Flavonoids are a big thing in cosmetics – but what are they and what do they do? Dr Haran Sivapalan explains.

Green tea, soybeans, red wine and thyme. What do these ingredients have in common (apart from perhaps all being components of a dish prepared by Heston Blumenthal)? The answer is that they all contain flavonoids, a class of over 4000 plant-derived chemicals (or ‘phytochemicals’) which share a similar molecular structure. Structurally, flavonoids are a type of “polyphenol” – a group of molecules that, in plants, play various roles including: aiding reproduction, fighting against infection and protecting against damage from UV (ultraviolet) radiation in sunlight. This latter function is of particular interest – if flavonoids can protect plants from the harmful effects of UV rays, can they do the same to human skin?

UV rays, skin damage and flavonoids

There are two types of UV rays in sunlight that have the potential to damage human skin: UV-A and UV-B rays. UV-A rays have a longer wavelength, are lower in energy and make up 95% of all the sun’s UV radiation reaching the Earth’s surface. Compared to UV-B rays, UV-A rays penetrate the skin more deeply, reaching well into the dermis (the middle layer of skin). While UV-A rays are responsible for tanning, they play a major role in aging of the skin too, causing it become less supple and develop wrinkles. There is also recent evidence that UV-A rays can damage keratinocytes at the bottom of the epidermis (outer layer of skin) and contribute to the development of certain types of skin cancers.

By contrast, UV-B rays have a shorter wavelength and penetrate less deeply – they cannot pass through glass and tend to mainly affect cells in the upper layer of skin, the epidermis. As UV-B rays have more energy however, they are more damaging than UV-A rays.  UV-B rays are predominantly responsible for sunburn. They can also directly damage the DNA of skin cells or alternatively cause indirect damage to cells by producing harmful, highly reactive molecules known as ‘reactive oxygen species.’ As a result of these types of damage, UV-B radiation plays a significant role in the development of skin cancer – particularly malignant melanoma and basal cell carcinoma (BCC).

So, UV rays damage the skin. But how do flavonoids fit into this? In plants, certain flavonoids act to safely absorb UV radiation (particularly UV-B radiation), thereby sparing UV-damage to surrounding cells. Flavonoids are also ‘antioxidants’ – they mop-up (or ‘scavenge’) and reduce the numbers of damaging ‘reactive oxygen species’ molecules. Finally, flavonoids may play a role in repairing damaged DNA.

Given these protective effects in plants, it seems plausible that flavonoids, when applied to human skin, might reduce sunburn, ageing, cancerous changes and other skin damage brought about by UV light.

Do studies in humans back up this hypothesis?

Flavonoids and sunburn

Flavonoids in green tea

Green tea is rich in flavonoids called ‘catechins’ – which includes catechin, epicatechin and epigallocatechin. In addition to being a drink, green tea extract is sold in capsule or tablet form as a dietary supplement and also as a topical cream to be applied directly to the skin. Studies have assessed the skin benefits of green tea flavonoids consumed in all three of these forms: beverages, oral supplements and topical applications.

• Green tea as a beverage

One study of 60 women looked at the effect of drinking 1 litre of green tea per day over a course of 12 weeks. Blood tests throughout the study found that circulating levels of catechins had significantly increased. Drinking green tea everyday, albeit in large volumes, may then be a viable way of getting flavonoids into your body.

When it came to benefits for the skin, the researchers found that, in those who consumed green tea, the skin became less red (erythematous) and sunburnt after being exposed to controlled doses of UV radiation. This, the researchers suggested, demonstrated the photo-protective effect of green tea – it was protecting the skin from the harmful effects of UV rays.

The researchers also found that skin elasticity, moisture, amount of wrinkles and local blood flow to the skin had all significantly improved in those drinking green tea compared to those drinking a placebo.

• Green tea supplements

Of course, drinking large volumes of green tea everyday may prove quite difficult to implement into one’s lifestyle. Popping a green tea pill with breakfast sounds infinitely easier. Studies have shown that swallowing green tea oral supplements does actually succeed in getting flavonoids into the bloodstream. Evidence that this is of benefit to the skin, however, is less convincing.

Researchers at the University of Manchester conducted a study on 50 subjects, half of whom were given a 1080 mg capsule of green tea catechins over the course of 12 weeks. The other half were given a placebo.

The researchers measured something called the “minimal erythemal dose” (MED). Put simply, the MED is the smallest amount of UV radiation needed to make the skin red (erythemal) and show signs of sunburn. Skin which is better protected from UV light (e.g. darker skin) will require higher doses of UV radiation to cause redness and sunburn – it will therefore have a higher MED. Reasoned this way, if catechins were having a significant protective effect, then those taking the green tea capsules would expect a higher MED than those taking a placebo.

