Polyphenols are a diverse group of naturally occurring compounds found almost exclusively in plants. These substances are responsible for functions within the plant, from providing pigmentation to protecting against environmental stressors like ultraviolet radiation and pests. There are thousands of distinct polyphenols, each with a unique chemical structure. They are not considered essential nutrients for humans in the way vitamins or minerals are, as they do not directly provide energy, but their significance lies in their complex interactions within the body.
Types of Polyphenols
The world of polyphenols is organized into four primary categories based on their chemical structures: flavonoids, phenolic acids, stilbenes, and lignans. Each of these main groups contains numerous subgroups, further highlighting the diversity of these plant compounds.
Flavonoids are the largest group of polyphenols, with over 6,000 types identified. This category is broken down into subgroups, including flavonols, flavanones, and anthocyanins. Quercetin, a prominent flavonol, is found in onions and apples, while anthocyanins are the pigments that give berries their deep colors.
Phenolic acids represent about a third of the polyphenols in our diet. These compounds are abundant in many plant-based foods and beverages. Well-known examples include caffeic acid in coffee and ferulic acid in whole grains like oats and wheat. They also contribute to the flavor and aroma of many foods.
The stilbenes group is less common in the diet but includes the well-known polyphenol resveratrol. This compound is associated with grapes and red wine, where it is concentrated in the skins. Lignans are another class, found in high concentrations in seeds, particularly flax and sesame seeds, and are structurally distinct.
Dietary Sources of Polyphenols
Polyphenols are widespread in a diet rich in fruits, vegetables, and whole foods. The concentration is often highest in the outer layers of plants, such as the skin of an apple or the bran of a grain. Because of this, the way a food is processed can influence its final polyphenol content.
Flavonoids are readily available from a colorful assortment of foods. For example, anthocyanins are abundant in blackberries and red cabbage, while onions and kale contain flavonols. Dark chocolate is a source of flavanols, and citrus fruits are rich in flavanones.
Phenolic acids are found in many common beverages and foods. Coffee and tea are significant sources, along with fruits like apples and plums. Whole grains and certain vegetables also contribute to the intake of these compounds.
Sources for other polyphenols are more specific. The stilbene resveratrol is found in the skins of red grapes and peanuts. Lignans are most densely concentrated in seeds, with flaxseed and sesame seeds being potent sources.
How Polyphenols Work in the Body
Once consumed, the mechanisms of polyphenols are complex. A primary function is their antioxidant capacity, where they help neutralize unstable molecules called free radicals. An excess of free radicals leads to oxidative stress, a state that damages cells and is linked to various health conditions. By mitigating this damage, polyphenols help maintain cellular health and a healthy balance of free radicals in the body.
Beyond their antioxidant activity, polyphenols can influence cellular signaling pathways involved in inflammation. While inflammation is a natural response to injury, chronic inflammation can contribute to long-term health issues. Some polyphenols interact with proteins and enzymes that regulate the body’s inflammatory response.
The gut microbiome plays a large role in how polyphenols work. Most polyphenols are not absorbed in the small intestine but travel to the colon. There, gut bacteria break them down into smaller, more active metabolites that can be absorbed. This interaction is a two-way street, as polyphenols also influence the composition of the gut microbiota.
Factors Influencing Polyphenol Absorption
The amount of polyphenols we consume does not directly translate to the amount our body can use, a concept known as bioavailability. This efficiency varies greatly among different polyphenols and is influenced by several factors, making their effects highly individual.
The food matrix, or the physical and chemical structure of a food, plays a large role. The presence of fiber can slow their release, while fats can enhance the absorption of certain types. Food preparation is another factor, as cooking can make some polyphenols more available, while degrading others.
An individual’s physiology is also a factor, including digestion efficiency and existing health status. The composition of a person’s gut microbiome is particularly important. Since gut bacteria break down most polyphenols into absorbable forms, the specific microbes in a person’s colon can alter what compounds are ultimately produced and absorbed.