2-ethylhexanoic acid, iron salt

Endpoint summary

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Additional information

In the absence of measured data on dermal absorption of iron, current guidance suggests the assignment of either 10 % or 100 % default dermal absorption rates. In contrast, the currently available scientific evidence on dermal absorption of metals (predominantly based on the experience from previous EU risk assessments) yields substantially lower figures, which can be summarised briefly as follows:

Measured dermal absorption values for metals or metal compounds in studies corresponding to the most recent OECD test guidelines are typically 1 % or even less. Therefore, the use of a 10 % default absorption factor is scientifically not supported for metal salts. This is corroborated by conclusions from previous EU risk assessments (Ni, Cd, Zn), which have derived dermal absorption rates of 2 % or far less from liquid media (but with considerable methodical deviations from existing OECD methods).

However, considering that under industrial circumstances many applications involve handling of dry powders, substances and materials, and since dissolution is a key prerequisite for any percutaneous absorption, a factor 10 lower default absorption factor may be assigned to such “dry” scenarios where handling of the product does not entail use of aqueous or other liquid media. This approach was adopted in the EU RA on zinc. A reasoning for this is described in detail elsewhere (Cherrie and Robertson, 1995), based on the argument that dermal uptake is dependent on the concentration of the material on the skin surface rather than its mass.

The following default dermal absorption factors for metal cations are therefore proposed (reflective of full-shift exposure, i.e. 8 hours):

From exposure to liquid/wet media: 1.0 %

From dry (dust) exposure: 0.1 %

This approach is consistent with the methodology proposed in the HERAG guidance for metals (HERAG fact sheet - assessment of occupational dermal exposure and dermal absorption for metals and inorganic metal compounds; EBRC Consulting GmbH, Hannover, Germany; August 2007).

This approach shall be applicable to iron contained in inorganic iron salts.

 

Information taken from EFSA (2004): Opinion of the Scientific Panel on Dietetic Products, Nutrition and Allergies on a request from the Commission related to the Tolerable Upper Intake Level of Iron, The EFSA Journal, 125, 1-34.

Iron is an essential trace element that has important metabolic functions, including oxygen transport and storage and many redox reactions. It is present in biological systems in one of two oxidation states, and redox interconversions of the ferrous (Fe2+) and ferric (Fe3+) forms are central to the biological properties of this mineral. Iron is an essential constituent of oxygen carriers, such as haemoglobin and myoglobin, and the iron contained within haem is essential for the redox reactions of numerous cytochromes.

The adult human body contains 2.2-3.8 g iron under iron-adequate conditions (Lynch, 1984). Homeostatic mechanisms have evolved that can alter intestinal iron absorption and supply iron preferentially to functional compartments in response to deficiency or excess.

Absorption and regulation of absorption

Tissue concentrations and body stores of iron are controlled at three different levels:

i. Luminal iron: the extent of uptake of iron by the cells of the gastrointestinal tract affects how much remains unabsorbed and passes to the lower bowel, prior to elimination in faeces;

ii. Mucosal iron: the mucosa is the main site of regulation of iron uptake in relation to liver stores and ferritin levels;

iii. Post-mucosal iron: relates to the impact of iron intake on iron status and body stores.

2.2.1.1 Luminal iron

Non-haem iron is present in foods largely as salts, which are made soluble in the stomach, and absorption from foods depends on its dissolution as ferric salts and subsequent reduction to the ferrous form. Any elemental iron in the diet is probably absorbed as non-haem iron following its dissolution in the acid stomach contents. The absorption of non-haem iron can be increased substantially by the presence of ligands, such as ascorbate, citrate and fumarate, as well as the presence of amino acids (e.g. cysteine) and oligopeptides resulting from meat digestion (Mulvihill et al., 1998). In contrast, very stable complexes, for example with phytates, phosphates and oxalates, impair non-haem iron absorption. Depending on the concentration of supportive or inhibitory ligands in the intestinal lumen the absorption of non-haem iron can vary by a factor of 10 in single-meal studies, but the effects are less pronounced in more long-term studies (Hallberg and Rossander, 1984; Rossander, 1987; Hunt and Roughead 2000).

