Kale: why is it used in skincare?

Oxidative damage caused by reactive oxygen species (ROS) on lipids, proteins, and nucleic acids can trigger various chronic diseases. However, kale has demonstrated a significant anti-radical activity , thus helping to maintain an adequate antioxidant status within cells.

Indeed, kale is a good source of antioxidants that can work together to reduce the level of ROS. Vitamin C, Vitamin E (α-tocopherol and γ-tocopherol), phenolic compounds (ferulic acid, caffeic acid, etc.) and carotenoids (lutein, zeaxanthin) are the main antioxidants in kale, due to their high content.

More specifically, theascorbic acid (vitamin C) is capable of trapping superoxide and hydroxyl radicals, as well as regenerating theα-tocopherol endogenous (vitamin E). As for polyphenolic compounds, they trap free radicals by forming stabilised chemical complexes, thus preventing their subsequent reactions, thanks to their electron-donating properties which depend on the number and location of hydroxyl groups (-OH).

A study has also demonstrated their ability to inhibit the production of nitric oxide (NO), thereby reducing the formation of ROS and promoting neovascularisation. The antioxidant action of carotenoids is based on their ability to trap singlet oxygen and neutralise peroxyl radicals. Finally, the predominant reaction responsible for the antioxidant activity of vitamin E is the donation of hydrogen atoms.

Consequently, kale extract protects cells against functional and morphological oxidative damage, and can therefore prevent and slow down premature skin ageing as well as delay the appearance of wrinkles.

Several studies have demonstrated that in the dermis, the collagen/elastin ratio, as well as their structures, change with age. These compounds play a significant role in the skin's resilience and elasticity. However, a 2017 study revealed that 29 female volunteers, aged between 40 and 56, who received a daily systemic dose of a kale extract supplement rich in carotenoids (totalling 1,650 µg of carotenoids), experienced a significant increase in collagen levels in the skin areas of the cheek and forearm, 10 months after the start of the study compared to the placebo.

Thus, the results obtained demonstrate that oral supplementation of a kale extract, containing a mixture of carotenoids, could potentially prevent the degradation of collagen I associated with dermal ageing. Similarly, the high content of vitamin K in the kale extract could also protect the skin from premature ageing. In the study, the topical application of vitamin K was found toincrease cell proliferation and collagen synthesis.

However, further studies aimed at elucidating the exact mechanism of action of kale on this property will be necessary to confirm it. But the question that now arises is whether a topical application of kale extract produces the same effects on the skin.

It has been reported that kale also contains a high content of phylloquinone (vitamin K). This latter is an essential cofactor to facilitate the post-translational conversion of specific glutamic acid residues into γ-carboxyglutamic acid in several proteins, particularly some factors involved in the blood coagulation system. Thus, it would facilitate the removal of extra-vascular blood from the skin, a beneficial action to accelerate the disappearance of blueish dark circles under the eyes.

Studies have reported that phenolic acid fractions extracted from kale leaves, rich in quercetin and kaempferol derivatives, have the ability to inhibit the growth of both Gram-positive bacterial strains (Staphylococcus aureus, Enterococcus faecalis, Bacillus subtilis), and the Gram-negative bacterial strain Moraxella catarrhalis, as well as fungi. Its mode of action is the inactivation of cellular enzymes and the alteration of membrane permeability, which depends on their chemical structure.

Organic compounds containing sulphur (glucosinolates) constitute another significant group of bioactive secondary metabolites with antimicrobial action in kale. Indeed, glucosinolates hydrolysed enzymatically generate certain specific products that affect the growth of various microorganisms. They have the unique ability to act in synergy with conventional antibiotics, thereby enhancing their effect. However, the authors have not suggested a mechanism of action. This effect could potentially be exploited to reduce the proliferation of bacteria associated with skin problems.

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• DARVIN M. E. & al. Influences of orally taken carotenoid-rich curly kale extract on collagen I/elastin index of the skin. Nutrients (2017).

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