Iron Absorption Essay, Research Paper
Human Biology Today
December 5, 1996
Iron Absorption from the Whole Diet:
Comparison of the Effect of Two Different Distributions of Daily Calcium Intake
Hypothesis – If a woman distributes her daily intake of calcium by having less of it in her lunch and dinner meals and more in her breakfast and evening meals, then this would reduce the inhibitory effects calcium has on heme iron and nonheme iron absorption.
Background Information – This experiment is one of many that addresses calcium s inhibitory affects on iron absorption. In 1994, the Consensus Development Panel in Optimal Calcium Intake suggested an increase of the current Recommended Dietary Allowances of calcium(Whiting, p.77). This goal of this increase was to aid in the prevention of osteoporosis and other bone diseases. Unfortunately, this attempt at prevention could have an adverse affect on the human body s ability to absorb iron.
Recent studies have shown that eating a normal daily allowance of calcium cuts iron absorption by as much as 50-60%(Hallberg et al. p.118). Other studies examine the affect of iron bioavailability on menstruating, pre-menopausal, and post-menopausal women(Rossander-Hulten et al and Gleerup et al). One of the fears of an increased amount of calcium intake is the increased possibility of anemia in women who are already susceptible to this condition. The iron inhibition by calcium is a classical example of how the correction of one nutritional problem can be the cause of another.
The physiological mechanism of this calcium-iron relationship remains a mystery, however there are two feasible theories. One states that calcium competes for an iron binding site on intestinal epithelial cells. It is believed calcium binds to the protein mobilferrin on the epithelial cells, which is the iron transport protein(Whiting, p.78). Another group of scientists theorizes that iron is able to be transported into the epithelial cells without problem, however the iron then has trouble getting into the blood stream. The presence of calcium inhibits iron s ability to leave the epithelial layer.
Another very interesting theory is not on the microscopic level but in the evolutionary plane. Eaton et al. state that one possibility for this phenomenon could lie in the Homo sapiens genetic ancestry. As little as 200 years ago humans had almost double the amount of calcium intake as they do in the present, because humans evolved in a high-calcium nutritional environment. With the decrease in calcium, there has also been a large decrease in physical activity(Eaton et al.). The inhibitory effect of calcium on iron absorption could be related to the low intakes of iron and calcium in conjunction with the present low-energy lifestyle(Glerrup et al. p. 103).
Extrinsic radioisotopic iron tracer – Radioisotopes of iron (59Fe and 55Fe) which can be traced from outside the body.
Heme – The heme molecule is a heterocyclic ring system of porphyrin derivation which has a molecule of iron in the center of the ring structure. Myoglobin and each of the four subunits of hemoglobin noncovalently bind to a single heme group. Heme is also the site at which each globin monomer binds one molecule of O2 (Voet et al, p. 216).
Heme iron – Iron which is located in the heme molecule.
Nonheme iron – Iron found in human tissue that is not a part of the heme molecule.
Oral reference dose – An oral dose of radio-labeled iron given to the subjects in order to examine their uninhibited iron absorption. This process was used as a control rate for each subject.
Experiment – The absorption of nonheme was measured from all meals during four 5-d periods (A1, A2, B1, and B2). Each day four meals were served to the subjects: breakfast, lunch, dinner, and an evening meal. The menu of the two B weeks were identical to the two A weeks, except for the distribution of dairy products. All meals, except for the evening meal, were served under supervision in the lab. A precise measyre of the iron content of each meal was required to enable the homogeneous labeling of the nonheme iron with radioisotopic iron. One wheat-rye roll served as the carrier of the radioisotope, and this was eaten throughout the course of the meal.
Before starting the 4-wk absorption study, a blood sample was drawn to determine hemoglobin and serum ferritin concentrations. Three weeks after the last serving, the total retention of 59Fe and 55Fe was measured by a whole-body counter to determine their ratio. An oral reference dose of 59Fe was also given to the subjects to determine the retention of absorbed iron. During the study, measurements were made of the blood menstrual losses in 19 of the subjects. One subject had no menustrationfor 4 years and the other had an irregular cycle.
