Introduction to Human Nutrition

210 Introduction to Human Nutrition

generally assumed that the body has no specifi c zinc
reserve and is dependent on a regular supply of the
element.
With essential roles in many fundamental cellular
processes (see below), it is not surprising that whole-
body zinc content is tightly controlled. Zinc in foods
is absorbed via a carrier-mediated transport process,
which under normal physiological conditions appears
not to be saturated. Zinc is absorbed throughout the
small intestine. Proximal intestinal absorption is effi -
cient, but it has a large enteropancreatic circulation;
the net intestinal absorption of zinc is achieved by the
distal small intestine. Zinc is transported in the plasma
by albumin and α 2 -macroglobulin, but only 0.1% of
body zinc is found in plasma. Body zinc content is
regulated by homeostatic mechanisms over a wide
range of intakes by changes in fractional absorp-
tion (normally 20–40%) and urinary (0.5 mg/day)
and intestinal (1–3 mg/day) excretion. For example,
during periods of low zinc intake, absorption is
enhanced and secretion of endogenous zinc into the
gastrointestinal lumen is suppressed. In contrast, high
zinc intake is associated with decreased absorption
and enhanced secretion of endogenous zinc. Within
cells, fl uctuations in zinc content are modulated
by changes in the amount of the metal associated
with the storage protein metallothionein but there
is a large number and variety of zinc homeostatic
proteins found throughout cells. Although zinc
transporters are very important for generating and
maintaining zinc gradients across membranes and
within cellular compartments, little is known about
many aspects of their functions and regulatory modes
of action.
The bioavailability of dietary zinc depends on
dietary enhancers and inhibitors and host-related
factors (Table 9.12). Diets can be roughly classifi ed as

having a low, medium, or high bioavailability, accord-

ing to the content of zinc, phytate, and animal protein.

From a mixed animal and plant product diet, 20–30%

zinc absorption can be expected. The lowest absorp-

tion, 10–15%, is seen from diets prevalent in develop-

ing countries that are based on cereals and legumes

with a high phytate content and with negligible

amounts of animal protein.

Metabolic function and essentiality

Zinc has three major groups of functions in the

human body: catalytic, structural, and regulatory.

Most biochemical roles of zinc refl ect its involvement

in the folding and activity of a large number (up to

10%) of proteins and over 100 different zinc metal-

loenzymes have been identifi ed, including RNA

nucleotide polymerase I and II, alkaline phosphatase

and carbonic anhydrases. Important structural roles

for zinc are in the zinc fi nger motif in proteins, but

also in metalloenzymes [e.g., copper/zinc superoxide

dismutase (Cu/Zn-SOD)]. Zinc is also required by

protein kinases that participate in signal transduction

processes and as a stimulator of transacting factors

responsible for regulating gene expression. Zinc plays

an important role in the immune system and, though

not a redox-active transition metal, is an antioxidant

in vivo.

Defi ciency symptoms

The clinical manifestations of severe zinc defi ciency

in humans are growth retardation, sexual and skeletal

immaturity, neuropsychiatric disturbances, derma-

titis, alopecia, diarrhea, increased susceptibility to

infections, and loss of appetite. Many of these fea-

tures, by and large, represent the dependence on zinc

of tissues with a high rate of turnover. However, severe

zinc defi ciency in humans is rare, and more interest

has been focused on marginal zinc defi ciency. This is

more diffi cult to diagnose and often occurs with other

micronutrient defi ciencies including iron. The current

understanding of zinc defi ciency is largely based on

responses to zinc supplementation. Zinc supplemen-

tation has been reported to stimulate growth and

development in infants and young children, and

reduce morbidity (diarrhea and respiratory infec-

tions) in children, particularly in developing coun-

tries and can increase both innate and adaptive

immunity. In women, low serum zinc concentration

during pregnancy was found to be a signifi cant

Table 9.12 Factors affecting zinc absorption

Increased absorption Decreased absorption

Physiological factors
Depleted zinc status Replete zinc status
Disease state (acrodermatitis enteropathica)

Dietary factors
Low zinc intake High zinc intake
Certain organic acids Phytate
Certain amino acids Certain metals
Human milk