Introduction to Human Nutrition

212 Introduction to Human Nutrition

in chicken. However, in many parts of the world, most
zinc is provided by cereals. In cereals, most of the zinc
is found in the outer fi ber-rich part of the kernel. The
degree of refi nement, therefore, determines the total
zinc content. Wholegrain products provide 30–50 mg/
kg, but a low extraction rate wheat fl our contains
8–10 mg/kg. The bioavailability of zinc can be low
from plant-based diets, in particular from wholegrain
cereals and legumes, owing to the high content of
phytic acid, a potent inhibitor of zinc absorption.

Micronutrient interactions

A decrease in copper absorption has been reported in
the presence of excessive zinc. Data indicate that the
level necessary to impair bioavailability is >40–50 mg/
day; therapeutic levels (150 mg/day) over extended
periods produce symptoms of copper defi ciency. As
mentioned above, iron under certain circumstances
impairs zinc absorption. Animal studies have sug-
gested an interaction between calcium and zinc in

phytate-rich diets, but this has not been confi rmed in

human studies.

9.9 Copper

Copper occurs in the environment in three oxidation

states. Copper (0) metal is used widely in the building

industry (e.g., water pipes, electrical wires) because of

its properties of malleability, ductibility, and high

thermal and electrical conductivity. Brass, an alloy

of copper and zinc, is used for cooking utensils and

musical instruments, and bronze, an alloy of copper

and tin, has been used in castings since early times.

Copper-based alloys and amalgams are used in dental

bridges and crowns, and copper is a constituent of

intrauterine contraceptive devices. Copper com-

pounds are widely used in the environment as fertil-

izers and nutritional supplements and, because of

their microbicidal properties, as fungicides, algicides,

insecticides, and wood preservatives. Other industrial

uses include dye manufacturing, petroleum refi ning,

water treatment, and metal fi nishing. Copper com-

pounds in the cuprous (1) state are easily oxidized to

the more stable cupric (2) state, which is found most

often in biological systems.

The most important copper ores are chalco-

cite (Cu 2 S), chalcopyrite (CuFeS 2 ), and malachite

[CuCO 3 ⋅ Cu(OH) 2 ]. Copper concentrations in soil

vary from 5 to 50 mg Cu/kg and in natural water from

4 to 10 μg Cu/l. Concentrations of copper in water,

however, depend on acidity, softness, and the extent

Table 9.13 Zinc content of some common foods

Food source Description Zn content (mg/100 g)

Liver Raw, calf 7.8
Beef Lean (from six different
cuts)

4.3

Lamb Lean (from six different
cuts)

4.0

Pork Lean (from three
different cuts)

2.4

Chicken Raw, meat only 1.1
Cod, plaice,
whiting

Raw 0.3–0.5

Muscles Boiled 2.1
Oysters Raw 90–200
Crab Boiled 5.5
Eggs Chicken, whole, raw 1.3
Cheese Soft and hard varieties 0.5–5.3
Pulses Raw 0.2–5.0
Wheat fl our Whole fl our 2.9
Wheat fl our White fl our 0.6–0.9
Milk Cow’s (3.9, 1.6 and
0.1% fat)

0.4

Yoghurt Whole milk 0.5–0.7
Green leafy
vegetables

Raw 0.2–0.6

Rice Raw, white, polished 1.8
Potatoes Raw 0.2–0.3

Data from Holland et al. (1995). Reproduced with permission from
HMSO.

Table 9.14 Classifi cation of foods based on zinc energy density

Zinc energy mg Zn/1000 kcal Foods

Very poor 0–2 Fats, oils, butter, cream cheese,

confectionery, soft/alcoholic

drinks, sugar, preserves

Poor 1–5 Fish, fruit, refi ned cereal

products, biscuits, cakes,

tubers, sausage

Rich 4–12 Whole grains, pork, poultry,

milk, low-fat cheese, yoghurt,

eggs, nuts

Very rich 12–882 Lamb, leafy and root

vegetables, crustaceans, beef

kidney, liver, heart, molluscs

Adapted from Solomons, N.W. (2001) Dietary sources of zinc and

factors affecting its bioavailability. Food and Nutrition Bulletin, 22 ,

138–54