Introduction to Human Nutrition

Minerals and Trace Elements 205

may have dietary intakes of potassium exceeding
6 g/day.

Micronutrient interactions

As might be expected from the close metabolic inter-
actions among the major electrolytes, potassium and
sodium dietary interactions may be important in
determining the risk of coronary heart disease and
stroke. Another potentially important interaction
concerns calcium. Potassium appears to have positive
effects on calcium balance by regulating the acid–base
balance and ameliorating any effects of sodium on
calcium depletion.

9.7 Iron

Iron is a relatively abundant element in the universe.
It is found in the sun and many types of stars in con-

siderable quantity. The core of the Earth is thought to

be largely composed of iron and it makes up 4.7% of

the Earth’s crust. The most common ore is hematite,

which is frequently seen as black sands along beaches

and streams. Taconite is becoming increasingly

important as a commercial ore. Because iron is easy

to obtain, its discovery is lost in the history of man,

many thousands of years ago. The early Greeks were

aware of the health-giving properties of iron. Iron has

been used for centuries as a health tonic. It is therefore

paradoxical that although the need for iron was dis-

covered long ago and although it is the most common

and cheapest of all metals, iron defi ciency is probably

the most frequent defi ciency disorder in the world

and the main remaining nutritional defi ciency in

Europe. Iron can exist in oxidation states ranging

from −2 to +6. In biological systems, these oxidation

states occur primarily as the ferrous (Fe^2 +) and ferric

(Fe^3 +) forms and these are interchangeable.

Absorption, transport, and

tissue distribution

The iron content of a typical 70 kg adult man is

approximately 4–5 g. Of this content, approximately

two-thirds is utilized as functional iron such as

hemoglobin (60%), myoglobin (5%), and various

heme (cytochromes and catalase) and nonheme

(NADH hydrogenase, succinic dehydrogenase, aconi-

tase) enzymes (5%). The remaining iron is found in

body storage as ferritin (20%) and hemosiderin (10%),

the two major iron storage proteins. Only very minor

quantities of iron (<0.1%) are found as a transit

chelate with transferrin, the main iron transport

protein in the body.

The metabolism of iron differs from that of other

minerals in one important respect: there is no phy-

siological mechanism for iron excretion. The body has

three unique mechanisms for maintaining iron balance

and preventing iron defi ciency and iron overload:

● storage of iron (with ferritin being an important

reversible storage protein for iron)

● reutilization of iron (especially of iron in

erythrocytes)

● regulation of iron absorption.

In theory, therefore, when the body needs more iron,

absorption is increased, and when the body is iron

suffi cient, absorption is restricted. This control is not

perfect but is still of great importance for the preven-

tion of iron defi ciency and excess. Iron from food is

Table 9.8 Sodium and potassium content of various foods (mg/100 g
edible portion)

Food Na K

Legumes
Red kidney beans 18 1370
Soyabeans 5 1730
Lentils 12 940

Dried fruit
Raisins 60 1020
Figs 62 970

Nuts
Walnuts 7 450
Almonds 14 780

Fruit and vegetables
Banana 1 400
Melon 5–32 100–210
Potato 11 320
Spinach 140 500

Meat and fi sh
Beef, veal, lamb 52–110 230–260
Chicken 81 320
Herring 120 320
Halibut 60 410
Tuna 47 400
Mussels 290 320

Miscellaneous
Cow’s milk 55 140
Chocolate 11 300

Reproduced from Sánchez-Castillo and James in Sadler et al. Encyclo-
pedia of Human Nutrition, copyright 1999 with permission of
Elsevier.