bettendorff et al., 2014). 2.2.2 Health consequences of deficiency and excess 2.2.2.1 Deficiency Thiamin deficiency usually presents with symptoms of peripheral neuritis, cardiac insufficiency and a tendency for oedemas and may be accompanied by extreme fatigue, irritability, forgetfulness, poor coordination, gastrointestinal disturbances, constipation, laboured breathing, loss of appetite and weight loss (WHO, 1999). Thiamin deficiency leads to disorders that include several forms of beriberi, with mostly neurological and cardiovascular manifestations. Dry beriberi is predominately a neurological disorder with a sensory and motor peripheral neuropathy. Wet beriberi is the term used for thiamin deficiency that, in addition to the presence of peripheral neuropathy, involves cardiovascular manifestations that include congestive heart failure, cardiomegaly and tachycardia. A rapidly developing form of wet beriberi refers to the acute fulminant cardiovascular beriberi (Shoshin beriberi), or acute pernicious beriberi. Infantile beriberi can occur in breastfed infants of thiamin-deficient mothers at the age of 2–6 months and may be characterised by both neurologic and cardiac signs with lethal outcome due to heart failure (Roman-Campos and Cruz, 2014; Abdou and Hazell, 2015). Infantile thiamin deficiency was described in infants fed a soy-based thiamin-deficient infant formula (Fattal-Valevski et al., 2005). Lack of thiamin impairs metabolic functions of the brain and can lead to Wernicke's encephalopathy, which is clinically characterised by ocular abnormalities, ataxia, and disturbances of consciousness, and to Korsakoff's syndrome (psychosis) resulting in amnesia, disorientation and often confabulation (Harper et al., 1986; Gui et al., 2006; Sechi and Serra, 2007; Kopelman et al., 2009). Thiamin deficiency occurs predominantly in populations whose diet consists mainly of poor sources of thiamin (as milled white cereals, including polished rice and white wheat flour). It can also be related to diets that are rich in thiaminase, a natural thiamin-degrading enzyme, which is abundantly present in some raw or fermented fish, ferns and insects consumed primarily in Africa and Asia (WHO, 1999). In Western countries, thiamin deficiency is associated with alcoholism and drug abuse, and can occur in other risk groups including subjects after bariatric surgery, gastrectomy, or with chronic gastrointestinal and liver disorders (Lonsdale, 2012; Crook and Sriram, 2014). 2.2.2.2 Excess Reviewing the evidence to set a tolerable upper intake level (UL) for thiamin, the SCF noted that data on adverse effects of oral intake of thiamin in humans were limited and that dose–response studies were lacking (SCF, 2001). The SCF also noted that thiamin absorption declines for an intake higher than 5 mg/day and absorbed thiamin is actively excreted in the urine. No lowest-observed-adverse-effect level (LOAEL) or no-observed-adverse-effect level (NOAEL), and therefore, no UL, could be set for thiamin. 2.3 Physiology and metabolism 2.3.1 Intestinal absorption and bioavailability Thiamin in food exists mainly in phosphorylated forms in animal products, and in free form in foods of plant origin. Thiamin phosphate esters are hydrolysed in the intestinal lumen by phosphatases, mainly the alkaline phosphatase associated with brush-border membranes. Free thiamin is taken up through the mucosal membrane by a specific saturable transport system (Laforenza et al., 1997; Reidling et al., 2002). Two transporters, ThTR-1 and ThTR-2, encoded by SLC19A2 and SLC19A3 genes, are involved in intestinal thiamin uptake (Said et al., 2004). In case of low dietary thiamin intake, an enhanced expression of ThTR-2 is induced, but not of ThTR-1 (Laforenza et al., 1997; Reidling et al., 2002; Said et al., 2004). When two healthy young men received an oral dose of 0.67 mg 2-14C-thiazole-labelled thiamin (50 μCi) and a controlled diet providing a constant thiamin intake (range 1.35–2.10 mg/day, mean 1.75 mg/day), less than 1% of the radioactivity dose was found in the first and second day faecal samples (Ariaey-Nejad et al., 1970). Overall, less than 5% of the labelled dose was found in the 5-day faecal collection. In another study which involved 10 healthy individuals who received a dose of 1 mg of 2-14C-thiazole-labelled thiamin (10 μCi), mean faecal excretion was 4 ± 6.1% during the first 24 h after administration (Tomasulo et al., 1968). The efficiency of thiamin absorption is reduced upon