2.2.1.2 Mucosal iron

In addition, homeostatic regulation will influence the extent of non-haem iron absorption. Body iron content is linked to demand by regulated intestinal non-haem iron absorption which, in turn, is regulated to a major extent by the uptake of iron into the cells of the intestinal mucosa (Schümann et al., 1999a and b). This step is mediated by the divalent metal transporter (DMT-1) (Gunshin et al., 1997)[…].The activity of DMT-1 decreases after a period of high oral iron intake (Oates et al., 2000), and such down-regulation of iron uptake may be regarded as protection against iron overload. Transfer of information on hepatic iron stores to the gut may be mediated via an acute phase hepatic protein, hepcidin (Nicolas et al., 2001), or possibly the pro-hormone form pro-hepcidin (Kulaksiz et al., 2004), which influences the extent of iron absorption (Leong and Lonnerdal, 2004) and can result in a mucosal block. Due to the various exogenous and endogenous factors affecting iron absorption, a clear relationship is generally not found between total iron intake and iron status. Total daily iron absorption is about 0.9 mg in males and 0.5 and 0.6 mg higher in menstruating women and blood donors. The mucosal barrier can be overwhelmed by high iron doses, such as occurs in acute iron intoxication (Ellenham and Barceloux, 1988).

2.2.1.3 Post-mucosal iron

The concentration of free iron in blood is extremely low due to the high affinity binding of iron to transferrin. Transferrin is an 80 kDa plasma protein that binds two Fe3+ ions per molecule with high affinity (binding constant: 10-30). The total iron binding capacity (TIBC) of transferrin in the plasma of healthy adults is approximately 56μmol/L. About 30% and 10% of TIBC is occupied in normal iron status and in iron deficiency respectively. The plasma iron pool is approximately 3 mg, but its daily turnover is more than 30 mg/day (Bothwell et al., 1979). Iron is transferred from the plasma to the tissues via binding of iron-loaded transferrin to transferrin receptors (TfR) at the cell surface, which are subsequently internalised withinendocytotic vesicles. High serum TfR values indicate high activity of cellular iron uptake, which in most cases is due to high erythropoietic iron demand. Transferrin releases its iron under acidic conditions in the endocytotic vesicles, and the transferrin and TfR return subsequently to the cell surface. Within cells, iron is preferentially distributed to iron-dependent enzymes and iron-binding proteins. Excessive intracellular iron is sequestered in the storage protein ferritin (500 kDa) or in its degradation product called haemosiderin. One ferritin molecule can store up to 4500 iron atoms as ferric hydroxyphosphate micelles. Such iron can be remobilised and utilised on demand. The serum ferritin concentration is an indication of body iron stores, except when high levels arise due to inflammation (Ponka et al., 1998). Homeostatic mechanisms involving cytosolic iron-regulatory proteins (IRPs) have developed to maintain low concentrations of free iron, in order to provide iron for essential functions and to protect the cells from oxidative damage. IRP-1 is activated by cellular iron-deficiency, but also by reactive oxygen species (ROS). At normal cellular levels of free iron, IRP-1 contains a 4Fe-4S cluster but in iron-deficient cells the 4Fe-4S cluster is lost and the molecule binds to specific base loops of mRNAs for DMT-1, TfR and ferritin, called “iron-response elements”. In deficiency IRP binding to TfR-mRNA increases TfR expression, while the ferritin mRNA is broken down more rapidly, which restores the intracellular free iron concentration by a combination of increased uptake and decreased sequestration.

