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Iron absorption from standard Latin American meals
Table 5 shows non-haem iron absorption from four standard Latin American meals. The total phytate content was 669, 1,021, 370, and 560 mg in meals A, B. C, and D respectively. The total tannate content of black beans was 293, 348, 187, and 214 mg respectively. Iron absorption from meals A and B was the same in spite of the lower phytate content in meal A. Apparently, the high iron absorption from precooked-maizeflour bread observed when the food was given alone or in the breakfast is neutralized when given with other vegetable foods with high phytate content. Only meal C, which had a low phytate concentration and contained meat, showed a statistically significant higher absorption than meals A and B.
TABLE 3. Iron absorption from four breakfasts in 201 subjects
| Breakfast subjects | Number of subjects | Serum
ferritin concentration (µ/litre)a |
Iron absorption (%) | ||||
| Mean | SE | Non-haemb | Reference dose | ||||
| Mean | SE | Mean | SE | ||||
| A. Normal subjects | |||||||
| 1. Bread from whole maize, milk | 29 | - | 1.2 | 1.2 | |||
| 2. Bread from precooked maize flour, milk, coffee | 28 | 34 | 1 | 3.5 | 1.2 | ||
| 3. Bread from white wheat flour, milk, coffee | 28 | 37 | 1 | 1.8 | 1.4 | 12.6 | 1.1 |
| 4. Bread from white wheat flour, tea | 36 | 68 | 1 | 0.8 | 1.2 | ||
| B. Subjects with moderate iron deficiency | |||||||
| 1. Bread from whole maize, milk | 11 | - | 4.7 | 1.1 | |||
| 2. Bread from precooked maize flour, milk, coffee | 6 | 26 | 1 | 6.3 | 1.3 | ||
| 3. Bread from white wheat flour, milk, coffee tea | 8 | 32 | 1 | 6.5 | 1.2 | 45.9 | 1.0 |
| 4. Bread from white wheat flour, | 13 | 23 | 1 | 2.7 | 1.3 | ||
| C. Subjects with severe iron deficiency | |||||||
| 1. Bread from whole maize, milk | 14 | - | 8.6 | 1.3 | |||
| 2. Bread from precooked maize flour, milk, coffee | 17 | 8 | 1 | 10.0 | 1.1 | ||
| 3. Bread from white wheat flour, milk, coffee | 9 | 6 | 1 | 5.5 | 1.2 | 47.9 | 1.1 |
| 4. Bread from white wheat flour, tea | 10 | 6 | 1 | 2.7 | 1.2 | ||
a. Serum ferritin concentration was not determined in subjects tested for breakfast 1.
b. Data normalized according to iron absorption from the reference dose by each individual and by all subjects in the same iron-status category.
The statistically significant differences in iron absorption from the different breakfasts by subjects in the same iron-status category were as follows:
A1vs.A2,p<.001 B1vs.B4,p<.001 Clvs.C4,p<.002
A2vs.A3,p<.025 B2vs.B4,p<.001 C2vs.C3,p<.002
A2vs.A4,p<.001 B3vs.B4,p<.001 C2vs.C4,p< 001
The statistically significant differences in iron absorption from the breakfasts by subjects in different iron-status categories were as follows:
A1vs.B1,p<.001 Alvs.C1,p<.001 B1vs.C1,p<.001
A2vs.B2,p<.001 A2vs.C2,p<.001
A3vs.B3,p<.001 A3vs.C3,p<.001
A4vs.B4,p<.001 A4vs.C4,p<.001
Discussion
Several workers have established the progressive stages of iron deficiency [24, 28, 29]. The first stage of negative iron balance, iron depletion, is characterized by a fall of iron stores as indicated by a reduction in bone marrow haemosiderin and a reduction in the plasma concentration of ferritin from about 30 to 200 µg per litre in healthy subjects to nearly 20 µg per litre. The body counterbalances this fall in iron stores by enhancing iron absorption.
TABLE 4. Effect of eggs and ascorbic acid on iron absorption from a breakfast
| Study 1 (7 subjects) |
Study 2 (10 subjects) |
|||
| Mean | SE | Mean | SE | |
| Blood haemoglobin concentration (g/100 ml) | 13.6 | 0.6 | 13.4 | 0.8 |
| Serum transferrin saturation(%) | 26 | 3 | 30 | 4 |
| Serum ferritin concentration(µg/litre) | 22 | 1 | 15 | 1 |
| Iron absorption
from basic breakfast (with bread from precooked maize flour) (%)a |
6.3 | 1.3 | ||
| Iron absorption from en riched breakfastsb | ||||
| A. Basic breakfast +1 egg | 4.5 | 1.2 | - | |
| B. Basic breakfast +2 eggs | - | 4.9 | 1 .3 | |
| C. Basic breakfast +15 mg ascorbic acid | 6.9 | 1.3 | - | |
| D. Basic breakfast +30 mg ascorbic acid | - | 21.0 | 1.2 | |
a. Mean and standard error of absorption by all subjects in
the two studies.
b. Normalized according to absorption from the basic breakfast by
each individual and that by all subjects in the two studies.
The only statistically significant difference in the absorption from the different breakfasts was that between the basic breakfast and enriched breakfast D.
