Flavonoids – Natural antioxidants

Flavonoids have been marketed in recent years as all-natural antioxidants derived from plants.

In most people's minds, antioxidants are “something healthy that helps strengthen our immune system by attacking free radicals”.

Therefore, most people assume that these natural antioxidants must be healthy. However, the fact is that there is little scientific evidence on the effects of flavonoids on human health, and the studies that do exist have generally been small and without clear results.

In this article, we go through all the most important aspects of flavonoids' impact on health, but also what flavonoids are, what types there are, etc.

What are flavonoids?

Flavonoids are a group of phytonutrients that are part of an even larger group of antioxidants called polyphenols. The majority of flavonoids are natural colorants found in plants.

For example, flavonoids are the main colorant in flowers and also the dye that gives flower petals their red and blue colors.

In the past, flavonoids were called vitamin P, but this term is no longer used. This is mainly because flavonoids are chemically not vitamins.

However, flavonoids have several important nutritional effects on human health and it is these effects, among others, that we review in this article.

Which foods contain flavonoids?

Some flavonoids (such as catechin) are very common in the human diet and are found in virtually all plants. Others (such as quercitin) are also found in all plants, but in much smaller amounts. The abundance and variety of flavonoids means that humans and animals consume relatively large amounts of them in their diet. However, some foods are particularly high in flavonoids – these include:

Note that the concentration of flavonoids is very often higher in the peel than in the fruit, nut or plant itself. In peanuts, for example, it is the red peel (or shell) that has a particularly high content of flavonoids.

In addition, it should be mentioned that the flavonoids in cocoa are often removed from commercial products (such as cocoa powder and chocolate – including dark chocolate) because they have a bitter taste. However, the flavonoids are often found in milk chocolate, but some experts believe that the milk in the chocolate interferes with the absorption of the flavonoids, thereby limiting any health benefits that might have been gained.

The effect of flavonoids on health

Before the health benefits of flavonoids began to be recognized, medical science had long wondered why the incidence of cardiovascular disease in general was significantly lower among the Japanese and French than in the rest of the Western population.

When the high flavonoid content of green tea and red wine – which the Japanese and French drink a lot of, respectively – was discovered, it was understood for the first time that there was a link between flavonoid intake and the incidence of cardiovascular disease.

Scientific studies

Although there are now numerous scientific studies on the potential beneficial effects of flavonoids on human health, there is still no evidence that they can be used to boost our health or alleviate some types of health problems.

In recent years, many companies in the health food industry and elsewhere have claimed that flavonoids have almost “miraculous” effects on health, especially their role as antioxidants. But while it sounds great that flavonoids are “natural antioxidants”, the fact is that little is known in medical science about either flavonoids or antioxidants and their effect on human health – and as mentioned, there is no scientific research to support the companies' claims.

Antioxidants

Although the antioxidant properties of flavonoids have been used to argue that flavonoids are beneficial to health, several studies have shown that flavonoids are generally poorly absorbed by the human body and that those that are absorbed are quickly excreted.

The results of the above studies indicate that the antioxidant activity of flavonoids in the body is extremely limited and that the increase in antioxidant activity in the blood seen after consumption of flavonoid-containing foods is not directly caused by the flavonoids, but instead by increased uric acid production due to flavonoid depolymerization and excretion.

Inflammation

Inflammation is involved in a wide range of diseases and is also suspected to be the cause of some systemic diseases and the like. Examples include cancer, cardiovascular disease, diabetes and celiac disease.

Some studies have shown that flavonoids may affect the body's anti-inflammatory mechanisms via their ability to inhibit reactive oxygen and nitrogen compounds. Some studies have also suggested that flavonoids may inhibit the pro-inflammatory activity of enzymes involved in free radical production and alter intracellular signaling pathways in immune cells.

However, not all of these properties are considered scientifically proven and it is not yet known if they can be used in the treatment of the aforementioned diseases (some of the diseases are discussed in more detail below).

Cardiovascular diseases

There are relatively many studies on the effects of flavonoids on cardiovascular diseases and the research so far has shown that flavonoids can:

• Inhibit coagulation, thrombus formation and platelet aggregation
• Reduce the risk of atherosclerosis (hardening of the arteries)
• Reduce blood pressure and the risk of high blood pressure
• Reduce oxidative stress and related signaling pathways in blood vessel cells
• Alter inflammatory mechanisms in the blood circulation
• Improve endothelial and capillary function
• Alter blood lipid levels
• Regulate carbohydrate metabolism
• Alter certain age-related mechanisms

Polyphenols (which includes flavonoids) and their effect on human health have also been studied several times, but researchers do not yet have enough data to use them in the treatment of cardiovascular disease.

Cancer

The clinical studies available on the effects of flavonoids on cancer have shown conflicting results for most cancers. In addition, most studies have been too small and retrospective. The only exceptions are:

• stomach cancer, where flavonoids are thought to reduce the risk in women and
• Smoking-related cancers, where flavonoids should reduce the risk in women as well as men (i.e. all smokers)

Antibacterial properties

It is scientifically proven that flavonoids:

• have antibacterial properties
• can be used in conjunction with antibiotics
• can suppress bacterial virulence factors

However, more clinical studies are needed to determine whether flavonoids can be used to treat bacterial infections in e.g. pharmaceuticals.

Functions of flavonoids in plants

Flavanoids are found in many different plants where they have many different functions. Flavanoids are the main plant dyes in flowers, for example, where they create the blue and red colors seen in flower petals. Flower colors are especially important for attracting insects and birds to spread their pollen.

In addition, flavonoids act as a form of UV filter in plants, i.e. they absorb some of the harmful radiation that comes with sunlight and which can otherwise destroy the plant's DNA.

Flavonoids can also act as signaling substances or physiological regulators. They can also have an inhibitory effect on, for example, plant cell cycles or on foreign pathogenic organisms such as the fungus Fusarium oxysporum.

Types of flavonoids

The chemical structure of flavanoids is generally 15 carbon atoms, consisting of 2 benzene rings and 1 heterocyclic ring. Their structure can be abbreviated as C6-C3-C6. According to the IUPAC nomenclature, they can be classified as follows:

• Flavonoids or bioflavonoids
• Isoflavonoids
• Neoflavonoids

As you can see from the list above, bioflavonoids are just another name for flavonoids – and not a “special form” of flavonoids as some health food companies claim.

Subgroups

More than 5000 naturally occurring flavonoids are known, all derived from different plants. They are classified according to their chemical structure and are usually divided into the following subgroups:

• Antoxantins
• Flavanones
• Flavanols
• Flavans
• Anthocyanins