Under physiological conditions iron status is almost exclusively regulated by adaptation of intestinal iron absorption according to the demand. In healthy iron-replete humans with iron stores of 800-1200 mg, non-haem iron absorption increases when iron stores are depleted and vice versa (Finch, 1994). This mode of regulation is highly predictable in normal subjects, and iron stores as determined by plasma ferritin values have been used to predict iron absorption (Magnusson et al., 1981). Duodenal non-haem iron absorption is linked to body iron status via the supply of serum iron to intestinal crypt cells. At adequate supply levels, the activity of IRPs in these cells is low, whereas in iron-deficiency IRP activity in the enterocytes is increased and intestinal iron absorption is high. Iron absorption adapts to changes in plasma iron concentration with a lag time of 48h, which corresponds to the time required for young enterocytes to express enzymes and transport proteins, such as DMT-1 and iron regulated gene (IREG), and to migrate up the villi to the site of absorption (Schümann et al., 1999b).

In anaemia, an undefined and unidentified “erythrocyte regulator” can increase iron absorption to 20-40 mg Fe/day from oral iron preparations (Finch, 1994). In addition, iron absorption is increased by an unknown mechanism to up to 66% in pregnant women and returns to normal at 16-24 weeks after delivery (Barrett et al., 1994). Iron absorption is also increased during lactation and growth.

Regulation of non-haem iron absorption protects from iron overload at normal and moderately increased iron-intake levels. A group of 12 Swedish male blood donors and 19 non-donors received standard meals with radioactively labelled non-haem iron (12 mg Fe/day) and haem-iron (2 mg/day). Total iron absorption increased when serum ferritin concentrations were less than 60 μg/L, but decreased when serum ferritin exceeded this level, and the homeostatic equilibrium point of the system was estimated to be 60 μg ferritin/L (Hallberg et al., 1997). This study did not investigate the maximum regulatory capacity by the administration of high levels of additional iron, but such data are available from fortification studies. Fortification of curry powder with sodium-iron-EDTA giving additional intakes of 7.5 mg Fe/day for 2 years in iron-replete male subjects (Ballot et al., 1989) did not increase serum ferritin levels. There were no changes in iron stores as estimated by serum ferritin following the addition of 10 mg Fe/day as ferrous sulphate to the food of a healthy male subject for 500 days, or as determined via serial phlebotomies after the end of the iron substitution period (Sayers et al., 1994). In these two studies serum ferritin levels did not change despite long-term challenge with 7.5-10 mg Fe/day in addition to normal dietary iron intake of approx. 10 mg Fe/day. Iron status in the elderly, as indicated by elevated serum ferritin levels, was increased after intake of an additional 30 mg Fe/day as supplements (Fleming et al., 2002), in a study in which subjects with abnormal results for blood leukocytes, C-reactive protein (CRP), and 3 liver enzymes, as indicators of inflammation and liver diseases, were excluded. Theoretical calculations of the accumulation of iron in a fertile woman given different daily intakes (Borch-Iohnsen and Petersson Grawe, 1995) indicated that a daily intake of 60 mg for 5 years would lead to a serum ferritin value close to that seen in iron overload.

Elimination

Iron excretion via the kidneys is very low, and body iron is highly conserved. Renal elimination is not controlled as part of iron homeostasis or the control of excess body stores. Normally, only about 0.1 mg is lost daily in urine. The sloughing of mucosal enterocytes results in elimination of absorbed iron before it reaches the systemic circulation and accounts for the loss of 0.6mg per day into the intestinal lumen. About 0.2-0.3 mg is lost daily from the skin. The total daily loss is equivalent to about 0.05 % of body iron content (Green et al., 1968). Menstrual losses are variable and may be almost as high as the total loss in non-menstruating women (FNB, 2001).(EFSA, 2004)

