The antioxidant power of zinc.

An antioxidant is an agent that possesses the property ofpreventing or slowing down the oxidation process of a substrate, and thus the production of free radicals. Within the body, there is a constant balance between oxidative defences and pro-oxidant species: in other words, antioxidants are continually eliminating free radicals.Although it has been proven that the presence of small amounts of free radicalsare necessary for the proper functioning of metabolism (defence mechanism against microbes, elimination of old or defective cells, etc...), they can become harmful if they are produced in excess.

An imbalance then sets in (excessive production of free radicals or absence/deficiency in antioxidants) and reactive oxygen species (ROS) begin to cause damage to vital components of cells (DNA, proteins, lipids...) with the result being adisruption in the skin's natural ability to regenerate and in cellular communication, apremature ageing of the skinor even in the long term being the cause ofskin cancers.Indeed, free radicals will activate a myriad of signalling pathways which among other things lead to a reduction in collagen production, and the synthesis and activation of matrix metalloproteinases (MMP) responsible for the degradation of connective tissue, which then promotes skin ageing.

Although free radicals are a natural by-product of processes occurring within the body, these incomplete molecules are not solely generated by the body itself. Numerous external factors (pollution, stress, lack of sleep, sedentary lifestyle, UV rays, smoking, alcohol consumption, electromagnetic waves, allergens, etc...) are known to trigger or increase their production, which then manifests as signs of ageing (age spots,skin sagging, fine lines, etc...), a loss of radiance and an increase in skin sensitivity (photosensitivity, redness, irritations…).

Zinc is a crucial micronutrient, found in small quantities within the human body, at less than 50 mg/kg. It is vital for the body's health due to its critical roles in growth and development, bone metabolism, the central nervous system, immune function, and wound healing, which is the focus of this article.

Zinc is a vital cofactor for the function of over 10% of proteins encoded by the human genome, representing no less than 3,000 proteins and enzymes ! Zinc-dependent proteins play numerous indispensable roles within cells, such as transcriptional regulation, DNA repair, cell death, metabolic processing, regulation of the extracellular matrix (ECM), and antioxidant defence.

It should be noted : this trace element cannot be stored by the body. Therefore, to reap its benefits, regular consumption of zinc-rich foods is essential. The food richest in zinc is the oyster, but there are others such as the offal, red meat, wholemeal bread and eggs. The vast majority of zinc present in the human body is stored in skeletal muscles (60%), but reserves are also present in the bones (30%), the skin (5%), the liver and other organs (2 to 3%).

Zinc is particularly important for the skin. The skin contains a relatively high zinc content (about 5% of the body content), mainly associated within the epidermis (50-70 μg/g of dry weight). Due to its abundance in the epidermis, it is observed that a slight zinc deficiency results in rough skin and an alteration in wound healing.

A study has shown that zinc supplementation increases the antioxidant power of plasma, reduces inflammatory cytokines in plasma, and oxidative stress biomarkers in older subjects. Several mechanisms have been highlighted to demonstrate the antioxidant power of this trace element.

1. Zinc competes with iron (Fe) and copper (Cu) ions to bind to cellular membranes and proteins. This consequently limits the action of these pro-oxidant metals (Fe and Cu), which catalyse the production of the radical hydroxyl HO• from H2O2.

2. Zinc binds to the sulfhydryl (SH) groups of biomolecules, thereby protecting them from oxidation.

3. Zinc enhances the activation of proteins, molecules, and antioxidant enzymes such as glutathione (GSH), catalase, and superoxide dismutase (SOD), and also reduces the activities of enzymes promoting oxidation such as inducible nitric acid synthase (iNOS) and NADPH oxidase. Furthermore, zinc inhibits the generation of lipid peroxidation products.

4. Zinc induces the expression of a metal-binding protein, metallothionein (MT), which is very rich in cysteine and serves as an excellent scavenger of ⋅OH radical ions.

Important to note : topical application of zinc results in fewer side effects than internal use, but it is not considered as effective as oral supplementation. However, this does not mean it is completely ineffective, quite the contrary. Topically applied zinc can help the skin defend itself against attacks from free radicals. You can find this compound in various active ingredients such as the zinc PCA, the zinc oxide or even zinc gluconate.

• PRASAD A. S. Zinc is an antioxidant and anti-inflammatory agent: its role in human health. Frontiers in Nutrition (2014).