In the second stage, iron-deficiency erythropoiesis, iron stores are exhausted, bone marrow haemosiderin is absent, the ferritin concentration is reduced to below 12 µg per litre, and the plasma iron concentration is reduced and the transferrin is increased, resulting in a reduction of the transferrin saturation to below 16%. In this stage, the erythrocyte protoporphyrin concentration is above 80 mg per litre.
In the third stage, iron-deficiency anaemia, the blood haemoglobin concentration, which is within normal limits in the two previous stages, starts to decrease, and the normal red cell population is gradually replaced by microcytic and hypochromic erythrocytes.
The results of the studies presented here on iron absorption from whole-maize bread and from four types of breakfasts provide further information on the various stages of iron deficiency. First, increased iron absorption is observed in some subjects with serum ferritin concentration above 20 µg per litre and transferrin saturation 16% or greater. Thus, in the 27 subjects with moderate iron deficiency in the study using breakfasts (table 3), ferritin concentration was above 20 µg per litre in 18 and above 30 µg per litre in 8. Several authors have reported that even in persons with normal transferrin saturation and ferritin
TABLE 5. Iron absorption in 15 subjects from four standard meals
| Mean | SE | |
| Blood haemoglobin concentration (g/100 ml) | 11.9 | 0.5 |
| Serum transferrin saturation (%) | 20 | 2 |
| Serum ferritin concentration (µg/litre) | 21 | 1 |
| Iron absorption from standard meals (%) | ||
| A. Vegetable meal (precooked maize flour, rice, black beans) | 2.7 | 1.4 |
| B. Vegetable meal (whole maize, rice, black beans) | 2.8 | 1.4 |
| C. Vegetable and meat meal(precooked maize flour, rice, black beans, meat) | 6.4 | 1.4 |
| D. Vegetable and meat meal(whole maize, rice, black beans, meat) | 3.5 | 1.4 |
The statistically significant differences in the absorption of non-haem iron from the meals were as follows: A vs. C, p<.001; B vs. C, p<.001. concentration, absorption in a reference dose of ferrous sulfate and in vegetable foods can increase [30-32].
Second, the absorption of non-haem iron from the reference dose and from meals observed in subjects with moderate iron deficiency, which is very similar to the stage of iron depletion previously mentioned, was more than twice that observed in healthy subjects. This increase did not continue to the next stage of iron depletion, in which the ferritin concentration falls to below 12 µg per litre and the transferrin saturation is below 16%. Furthermore, in this stage of severe iron deficiency, the absorption was the same whether or not the subjects had anaemia. These results suggest that the increase of iron absorption is stimulated by the reduction in iron stores but is independent of the severity of iron deficiency.
When maize is given alone, it is clear that iron absorption from bread made with precooked maize flour is higher than that from bread made with other types of maize flour. The low phytate content of this maize flour, half that of polished maize and one-third that of whole maize, is probably the cause of its high iron absorption. Its nutrient content is very similar to that of the other types except for iron: it has about one-third less than that of either white or yellow whole-maize grain. In spite of the increased consumption of precooked maize flour by Latin American populations, it is surprising that no steps have been taken at a government level to oblige industry to replace the iron lost during the preparation of this flour, as was done with white wheat flour 40 years ago.
The results with the four breakfasts suggest three factors that must be taken into consideration with respect to non-haem iron absorption. The first is to emphasize the advantage of using precooked-maize-flour bread instead of whole-maize bread. Because this type of bread has a lower iron content, however, it would be necessary to accompany it with other foods of a higher iron content, such as beans or eggs, to optimize iron use.
The second factor is related to the beverage consumed with the breakfast. The high concentration of tannate in coffee (5.0 mg/100 g) reduces non-haem iron absorption from meals by about 30-50% [8, 31] and the concentration in tea 112.0 mg/100 g) reduces it by 60-70% [30, 33]. In breakfast no. 4, consisting of white bread and tea, absorption was reduced by about 65% compared with breakfast no. 3, which also contained white wheat bread but in which tea was replaced by coffee.
The third factor is related to the amount of ascorbic acid that is necessary to improve non-haem iron absorption. In our study, 15 mg of ascorbic acid was not sufficient to significantly increase the absorption of 2.2 me non-haem iron provided by the breakfast, but 30 mg increased absorption by about a factor of three. According to these data, in this type of breakfast, about 10-15 mg ascorbic acid is necessary to significantly increase the absorption of 1 mg of non-haem food iron. This observation is in accord with the results of other studies [34, 35].
The data on non-haem iron absorption from standard Latin American meals reflect interaction in the lumen of the gut between the inhibiting substances (phytates, tannates) and the enhancing substances present in meat that regulate iron absorption. This explains why exchanging whole-maize bread for precooked-maize-flour bread in the meals containing only vegetable foods did not promote a statistically significant increase in iron absorption, as observed in the breakfast already mentioned, because the amount of phytate, 493 mg, was still very high. And it suggests why the enhancing effect of meat on iron absorption was detected in the meal with a low phytate content. Hallberg and Rossander [11] also found an increase in iron absorption of the basal meal containing whole-maize bread when eaten with 50 mg ascorbic acid or 75 9 beef.
Acknowledgements
The work of this paper was supported in part by the Consejo Nacional de Investigaciones Científicas y Tecnológicas, CONICIT, and by the United Nations University. We also want to express our gratitude to Dr. Virgilio Urbina from Fundación Polar, for his help in procuring information on precooked maize flour.
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