208 Introduction to Human Nutrition
fortifi ed iron. Individuals taking iron at high
levels (>45 mg/day) may encounter gastrointes-
tinal side-effects (constipation, nausea, vomiting,
and diarrhea), especially when taken on an empty
stomach. Based largely on the data on gastrointestinal
effects following supplemental elemental iron intake
in apparently healthy adults, the US Food and Nutri-
tion Board established a tolerable UL of iron of
45 mg/day.
Genetic diseases
Primary idiopathic hemochromatosis is a hereditary
disorder of iron metabolism characterized by an
abnormally high iron absorption owing to a failure of
the iron absorption control mechanism at the intes-
tinal level. High deposits of iron in the liver and the
heart can lead to cirrhosis, hepatocellular cancer, con-
gestive heart failure, and eventual death. Sufferers of
this disorder can develop iron overload through con-
sumption of a normal diet, but would be at much
higher risk if consuming iron-fortifi ed foods. Thus,
early detection of the disease via genetic screening
followed by regular blood removal has proven to be a
successful treatment.
Assessing status
Several different laboratory methods must be used in
combination to diagnose iron defi ciency anemia cor-
rectly. The most commonly used methods to assess
iron status include:
● serum ferritin
● transferrin saturation
● erythrocyte protoporphyrin
● mean corpuscular volume
● serum transferrin receptor
● hemoglobin or packed cell volume.
Iron defi ciency anemia is usually defi ned as a hemo-
globin level below the cut-off value for age and sex
plus at least two other abnormal iron status measure-
ments. The most commonly used are probably
low serum ferritin, high protoporphyrin, and, more
recently, high serum transferrin receptor.
Requirements and dietary sources
Daily (absorbed or physiological) iron requirements
are calculated from the amount of dietary iron neces-
sary to cover basal iron losses, menstrual losses, and
growth needs. They vary according to age and sex,
and, in relation to body weight, they are highest for
the young infant. An adult man has obligatory iron
losses of around 1 mg of iron/day, largely from the
gastrointestinal tract (exfoliation of epithelial cells
and secretions), skin, and urinary tract. Thus, to
remain replete with regard to iron, an average adult
man needs to absorb only 1 mg of iron from the diet
on a daily basis. Similar obligatory iron losses for
women amount to around 0.8 mg/day. However,
adult women experience additional iron loss owing
to menstruation, which raises the median daily iron
requirement for absorption to 1.4 mg (this covers
90% of menstruating women; 10% will require daily
absorption of at least 2.4 mg iron to compensate for
their very high menstrual losses). Pregnancy creates
an additional demand for iron, especially during the
second and third trimesters, leading to daily require-
ments of 4–6 mg. Growing children and adolescents
require 0.5 mg iron/day in excess of body losses to
support growth. Physiological iron needs can be
translated into dietary requirements by taking into
account the effi ciency at which iron is absorbed from
the diet (typically around 10%). Current RDAs for
iron (recommended by the US Food and Nutrition
Board in 2001) are infants 0.27 mg (fi rst 6 months;
this is an adequate intake value), 11 mg (7–12
months), children 7 and 10 mg (1–3 and 4–8 years,
respectively), teenage boys 8 and 11 mg (9–13 and
14–18 years, respectively), adult men 8 mg (19 years
and older), teenage girls 8 and 15 mg (9–13 and 14–18
years, respectively), adult women 18 and 8 mg (19–50
years and 51 years and older, respectively), pregnant
women 27 mg and lactating women 10 and 9 mg
(younger than 18 years and 19–50 years,
respectively).
Iron is widely distributed in meat, eggs, vegetables,
and cereals, but the concentrations in milk, fruit, and
vegetables are low (Table 9.10). The iron content per
se of individual foods has little meaning as iron
absorption varies considerably. There are two types of
food iron: nonheme iron, which is present in both
plant foods and animal tissues, and heme iron, coming
from the hemoglobin and myoglobin in animal prod-
ucts. Heme iron represents 30–70% of the total iron
in lean meat and is always well absorbed. Nonheme
iron from meat and vegetable foods enters a common
nonheme iron pool in the gastric juice, from which
the amount of iron absorbed depends to a large extent
on the presence of enhancing and inhibiting sub-