After 12 weeks, the researchers found no difference in MED between the groups. This suggests that the green tea capsules were ineffective in protecting the skin from UV rays.

• Topical applications

In theory, applying creams and ointments directly to the skin ought to be a good way of getting flavonoids into the skin. By not ingesting the flavonoids, you avoid what scientists call “first-pass metabolism” – chemical processes in the digestive system and liver that reduce the amount of an active ingredient available for use by the body. Problems arise, however, when it comes to getting the flavonoids to penetrate and be absorbed by the skin. While some studies have shown adequate penetration and absorption of flavonoids applied to the skin, these processes are highly affected by other ingredients in creams and ointments. For example, incorporating the flavonoid epigallocatechin into an ointment causes it to become unstable and poorly absorbed by the skin.

At least five published studies have looked at topical applications containing green tea extract. These typically have involved applying the extract to the skin, exposing the area to UV light and then assessing for signs of UV-damage (redness of skin, elevated markers of inflammation, higher numbers of damaged keratinocytes or ‘sunburn cells.’) Although all these studies reported a beneficial, photo-protective effect, they are also all undermined by very low numbers of test subjects (varying between 8 and 20 people). Larger studies are needed before claiming that topical green tea extracts can protect the skin from UV light.

• Flavonoids in chocolate

Chocolate, particularly dark chocolate, is rich in flavonoids including catechins. While it would be great if studies gave us an added excuse to eat more chocolate, evidence is scant that eating chocolate is beneficial to the skin. One study saw people eat chocolate with a high-flavonoid content over the course of 12 weeks. The minimal erythemal dose (MED) significantly increased after 12 weeks, suggesting greater protection from UV light. Crucially though, there was no proper control group in this study, highlighting the need for higher quality studies before rushing to the nearest chocolatier.

Flavonoids and skin ageing

• Green tea supplements

Stanford University conducted a two-year trial whereby they gave subjects either 250mg oral supplement of green tea extract or a placebo twice daily. Dermatologists monitored the subjects’ skin at regular intervals for signs of ageing: wrinkles, roughness, solar spots (lentigines), changes in pigmentation. At the end of two years, there was no significant difference between the group receiving green tea and the placebo group.

• Topical applications

A study by Dr. Annie Chiu and colleagues at Emory University gave subjects a 10% green tea extract cream, which was applied to the skin, together with a green tea oral supplement. Compared to a placebo group, the researchers failed to observe any significant differences with regards to signs of skin ageing. If there is an elixir of life then, the current evidence suggests that it probably isn’t green tea.

• Flavonoids in soy-extract

Soybeans contain genistein, a flavonoid that can also have an effect similar to oestrogen. This is of interest to dermatologists because a decrease in oestrogen levels after the menopause can contribute towards aging skin. In this context, a Brazilian study that saw 30 post-menopausal subjects receive 100 mg of soy-extract daily over six months, found significant improvements in skin thickness, elasticity and collagen content at the end of the study. The lack of control group, however, makes this a poor quality study.

•  Flavonoids and cancer

Although some animal studies have shown a potential for flavonoids to counteract biochemical processes that cause cancer, there is no evidence to suggest they can help prevent or treat cancer in humans. This includes skin cancer.

A Cochrane Review, widely considered the gold standard when it comes to evidence-based medicine, amassed the results of 51 studies (spanning a total of 1.6 million subjects) on green tea and cancer. It concluded, “there is insufficient and conflicting evidence to give any firm recommendations regarding green tea consumption for cancer prevention.”

Similarly, the Linus Pauling Institute Micronutrient Information Centre states, “to date, there is little evidence that flavonoid-rich diets might protect against various cancers.”

Take home message

While there is some evidence to show that drinking green tea or applying green tea extract to your skin can protect against UV-damage, more research is needed. The evidence base for other types of flavonoids is noticeably bare.

Although supplements and topical applications may increase levels of flavonoids in the skin, the ability to do this varies considerably from product to product. For example, some green tea extract creams may penetrate and be absorbed the skin, whereas others may not. Regardless of whether or not you use flavonoid-containing products, it is advisable to always use sunscreen and take other sun protection precautions before being exposed to sunlight.

Despite a lack of overwhelming evidence for skin benefits, dietary flavonoids have been proven safe to consume on a regular basis. If you enjoy drinking green tea, eating soybeans and cooking with thyme, then you should continue to do so. With regards to green tea specifically, the Cochrane Review suggests a intake of between three to five cups (1200 ml) a day will provide you with 250mg catechins a day; whether those catechins will benefit your skin remains to be conclusively proved. Anyway, as a Brit, I’ll be sticking to my cuppas of English Breakfast Tea!