Requirements and recommended intakes

The recommended daily intakes for different groups of the population are based on the amount of ingested iron necessary for absorption of the estimated average amounts of iron lost each day. During the first year of life the body requires approximately 260 mg of iron for metabolism and growth, i.e. 0.6-0.8 mg Fe/day, which corresponds to a dietary intake of 6-8 mg Fe/day, assuming 10% absorption. These data are the rationale for the recommendation of 1 mg Fe/kg body weight per day for children between the 4th month and the 3rd year of life (Oski, 1993). The Scientific Committee on Food recommended daily intakes of 6 mg and 4 mg for infants aged 0.5-1 year and 1-3 years respectively, assuming 15% absorption of the daily intake (SCF, 1993). An adult male loses approx. 1 mg Fe/day, mostly from the intestine (Green et al., 1968), and a daily intake of approx. 10 mg Fe is needed to replace these basal losses, assuming 10% absorption, and the recommended dietary iron intake has been estimated as between 8 and 10 mg Fe/day (SCF, 1993; FNB, 2001; Arbeitsgruppe “Referenzwerte für Nährstoffzufuhr, 2000). Menstrual iron losses are below 1.6 mg Fe/day in 95% of women, which leads to an average total loss of approx. 2.5 mg Fe/day (Baynes and Bothwell, 1990). Assuming 10-20% absorption in iron deficiency, an intake of 15-20 mg Fe/day is recommended for women of reproductive age (SCF, 1993; FNB, 2001; Arbeitsgruppe “Referenzwerte für Nährstoffzufuhr, 2000). During pregnancy, 450 mg Fe is needed to allow increased erythropoiesis, while 270-300 mg and 50-90 mg are transferred to the foetus and placenta, which gives a total extra demand of 770-840 mg. This demand corresponds to approx. 3 mg Fe/day and will be provided by an intake of 30 mg Fe/day and is the rationale for the recommended higher iron intake in pregnancy (FNB, 2001; Arbeitsgruppe “Referenzwerte für Nährstoffzufuhr, 2000).(EFSA, 2004)

 

The following information is taken into account for any hazard / risk assessment:

Bioaccumulation potential is not relevant since iron is an essential element in human nutrition.

Theoretical calculations of the accumulation of iron in a fertile woman given different daily intakes (Borch-Iohnsen and Petersson Grawe, 1995) indicated that a daily intake of 60 mg for 5 years would lead to a serum ferritin value close to that seen in iron overload.(EFSA, 2004)

 

References:

Arbeitsgruppe: Referenzwerte für die Nährstoffzufuhr (2000). Umschau Braus Verlag, Frankfurt, pp174-179.

 

Ballot DE, MacPhail AP, Bothwell TH, Gillooly M, Mayet FG (1989). Fortification of curry powder with NaFe(III) EDTA in an iron-deficient population: Report of a controlled iron-fortification trial. Am J Clin Nutr 49: 156-161.

 

Barrett JFR, Whittaker PG, Eilliams JG, Lind T (1994). Absorption of non-haem iron from food during normal pregnancy. Br Med J 309: 79-82.

 

Baynes RD and Bothwell TH (1990). Iron deficiency. Ann Rev Nutr 10: 133-148.

 

Borch-Iohnsen B and Ptersson Grawe K (1995). In Risk evaluation of essential trace elements. Ed. Oskarsson A. Nordic Council of Ministers, Copenhagen, 1995, pp 67-117.

 

Bothwell TH, Charlton RW, Cook JD, Finch CA (1979). Internal iron kinetics In: Iron metabolism in man. Blackwell Scientific Publishers, Oxford, London, pp 327-353.

 

EFSA (2005): Opinion of the Scientific Panel on Dietetic Products, Nutrition and Allergies on a request from the Commission related to the Tolerable Upper Intake Level of Iron, The EFSA Journal, 125, 1-34.

 

Ellenham MJ and Barceloux DG (1988). Iron. In: Medical Toxicology. Elsevier, New York, Amsterdam, London, pp 1023-1030.

 

Finch C (1994). Regulators of iron balance in humans. Blood 84: 1697-1702.

 

Fleming DJ, Tucker KL, Jaques PF, Dallal GE, Wilson PWF, Wood RJ (2002). Dietary factors associated with risk of high iron stores in the elderly Framingham Heart Study Cohort. Am J Clin Nutr, Am J Clin Nutr 76: 1375-1384.

