The Role of Iron in Human Metabolic Processes

Reference from the joint venture report of FAO/WHO expert consultation on Human Vitamins and Minerals verbatim.

Bothwell, T.H. 1979. Iron metabolism in man. London, Blackwell Scientific Publications.

Hallberg, L. 1982. Iron absorption and iron deficiency. Hum Nutr:Clin. Nutr., 36:259-278.

Dallman, P.R. 1986. Biochemical basis for the manifestations of iron deficiency. Ann. Rev. Nutr., 6: 13-40.

Brock, J.H., Halliday, J.W., M.J.P. & Powell, L.W. 1994. Iron metabolism in health and disease, London, W.B. Saunders Company Ltd.

Kühn, L.C. 1996. Control of cellular iron transport and storage at the molecular level. In: Hallberg LA, et al., eds. Iron nutrition in health and disease. p. 17-29. London, John Libbey & Company.

Mascotti, D.P., Rup, D. & Thach, R.E. 1995. Regulation of iron metabolism: Translational effects medicated by iron, heme and cytokines. Ann. Rev. Nutr., 15: 239-61.

Iron has several vital functions in the body. It serves as a carrier of oxygen to the tissues from the lungs by red blood cell haemoglobin, as a transport medium for electrons within cells, and as an integrated part of important enzyme systems in various tissues. The physiology of iron has been extensively reviewed.

Most of the iron in the body is present in the erythrocytes as haemoglobin, a molecule composed of four units, each containing one heme group and one protein chain. The structure of haemoglobin allows it to be fully loaded with oxygen in the lungs and partially unloaded in the tissues (e.g., in the muscles).  

The iron-containing oxygen storage protein in the muscles, myoglobin, is similar in structure to haemoglobin but has only one heme unit and one globin chain. Several iron-containing enzymes, the cytochromes, also have one heme group and one globin protein chain. These enzymes act as electron carriers within the cell and their structures do not permit reversible loading and unloading of oxygen.

Their role in the oxidative metabolism is to transfer energy within the cell and specifically in the mitochondria. Other key functions for the iron-containing enzymes (e.g., cytochrome P450) include the synthesis of steroid hormones and bile acids; detoxification of foreign substances in the liver; and signal controlling in some neurotransmitters, such as the dopamine and serotonin systems in the brain. Iron is reversibly stored within the liver as ferritin and hemosiderin whereas it is transported between different compartments in the body by the protein transferrin.

Iron requirements

Basal iron losses

Green, R. 1968. Body iron excretion in man. A colloborative study. Am. J. Med., 45: 336-353.

Iron is not actively excreted from the body in urine or in the intestines. Iron is only lost withcells from the skin and the interior surfaces of the body – intestines, urinary tract, andairways. The total amount lost is estimated at 14 μg/kg body weight/day. 

FAO/WHO. 1988. Requirements of vitamin A, iron, folate and vitamin B12. Report of a Joint FAO/WHO Expert Consultation.. Rome: FAO. (FAO Food and Nutrition Series No. 23).

In children, it is probably more correct to relate these losses to body surface. A non-menstruating 55-kg women loses about 0.8 mg Fe/day and a 70-kg man loses about 1 mg. The range of individual variation has been estimated to be ±15 percent.

Brune, M. 1986. Iron losses in sweat. Am. J. Clin. Nutr., 43: 438-443.

Earlier studies suggested that sweat iron losses could be considerable, especially in a hot, Humid climate. However, new studies which took extensive precautions to avoid the interference of contamination of iron from the skin during the collection of total body sweat have shown that these sweat iron losses are negligible.

Growth

The newborn term infant has an iron content of about 250–300 mg (75 mg/kg body weight). During the first 2 months of life, haemoglobin concentration falls because of the improved oxygen situation in the newborn infant compared with the intrauterine foetus. This leads to a considerable redistribution of iron from catabolised erythrocytes to iron stores.

This iron will cover the needs of the term infant during the first 4–6 months of life and is why iron requirements during this period can be provided by human milk, that contains very little iron. Because of the marked supply of iron to the foetus during the last trimester of pregnancy, the iron situation is much less favourable in the premature and low-birth-weight infant than in the term infant. An extra supply of iron is therefore needed in these infants even during the first 6 months of life.

European Communities. 1993. Nutrient and energy intakes for the European Community. EG-Report. Brussels Luxembourg: Commission of the European Communities.

In the full-term infant, iron requirements will rise markedly after age 4–6 months and amount to about 0.7–0.9 mg/day during the remaining part of the first year. These requirements are therefore very high, especially in relation to body size and energy intake (Table 39).

In the first year of life, the full-term infant almost doubles its total iron stores and triple its body weight. The change in body iron during this period occurs mainly during the first 6–12 months of life. Between 1 and 6 years of age, the body iron content is again doubled. The requirements for absorbed iron in infants and children are very high in relation to their energy requirements. For example, in infants 6–12 months of age, about 1.5 mg of iron need to be absorbed per 4.184 MJ and about half of this amount is required up to age 4 years.