The unique nutrient richness of every whole, natural food can be showcased in a variety of ways. But there is no better way to highlight the unique nutrient richness of foods than to focus on their flavonoid content! Flavonoids are a quite remarkable group of phytonutrients that fall into the chemical category of polyphenols. They're perhaps most famous for their rich diversity of color-providing pigments (including the deep blues of blueberries and rich reds of raspberries). The name of these phytonutrients actually derives from their color-related chemistry, with the Latin word flavus meaning "yellow." As a group, however, flavonoids are highly bioactive and play a wide variety of different roles in the health of plants, animals, and human health.
The flavonoid nutrient family is the largest nutrient family known to scientists. Over 6,000 unique flavonoids have been identified in research studies, and many of these flavonoids are found in plants that are routinely enjoyed in delicious cuisines throughout the world. In terms of nutrient richness, we get far more flavonoids from plant foods than from animal foods, and in particular, vegetables and fruits can be especially nutrient-rich in this type of phytonutrient.
Some of the most widely-studied flavonoids are nutrients you may already have heard about not in particular connection with any specific food—for example, the flavonoid quercetin. Other flavonoids you may have heard about due to their association with a particular food—for example, the catechins in green tea. Sometimes a flavonoid is actually easy to link up with its most nutrient-rich food sources— for example, the tangeritin found in tangerines (as well as other citrus fruits).
Role in Health Support
Because many flavonoids—and especially those belonging to two flavonoid subgroups called flavonols and flavan-3-ols—can be effective in reducing free radical damage to cells and other components in body tissue, they provide antioxidant benefits. It is not clear, however, if we should be thinking about flavonoids as falling into the same category as more widely known antioxidant nutrients like vitamin C or vitamin E.
One reason for this is because their concentration in the bloodstream is so much lower. Another reason lies in the fact that many of the antioxidant functions of the flavonoids are not performed by the flavonoids themselves, but by forms of the flavonoids that have been altered by our metabolism. Even though we do not know all the details about the way flavonoids function as antioxidants, however, studies have documented better protection of certain cell types—for example, red blood cells—following consumption of flavonoid-rich foods. Blueberries, for example, have been repeatedly studied in this context for their flavonoid-related antioxidant benefits.
In this antioxidant context, it is also worth pointing out the potentially unique relationship between flavonoids and vitamin C. Recent studies have shown the ability of flavonoids to alter the transport of vitamin C, as well as to alter function of an enzyme called ascorbate oxidase, which converts vitamin C into a non-vitamin form (monodehydroascorbate). While we do not yet know the full meaning of these relationships, it is clear that the transport and cycling of vitamin C are flavonoid-related. This association makes sense to us, since so many foods high in vitamin C (such as our top five WHFoods for vitamin C are papaya, bell peppers, broccoli, Brussels sprouts, and strawberries) are also high in flavonoids.
Much of the research on flavonoids as anti-inflammatories has involved their ability to block the production of messaging molecules that promote inflammation. In metabolic terms, this activity of flavonoids involves the inhibition of cyclo-oxygenase (COX) and lipoxygenase (LOX) enzymes. Not only have specific flavonoids (for example, quercetin) been shown to provide these benefits but so also have flavonoid-containing extracts from a variety of foods, spices, and herbs. In addition to the metabolic activities described above, food flavonoids have also been shown to suppress inflammatory signaling in another metabolic pathway called the nuclear factor kappa-B (NF-kB) pathway.
Cardiovascular System Benefits
Not surprisingly, since many problems in the cardiovascular system involve problems with oxidative stress and inflammation, the antioxidant and anti-inflammatory benefits from food flavonoids provide direct support for this body system. In the bloodstream, flavonoids have been shown to help protect LDL cholesterol molecules from oxygen-related damage. This LDL protection, in turn, helps to lower risk of atherosclerosis. Flavonoids including rutin and hesperidin have also been shown to increase the strength and integrity of the blood vessel walls, lowering risk of blood vessel problems. In one study, adding a spice mix to a meal of beef—a mix that contained such flavonoid-rich herbs as oregano, rosemary, garlic, ginger, and black pepper—led to a significant improvement in vascular function over the next several hours. Yet herbs and spices are by no means the only foods studied in this regard; similar effects have been demonstrated for soy foods, chocolate, pomegranate juice, and grape juice.
Finally, numerous flavonoids—including quercetin and rutin—have been shown to help prevent excessive clumping together of platelet cells that could otherwise lead to unwanted clogging of the blood vessels. This property of flavonoids is called an "anti-aggregatory" property, and it's yet another way in which these phytonutrients help support the cardiovascular system.
In 2014, a research group looked at cardiovascular benefits related to the flavonoid content of fruits and vegetables. These researchers were able to determine that six total fruit and vegetable servings did a better job at protecting cardiovascular health than four total servings. They also decided upon six total servings of fruits-plus-vegetables as their minimal recommendation for heart health. Many of our daily sample menus at WHFoods go beyond this recommendation and include between 6-10 total servings from these two food groups.
