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Iron: food sources, deficiencies and physical performance
Iron: food sources, deficiencies and physical performance

Iron: food sources, deficiencies and physical performance

Date: October 04, 2021

Iron is very important for human life; it plays a central role in the hemoglobin of red blood cells, where it participates in the transport of oxygen from the lungs to the tissues and carbon dioxide from the tissues to the lungs. Iron is also an integral part of several key enzymes in energy production and metabolism, including DNA synthesis.


Food sources

The recommended daily dose of iron is 10 mg for males and 15 mg for females. There are two forms of dietary iron, heme and non-heme . Heme iron is iron bound to hemoglobin and myoglobin. It is found in foods of animal origin and is the most easily absorbed form (see table 26.1). Non-heme iron is found in plants and is poorly absorbed compared to heme iron.


Signs and symptoms of deficiency

Iron deficiency is the most common form of nutritional deficiency in the United States. Children under 2 years of age, adolescents, pregnant women and the elderly are at higher risk; in these groups the percentage of affected subjects fluctuates between 30 and 50%. Iron deficiencies are present between 35 and 58% of healthy women and during breastfeeding this percentage increases (1,2) .


Kelp seaweed 100.0
Brewer's yeast 17.3
Molasses 16.1
wheat bran 14.9
Pumpkin seeds 11.2
Wheat germ 9.4
Ox liver 8.8
Sunflower seeds 7.1
Mile 6.8
Parsley 6.2
Clams 6.1
Almonds 4.7
Dried plums 3.9
Anacardi 3.8
Raisins 3.5
Brazilian nuts 3.4
Topinambur 3.4
Beetroot leaves 3.3
Thistles 3.2
Walnuts 3.1
Dandelion 3.1
Dates 3.0
Cooked dried beans 2.7
Pecan nuts 2.4
Shelled sesame seeds 2.4
Peanuts 2.1
Lentils 2.1
Tofu 1.9
Green peas 1.8
Olives 1.6
Brown rice 1.6
Artichoke 1.3
Broccoli 1.1
Cauliflower 1.1
Whole grain bread 1.1
Currant 1.1


A deficiency of iron can be caused by an increased need for this mineral, a reduced intake with the diet, a lower absorption or utilization, a bleeding or a combination of several factors. The increased need for iron occurs during the growth peaks of childhood and adolescence and during pregnancy and breastfeeding. Currently, the vast majority of pregnant women regularly take iron supplements because the dramatic increase in iron requirements cannot simply be met by diet.

Inadequate iron intake is common in many countries, especially in areas with a predominantly vegetarian diet. The typical diet of infants in developed countries (rich in milk and cereals) is also low in iron. In addition, deficits can occur among adolescents whose diet is based mostly on junk food (literally 'junk food', based on high-calorie and nutrient-poor snacks and sweets).

The segment of the population at greatest risk of dietary iron deficiency is however represented by the elderly (3) . Decreased iron absorption is extremely common in old age (4) and is often due to insufficient secretion of hydrochloric acid in the stomach (5) , a condition quite common in old age. Other causes of impaired absorption include chronic diarrhea or malabsorption, surgical removal of the stomach, and the use of antacids. Blood loss is the most common cause of iron deficiency in women of childbearing age and is usually caused by heavy periods (6,7). Other common causes of blood loss are peptic ulcer, hemorrhoids, and blood donations.


Consequences of iron deficiency

The negative effects of iron deficiency are caused by the impairment of the oxygen transfer process to the tissues and the activity of enzymes (containing iron). The deficiency can cause anemia, menorrhagia (heavy menstrual bleeding), learning disabilities , impaired immune functions and decreased energy and physical performance (1,2,8) .

Anemia is a decrease in the number of red blood cells or in the amount of hemoglobin (which binds iron) contained in the red blood cells.

The main function of red blood cells is to transport oxygen from the lungs to the body's tissues in exchange for carbon dioxide. Symptoms of anemia (such as excessive fatigue) reflect the reduced oxygen supply to the tissues and the accumulation of carbon dioxide.

