Zinc is a trace mineral that is essential for plants, animals and human beings. It is available only from the diet.
Relatively large amounts of zinc are deposited in bone and muscle but these stores are not readily available to the rest of the body.
The skin, with 20% of body zinc, has a more easily accessible supply of the mineral and acts as an immediate reservoir. The body pool of zinc has a rapid turnover rate which explains the prompt appearance of deficiency signs.
The first of these is depressed cell growth and repair which suggests an essential role for zinc in general metabolism. Total body content of zinc in a person weighing 70 kg is about 2.3 g.
In a mixed diet, about 70% of zinc is derived from meat, fish and poultry. Most of the zinc consumed in plant products comes from cereals. Drinking water contains only negligible quantities of zinc.
Meat, liver, eggs and seafood (especially oysters) are good sources of available zinc, whereas wholegrain products contain the element in a less available form.
There appears to be a link between dietary protein and zinc. High protein intakes increase zinc requirements and improves zinc utilisation. As a general rule, the higher the protein level of an unrefined food, the more zinc it provides.
Zinc is more efficiently absorbed from foods containing proteins of animal origin (meat, fish, poultry, eggs, milk) than from plant foods such as cereals and soya beans.
The amino acid cysteine, abundant in animal-derived foods, is believed to enhance absorption of the mineral. This is why amino acid chelated zinc is better absorbed than conventional zinc salts. Zinc in vegetable products is of the non-haem variety.
Factors in the diet that reduce zinc availability are:
- Phytic acids widely distributed in cereals, legumes, nuts, tubers and some fruits.
- Textured Vegetable Protein (TVP) is high in phytic acid and despite the fact that it supplies good levels of zinc, the mineral is poorly absorbed from this source.
- Dietary fibre which contains substances in addition to phytic acid which bind to zinc and prevent its absorption.
- High intakes of wholegrains, cereals, fruits, vegetables and nuts will supply much dietary fibre but the adverse effects of both this and phytic acid can be reduced to some extent by cooking.
- Polyphosphates, which are added to many processed and frozen foods, will bind to zinc and render it insoluble.
- Ethylenediaminetetraacetic acid (EDTA) is added to some foods and this will chelate with zinc to produce a complex that cannot be absorbed.
- Food processing and refining removes zinc from wholesome foods. For example, production of white flour from wholemeal flour causes 77% loss in zinc; refining unpolished brown rice to produce white rice causes 83% reduction in zinc levels; processing cereals from whole grains causes 80% loss of the mineral.
The signs and symptoms of dietary zinc deficiency in human beings include loss of the sense of taste, loss of appetite, growth retardation, skin problems and a weakened immune system.
Zinc deficiency during pregnancy in animals may lead to physical and mental disorders in the offspring.
Cases of zinc deficiency causing retarded growth and sexual development in men have been reported from the Middle East.
Studies in the USA indicate that some sectors of the population suffer zinc deficiency. In all cases, supplementary zinc and improved diet normalised development.
In people with low plasma zinc levels, a slow rate of wound healing has been accelerated simply by giving extra zinc.
Marginal zinc deficiency in apparently healthy children gave rise to sub-optimal growth, poor appetite and impaired taste acuity. Doubling zinc intakes brought marked improvement.
Supplementation of infant formulae to increase zinc levels resulted in increased growth rates in male but not female infants.
It is claimed that white spots on the nails are indicative of zinc deficiency.
Zinc deficiency has been noted in the following conditions:
Crohn’s disease, ulcerative colitis, coeliac disease, cystic fibrosis and other inflammatory conditions of the intestinal tract
High carbohydrate, low protein diets
Long-term kidney dialysis
Diseases of the pancreas, liver and kidney
Stress following surgery, accidental injury and burns
Some types of anaemia, e.g. thalassaemia, sickle cell anaemia
Use of oral contraceptives
Treatment with high dose steroid therapy for prolonged periods
Mild skin conditions
Skin ulcers resistant to healing
Zinc is needed to release vitamin A from liver stores.
Excessive iron supplementation depresses zinc absorption.
High zinc intakes depress blood copper levels.
High zinc intake reduces iron absorption.
High cadmium intake impairs zinc absorption.
There are reactions between calcium and zinc in the bone where zinc is deposited in the soft protein matrix before mineralisation with calcium.
Increased deposition of calcium in bone leads to increased intake of zinc by that bone. When calcium is released from the bone, so is zinc. Zinc is required by the enzyme alkaline phosphatase, which is needed for calcification of bone.
Excessive intakes of zinc can lower body levels of iron.