 

FNB (Food and Nutrition Board) (2001). Dietary Reference Intakes: Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodide, Iron, Manganese Molybdenum, Nickel, Silicon, Vanadium and Zinc. Institute of Medicine, National Academy Press, Washington D.C.

 

Green R, Charlton R, Seftel H, Bothwell TH, Maget F (1968). Body iron excretion in man. A collaborative study. Am J Med 45: 336-353.

 

Gunshin H, Mackenzie B, Berger UV, Gunshin Y, Romero MF, Boron WF, Nussberger S, Gollan JL, Hediger MA (1997). Cloning and characterization of a mammalian proton-coupled metal-ion transporter. Nature 388: 482-488.

 

Hallberg L and Rossander L (1984). Improvement of iron nutrition in developing countries: comparison of adding meat, soy protein, ascorbic acid, citric acid, and ferrous sulphate on iron absorption from a simple Latin American-type of meal.Am J Clin Nutr 39: 577-583.

 

Hallberg L, Hulthen L, Garby L (1997).Iron stores in man in relation to diet and iron requirements. Eur J Clin Nutr 52: 623-631.

 

Hunt JR and Roughead ZK (2000). Adaptation of iron absorption in men consuming diets with high or low iron bioavailability.Am J Clin Nutr 71: 94-102.

 

Kulaksiz H, Gehrke SG, Janetzko A, Rost D, Bruckner T, Kallinowski B, Stremmel W. (2004).Pro-hepcidin: expression and cell specific localisation in the liver and its regulation in hereditary haemochromatosis, chronic renal insufficiency, and renal anaemia. Gut 53: 735-743.

 

Leong WI and Lonnerdal B (2004). Hepcidin, the recently identified peptide that appears to regulate iron absorption. J Nutr 134: 1-4.

 

Lynch SR (1984). Iron. In: Solomons NW, Rosenberg ICH, eds., Absorption and Malabsorption of Mineral Nutrients. Alan R Liss Inc., New York, pp 89-124.

 

Magnusson B, Bjorn-Rassmussen E, Hallberg L, Rossander L (1981).Iron absorption in relation to iron status. A model proposed to express results of food absorption measurements. Scand J Haematol 27: 201-208 .

 

Mulvihill B, Kirwan FM, Morrissey PA, Flynn A (1998). Effects of myofibrillar muscle protein on the in vitro bioavailability of non-haem iron. Int J Food Sci Nutr 49: 187-192.

 

Nicolas G, Bennoun M, Devaux I, Beaumont C, Grandchamp B, Kahn A, Vaulont S (2001). Lack of hepcidin gene expression and severe tissue iron overload in upstream stimulatory factor 2 (USF2) knockout mice. PNAS 98: 8780-8785.

 

Oates PS, Trinder D, Morgan EH (2000). Gastrointestinal function, divalent metal transporter-1 expression and gastrointestinal iron absorption. Pflügers Arch.-Eur J Physiol 440: 496-502.

 

Oski FA (1993). Iron deficiency in infancy and childhood. N Engl J Med 329: 190-193.

 

Ponka P, Beaumont C, Richardson DR (1998). Function and regulation of transferrin and ferritin. Semin Hematol 35: 35-54.

 

Rossander L (1987). Effect of Fiber on iron absorption in man. Scand J Gastroenterol 22 (Suppl) 129: 68-72.

 

Sayers MH, English G, Finch CA (1994). The capacity of the store regulator in maintaining iron balance. Am J Hematol 47: 194-197.

 

SCF (Scientific Committee on Food) (1993). Reports of the Scientific Committee on Food (31st series). Commission of the European Community, Luxembourg, pp. 177-189.

 

Schümann K, Elsenhans B, Forth W (1999a). Kinetic analysis of 59Fe movement across the intestinal wall in duodenal rat segments ex vivo.Am J Physiol 276: G431-G440.

 

Schümann K, Moret R, Künzle H, Kühn LC (1999b).Iron regulatory protein as an endogenous sensor of iron in rat intestinal mucosa. Eur. J. Biochem 260: 362-372.