Iron deficiency is the most common cause of anemia, which is, however, only the last stage of this deficiency. Iron-dependent enzymes involved in energy production and metabolism are the first to suffer from low iron levels. The determination of ferritin in serum is the best laboratory test to determine the state of the body's iron reserves.


Physical performance

Several researchers have shown that even mild anemia due to iron deficiency causes a reduction in physical work capacity and productivity (9-11) . Nutritional statistics from the United States indicate that iron deficiency causes significant damage to work skills and health, and consequently causes severe economic loss to the individual and the country. In these cases, iron supplements can lead to a rapid improvement in working skills in individuals with deficiency.

The alterations in physical performance produced by the lack of iron do not necessarily derive from anemia. It must be remembered once again that the iron-dependent enzymes involved in energy production and metabolism are affected long before anemia occurs.

Reduced iron absorption is often caused by insufficient gastric secretion of hydrochloric acid (5) , a condition quite common in the elderly. Other causes that reduce iron absorption include chronic diarrhea or malabsorption, surgical removal of the stomach, and use of antacids.


Forms available

As already mentioned, the most easily absorbed form of iron is represented by the iron itself. The absorption rate of non-heme iron supplements, such as ferrous sulphate and ferrous fumarate, is 2.9% on an empty stomach and 0.9% on a full stomach; that of heme iron, on the other hand, reaches as much as 35% (1,2) . Add to this the fact that heme iron lacks the side effects (nausea, flatulence and diarrhea) associated with non-heme iron.

Furthermore, unbound non-heme iron tends more than heme iron to release pro-oxidant substances capable of forming free radicals. Per this reason, many experts, when recommending a supplement, choose heme iron over non-heme iron.

Despite the superiority of heme iron, non-heme iron salts are the most popular form of supplements. It is easier to take high amounts of non-heme iron than of heme iron; the net amount of iron absorbed is roughly the same. In other words, if you take 3 mg of heme iron and 50 mg of non-heme iron, the net amount of iron absorbed is about the same. The best form of non-heme iron is ferrous succinate.


Specific dosages

In case of iron deficiency, it is advisable to take 30 mg of iron linked to succinate or fumarate, twice a day, between meals. If this causes abdominal discomfort, 30 mg with meals three times a day will suffice. Alternatively, high quality (hydrolyzed) liquid liver extract can be taken in sufficient quantity to provide a daily dose of 4-6 mg of heme iron.



1. Krause MV and Mahan KL, Food, Nutrition and Diet Therapy, 7th Edition. WB Saunders, Philadelphia. PA, 1984, pp. 128-131, 157-164, 585-599.

2. Fairbanks VF and Beutler E, Iron. In: Modem Nutrition in Health and Disease, 7th Edition. Shils ME and Young VR (eds.). Lee and Febiger, Philadelphia, PA, 1988, pp. 193-226.

3. Morley JE. Nutritional status of the elderly. Am J Med 81.679-695, 1986.

4. Jacobs AM and Owen GM, The effect of age on iron absorption. J Gerontol 24, 95, 96, 1969.

5. Bezwoda W, et al., The importance of gastric hydrochloric acid in the absorption of non-heme iron. J Lab Clin Med 92, 108-116, 1978.

6. Arvidsson B, et al., Iron prophylaxis in menorrhagia. Acta Ob Gyn Scand 60, 157-160, 1981.

7. Taymor ML, Sturgis SH, and Yahia C, The etiological role of chronic iron deficiency in production of menorrhagia. JAMA 187, 323-327, 1964.

8. Cook JD and Lynch SR, The liabilities of iron deficiency. Blood 68. 803-809, 1986.

9. Viteri FE and Torun B. Anaemia and physical work capacity. Clin Haematol 3, 609-626. 1974.

10. Basta SS, et al., Iron defìcienty anemia and the productivity of adult males in Indonesia. Am J Clin Nutr 32, 6-25, 1979.

11. Gardner GW, et al., Physical work capacity and metabolic stress in subjects with iron deficiency anemia. Am J Clin Nutr 30, 910-917, 1977.

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