Introduction

Zinc is an essential mineral (and trace element) that is found in almost every cell. It stimulates the activity of approximately 100 enzymes, which are substances that promote biochemical reactions in the human body. Zinc supports a healthy immune system, is needed for wound healing, helps maintain the body's sense of taste and smell, and is needed for DNA synthesis. Zinc also supports normal growth and development during pregnancy, childhood, and adolescence.

Food Sources

Zinc is found in a wide variety of foods. Oysters contain more zinc per serving than any other food. Other good food sources include red meat, poultry, shellfish, beans, nuts, certain seafood, whole grains, fortified breakfast cereals, and dairy products. Zinc absorption is greater from a diet high in animal protein than a diet rich in plant proteins.

Zinc bioavailability (the fraction of zinc retained and used by the body) is relatively high in meat, eggs, and seafood because of the relative absence of compounds that inhibit zinc absorption and the presence of certain amino acids (cysteine and methionine) that improve zinc absorption. Phytates, which are found in whole grain breads, cereals, legumes and other products, can decrease zinc absorption. The zinc in whole grain products and plant proteins is therefore less bioavailable. The enzymatic action of yeast reduces the level of phytic acid in foods; therefore, leavened whole grain breads have more bioavailable zinc than unleavened whole grain breads.

Some important food sources of zinc:

Oysters	Alfalfa	Liver	Pumpkin seeds	Corned beef
Roast beef	Lamb	Crab	Pork cutlets	Sardines

Recommended Dietary Allowance (RDA)

The European Union RDA for the general population is set at 15 mg/day.

Because a sensitive indicator of zinc nutritional status is not readily available, the RDA for zinc was based on a number of different indicators of zinc nutritional status and represents the daily intake likely to prevent deficiency in nearly all individuals in a specific age and gender group

Inhibitors/stimulators:

The following food components have been found to stimulate the absorption of zinc.

Cysteine, Methionine – these amino acids increase zinc absorption.

The following food components have been found to inhibit the absorption of zinc.

Phytates – decrease the absorption of zinc.

Iron – High doses of iron supplements taken together with zinc supplements on an empty stomach can inhibit the absorption of zinc. When taken with food, supplemental iron does not appear to inhibit zinc absorption. Iron-fortified foods have no effect on zinc absorption.

This interaction is of concern in the management of iron supplementation during pregnancy and lactation and has led some experts to recommend zinc supplementation for pregnant and lactating women taking more than 60 mg/day of elemental iron.

Calcium – High levels of dietary calcium may possibly impair zinc absorption in humans. Calcium in combination with phytic acid reduces zinc absorption. This effect is particularly relevant to individuals consuming a diet that is highly dependent on tortillas made with lime (calcium oxide)

Functions in the Body

Numerous aspects of cellular metabolism are zinc-dependent. Zinc plays important roles in growth and development, the immune response, neurological function, and reproduction. On the cellular level, the function of zinc can be divided into three categories: 1) catalytic, 2) structural, and 3) regulatory.

Catalytic role of zinc

Nearly 100 different enzymes depend on zinc for their ability to catalyze vital chemical reactions. Zinc-dependent enzymes can be found in all known classes of enzymes.

Structural role of zinc

Zinc plays an important role in the structure of proteins and cell membranes. A finger-like structure, known as a zinc finger motif, stabilizes the structure of a number of proteins. For example, copper provides the catalytic activity for the antioxidant enzyme copper-zinc superoxide dismutase (CuZnSOD), while zinc plays a critical structural role.  The structure and function of cell membranes are also affected by zinc. Loss of zinc from biological membranes increases their susceptibility to oxidative damage and impairs their function.

Regulatory role of zinc

Zinc finger proteins have been found to regulate gene expression by acting as transcription factors (binding to DNA and influencing the transcription of specific genes). Zinc also plays a role in cell signalling and has been found to influence hormone release and nerve impulse transmission. Recently zinc has been found to play a role in apoptosis (gene-directed cell death), a critical cellular regulatory process with implications for growth and development, as well as a number of chronic diseases.

Deficiency

Zinc deficiency at such is rare, but most often occurs when zinc intake is inadequate or poorly absorbed, when there are increased losses of zinc from the body, or when the body's requirement for zinc increases (at 14-16 years). Zinc deficiency contributes to many health problems, which can be very serious if zinc deficiency is severe.

The consequences of zinc deficiency are several and they impact on human health severely. Growth retardation, male hypogonadism, neuro-sensory changes (abnormal dark adaptation and changes in taste acuity), delayed wound healing, abnormal immune functions, and impaired cognitive functions are some of the major effects of human zinc deficiency which are reversible with zinc supplementation. A mild deficiency of zinc in pregnant women is associated with increased maternal morbidity, abnormal taste sensation, prolonged gestation, inefficient labour, atonic bleeding, and increased risks to the foetus.

Toxicity

Zinc toxicity from a normal diet is highly unlikely.

Isolated outbreaks of acute zinc toxicity have occurred as a result of the consumption of food or beverages contaminated with zinc released from galvanized containers. Signs of acute zinc toxicity are abdominal pain, diarrhea, nausea, and vomiting. Single doses of 225 to 450 mg of zinc usually induce vomiting. Milder gastrointestinal distress has been reported at doses of 50 to 150 mg/day of supplemental zinc. Metal fume fever has been reported after the inhalation of zinc oxide fumes. Profuse sweating, weakness, and rapid breathing may develop within 8 hours of zinc oxide inhalation and persist 12-24 hours after exposure is terminated.

Regulation

Zinc is found in highest concentration in the liver, with lesser amounts found in the pancreas, kidney, and pituitary gland. Zinc absorption occurs primarily in the small intestine. Zinc-binding ligand molecules act to transport zinc across the mucosal cells of the intestine, where it is picked up by albumin molecules for transport to the liver and other organs.

Excess zinc is excreted by the kidneys.