consumption of thiamin above 5 mg/day, (Friedemann et al., 1948; Davis et al., 1984). When thiamin was infused directly into the lumen of the small intestine of humans and animals, it was absorbed by an active process at low concentrations (0.2–2.0 μM (0.05–0.50 mg/L)) and by a passive process at higher concentrations (5.0–50.0 μM (1.3–13 mg/L)) (Hoympa et al., 1975; Hoympa et al., 1982; Hoympa, 1982). Chronic alcohol consumption impairs the intestinal absorption of thiamin, possibly through the inhibition of thiamin transporters (Subramanya et al., 2010). In the above-mentioned study from Tomasulo et al. (1968), significantly lower absorption of thiamin was found in 20 chronic alcoholic individuals (mean faecal excretion of labelled thiamin 21 ± 13.9%), compared to the 10 healthy controls (4 ± 6.1%). Bioavailability of dietary thiamin can also be impaired by different types of anti-thiamin factors present in some foods. These factors degrade or modify thiamin so that it cannot be absorbed or loses its function. Sulfites, which are added to foods as a preservative, destroy thiamin at the methylene bridge. Thiamin can also be degraded by thermolabile thiaminases present in some raw or fermented fish, ferns and insects (Combs, 1992; WHO, 1999). Plants may contain heat-stable thiamin antagonists, such as some aromatic acids (e.g. caffeic acid, chlorogenic acid, and tannic acid), which can oxidise the thiazole ring, making thiamin absorption impossible. Flavonoids, quercetin and rutin, have also been implicated as thiamin antagonists (Kositawattanakul et al., 1977; Hilker and Somogyi, 1982; Vimokesant et al., 1982). The bioavailability of thiamin was found to be reduced in controlled studies comparing diet with and without tea (Wang and Kies, 1991; Saeed and Zaheer-ud-Din, 1996). Microbiota of the large intestine can synthesise thiamin in the form of TDP. In vivo experiments suggested that thiamin derived from bacterial synthesis is not used as a source of the vitamin (Alexander and Landwehr, 1946; Denko et al., 1946). More recently, free thiamin was found to be taken up by isolated human colonic epithelial cells via a process similar to the one occurring in the small intestine. A specific regulated high-affinity carrier-mediated uptake system (encoded by SLC44A4 gene) for TDP was also identified (Nabokina et al., 2015). Further studies are needed to determine whether TDP synthesised by microbiota may be used by colonocytes. The Panel notes that data on the efficiency of thiamin absorption are limited. In healthy subjects, thiamin absorption was found to be above 95% of daily thiamin intake lower than 2 mg, as determined by the absorption of 14C-labelled thiamin. The Panel notes that alcohol and anti-thiamin factors (such as some phenolic compounds, sulfites and thiaminases) can reduce thiamin bioavailability. 2.3.2 Transport in blood Thiamin is transported by a high-affinity transporter into erythrocytes, where it is phosphorylated to TDP, a fraction of which is further converted to TTP (Gangolf et al., 2010). As a result, thiamin in blood is mainly found in erythrocytes (> 80% of total thiamin in the blood) in the form of TDP and TTP, while low amounts of the vitamin are present in plasma, as free thiamin, TMP and protein-bound TDP (Gangolf et al., 2010). 2.3.3 Distribution to and content in tissues Thiamin is taken up by cells of the blood, liver, heart and other tissues, including the placenta and brain, by active transport, mostly through thiamin transporters ThTR-1 and ThTR-2. In addition, the reduced folate carrier-1 (encoded by SLC19A1 gene) provides a minor access route for TMP, TDP and TTP but not free thiamin. Members of the human extraneuronal monoamine transporter proteins, including the organic cation transporter proteins, are active in the transport of amine forms of nutrients and neurotransmitters, including thiamin, to the neurons (Zhao and Goldman, 2013; Manzetti et al., 2014). The total thiamin content of the adult body has been estimated to be about 25–30 mg, located mostly in the skeletal muscles, heart, brain, liver and kidneys (Ariaey-Nejad et al., 1970; Manzetti et al., 2014). Analysis of biopsies from various human tissues shows that TDP is the most abundant thiamin compound, with the highest content in the heart and skin, followed by the kidney, lung, colon, adipose tissue, skeletal muscle and vascular samples (content from 9 ± 6 to 66 ± 44 pmol/mg protein). The content of other forms is low: TMP content range