Zinc in foodstuffs and beverages

With a total amount of about 2 g per person, zinc and iron are quantitatively the most important trace elements for humans, easily forming coordinative bonds with amino acids. As zinc has similar physical/chemical properties to copper, this frequently leads to antagonistic interactions between the metals.

Physiological importance of zinc

The effects of zinc are based on its involvement in more than 300 enzymatic reactions, in which it acts as a co-factor on the one hand and as an integral component on the other. When it is present in the active centre of enzymes, it increases their reactivity. Many electron-transferring enzymes - especially oxidoreductase - make use of the ability of zinc to form various stages of oxidation.

Zinc-dependent enzymes can, among other things, be found as part of the intermediate metabolism of carbohydrates, lipids, amino- and nucleic acids, as well as porphyrine biosynthesis. Some metabolic enzymes (e.g. carboxypeptidase A and B, aminopeptidase, alkaline phosphates) as well as dehydrogenase, which is required to break down alcohol, require zinc to function. As it forms part of enzyme catalysis, this trace element is also involved in the anti-oxidative mechanisms.

In addition, zinc is involved in the regulation of the acid/alkaline balance. Synthesis of the retinol-binding protein (RBP) in the liver depends on zinc, as does the retinol dehydrogenase reaction in the visual cycle. The immunomodulatory effects of zinc deserve special mention. Ultimately, the synthesis of testosterone and thus the development and maturing of the male sex organs and spermatogenesis also take place with the involvement of zinc.

Zinc and nutrition

Lean red meat, innards and hard cheese are good sources of zinc. Full grains, pulses, nuts and seeds also have a high zinc content, while vegetables, fruit and products made with superfine flour only contain small amounts of this trace element. Human zinc needs are mainly covered by animal products, as the availability of zinc in food depends on the presence of absorption-impeding and -inducing factors, as is the case with iron. The content of fibre and complexing agents (such as phytate) in plants, for example, means that zinc from vegetable-based food products is poorly absorbed.

In contrast with iron, the body's zinc storage capacity is relatively low. No large amounts of this trace element can be mobilised when required - also the zinc located in bones is not available in the short term. A continuous supply of zinc via nutrition is therefore essential.

Supply recommendations

Zinc requirements are mainly the result of obligatory losses, which are lower in women (1.6 mg/d) than in men (2.2 mg/d). To ensure a constant supply, men are recommended to have a daily zinc intake of 10 mg, while women should have 7 mg. This value increases to 10 or 11 mg/d during pregnancy and lactation. For children up to 15 years, the recommended dose - depending on age and sex - is between 3 and 9.5 mg/d.

Vegetarians have increased zinc requirements, as the availability from vegetable food sources is very low. The availability of zinc is strongly limited, especially in the case of lacto-(ovo-)vegetarians, as this type of nutrition is not only high in phytate and fibre, but also contains considerable amounts of casein, which has an additional adverse effect on absorption.

Inadequate zinc supply

Mild to moderate symptoms of a zinc deficit are mainly found in risk groups such as multimorbid seniors, alcoholics, as well as patients suffering from gastrointestinal diseases (e.g.  chronically inflamed intestine, short intestine syndrome, malabsorption syndrome) or HIV infection.

The immune system is very sensitive to even a slight lack of zinc. In children, a marginal deficit results in an increased susceptibility to infections. Other symptoms of a slightly inadequate supply are dermatitis, a weak immune system, hair loss and delayed wound healing.

Excessive supply of zinc

Zinc intoxications are very rare and tend to go unnoticed. Acute poisoning may occur after eating food stored in containers made of zinc-based materials. Poisoning takes the form of abdominal pain, vomiting, a feeling of anxiety, as well as headaches and fever. The individual dose required to trigger these symptoms is approximately 2 decimal powers above the daily recommended intake. The upper value for a safe long-term supply of zinc from both food and supplements (tolerable upper intake level UL) is deemed to be 25 mg zinc per day.

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