Iron Absorption from the Whole Diet: Comparison of the Effect of Two Different
Distributions of Daily Calcium Intake

Todd Bowen
Human Biology
Today December 5, 1996

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 calciumís
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 bodyís
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 ironís 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).

Terms -

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

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