The subject sample consisted of 21 healthy female volunteers in a fertile age period. Most of these subjects were senior students or staff members of the institute, and were all highly motivated to participate in the experiment.
All meals were prepared in the laboratory kitchen. All the amounts of food were weighed and prepared for the subjects. Subjects with higher energy requirements than provided by the meal served were allowed to eat more of the special unlabeled wheat roll. Each whole-grain roll was labeled from the radioisotopic iron standard solutionjust before serving I amounts that gave exactly the same specific activity of nonheme iron in all meals.
Fresh samples of raw and boiled vegetables and potatoes were analyzed for ascorbic acid on the same day that they were served. Weighed amounts from the meat dishes were analyzed for total iron and analysis of nonheme iron content of the meat. The mineral analyses of calcium, phosphorous, and magnesium were made after completion of the wet-ash method in sulfuric acid and hydrogen peroxide.
The measurement of nonheme iron absorption was done by using a liquid-scintillation spectrometer. The following equation wasused to determine heme iron absorption: heme iron absorption% = reference dose absorption % X 0.322 +15.71.
Results – The nonheme iron absorption of high calcium intake was 12.1 +- 2.20% (range, 1.8%-32.3%) and low calcium intake was 15.9 +- 2.50% (range, 1.6% – 40.6%). When log serum ferritin concentration was used as a measure of iron status, the r-squared values were 0.68 and 0.51, respectively.
The mean difference of the individual total iron-absorption figures from the two 10-d periods was 0.52 mg. In 16 of the 20 women , total iron absorption was higher when the calcium intake with the two main meals was low. No obvious explanation was found for the four subjects who responded differently to the higher calcium intake.
Conclusion – The absorption of heme and nonheme iron are both influenced by calcium to some extent. A main result of this study was that 30-50% more iron was absorbed when the intake of calcium was low at lunch and dinner compared with when the intake of calcium was high at these meals. This difference in iron absorption corresponds to 0.44 mgFe/day. In 5 of the original 21 women in our study, iron absorption was unexpectedly lower when milk was not served with the lunch and dinner meals. In 4 of the 5 subjects the difference was small and no obvious explanation could be found. In 11 of the 42 periods studied, nonheme iron absorption exceeded 20% and in four periods, 30%.
An important condition for this kind of long-term and rather complex absorption study is access to highly motivated and knowledgeable subjects who follow experimental instructions carefully.
Misgivings – The one facet of this experiment I felt was very lacking was the diversity of the sample. The entire sample had a very similar background and all came from the same place, the university. I was also disappointed in the size of the sample. It seems this sample could have been bigger, or they could have had another trial.
Personal Response – Overall, I really enjoyed reading this article. The questions about our diet that it poses in the conclusion were extremely interesting. I also thought it was written well. The terms and ideas were easy enough for the layman to understand, while it was also able to discuss complicated ideas.
Eaton SB, and Nelson DA. Calcium in evolutionary perspective. American Journal of Clinical Nutrition 1991;54:281S-287S.
Gleerup A, Rossander-Hulthen L, Gramatkovski E, and Hallberg L. Iron absorption fom the whole diet: comparison of the effect of two different distributions of daily calcium intake. American Journal of Clinical Nutrition 1995;61:97-104.
Hallberg L, Brune M, Erlandsson M, Sandberg A, and Rossander-Hulthen. Calcium: effect of different amounts on nonheme- and heme-iron absorption in humans. American Journal of Clinical Nutrition 1991;53:112-119.
Rossander-Hulten L, and Hallberg L. Iron requirements in menstruating women. American Journal of Clinical Nutrition 1991;54:1047-1058.
Voet D, and Voet JG. Hemoglobin Function. Biochemistry. New York, NY. 1995.
Whiting S. The Inhibitory Effect of Dietary Calcium on Iron Bioavailability: A Cause for Concern? Nutrition Reviews, Vol. 53, No. 3, pp. 77-80.