Support of the Nervous System
Protection of nerve cells from oxygen-based damage, and help during the slow and demanding process of nerve regeneration (outside of the brain and spinal cord), are both demonstrated benefits of flavonoid intake for the nervous system. There is some preliminary evidence that the onset of certain chronic neurodegenerative diseases—including age-related dementia and Alzheimer's disease—may be delayed when long-term intake of flavonoids has been strong.
Because flavonoids may help to improve blood flow in the brain, there is also preliminary evidence to suggest the possibility of better brain functioning in some areas, including areas involving cognitive function.
Other Health Benefits
In terms of their anti-cancer potential, research on flavonoids has been somewhat mixed. Due to their well-documented antioxidant and anti-inflammatory properties, flavonoids would be expected to lower risk of certain cancers since chronic oxidative stress and chronic unwanted inflammation can place cells at greater risk of becoming cancerous. Furthermore, because flavonoids are known to modify the body's detoxification pathways, it might be expected that flavonoids would help lower exposure to unwanted toxins that could pose increased cancer risk. In studies on animals and on isolated cell types, the above expectations seem to be fully met, with flavonoid intake improving detoxification, oxidative stress, unwanted inflammation, and initiation of cells into pre-cancerous states. However, in larger scale studies on humans and risk of human cancers, greater intake of flavonoids has not been consistently associated with decreased risk of cancer. To date, the strongest evidence appears to involve breast cancer and lung cancer were decreased risk is a more consistent finding.
We suspect that part of the mixed findings in this flavonoids-and-cancer area might involve the complex nature of flavonoids as a group. For example, it may be the case that certain subgroups of flavonoids are particularly helpful for lowering risk of certain types of cancer. It might also be the case that studies have had trouble accurately quantifying flavonoid intake. There are thousands and thousands of food flavonoids, and yet some studies have only focused on very select examples or limited types of foods.
Improved detoxification is a very likely benefit that we get from strong flavonoid intake; yet, like with the area of cancer risk, research here has been somewhat mixed. When the cells in our body detoxify unwanted contaminants, there are two key steps involved in the process. In a first step (called Phase 1), potentially damaging molecules are made more reactive so that they can be passed on to Phase 2. In this second, Phase 2 step, the activated molecules get neutralized by being combined with a second neutralizing molecule. Flavonoids can impact both steps in detoxification (Phase 1 and Phase 2). With Phase 2, these influences seem fairly consistent because they tend to promote the combining/neutralizing goal of Phase 2. However, with respect to Phase 1, the role of flavonoids is more complicated since they can switch Phase 1 either on or off. In other words, they can both facilitate and block this first step in detoxification. This complicated relationship between flavonoids and detoxification has resulted in some mixed research findings, although overall, most researchers have concluded that strong flavonoid intake modified detoxification in a helpful way and decreases our risk of problems from unwanted toxins.
A final potential health benefit we want to mention is better regulation of cell cycles. Most cells in our body go through stages of activity where they rest, divide, or go into a self-dismantling and self-recycling process called apoptosis. In the health of all our body systems, it is important for these cell cycle stages to stay in balance. Ample intake of food flavonoids appears to promote these cell cycle balances, most likely through regulation of signaling that takes place between cells and their surroundings.
Summary of Food Sources
Flavonoids are best known for their antioxidant and anti-inflammatory health benefits as well as the support of the cardiovascular and nervous systems. Because they also help support detoxification of potentially tissue-damaging molecules, their intake has often, although not always, been associated with decreased risk of certain types of cancers, including lung and breast cancer. However, it is important to note that the amount of flavonoids required to provide the above health benefits is not certain, and there are some conflicting research findings in this regard. You will find more details about the health benefits of flavonoids in the Role in Health Support section of this profile.
Flavonoids are produced by plants, and plant foods are by far our greatest source of these health-supporting phytonutrients. Among all plant food groups, by far it's been fruits and vegetables that have been best studied and most analyzed for their flavonoid content. There is also flavonoid data on nuts and seeds, grains, beans, and legumes, and select other foods and beverages (for example, green and black tea).
It's important to remember that flavonoids are a very large (more than 6,000 have been so far identified) and very diverse group of phytonutrients. The U.S. Department of Agriculture's (USDA) Flavonoid Database actually breaks down its flavonoid analyses into five of the basic flavonoid chemical subgroups, and it analyzes the best food choices in each of these subgroups. We like this approach to understanding the flavonoid content of food because it emphasizes the need to consume a wide variety of flavonoids that includes all of the different types. In keeping with this approach, the charts below will show you our top WHFoods in each of the flavonoid subcategories. The five subcategories shown in the charts below are: (1) flavonols (which include quercetin, kaempferol, myricetin, and isorhamnetin); (2) flavan-3-ols (which include catechins, epicatechin, gallocatechin, and theaflavins); (3) flavones (which include apigenin and luteolin); (4) flavanones (which include hesperetin, naringenin, and eriodictyol); and (5) anthocyanidins (which include cyanidin, delphinidin, malvidin, pelargonidin, peonidin, and petunidin).