Flavonoids are one of the largest nutrient families known to scientists, and include over 6,000 already-identified family members. Some of the best-known flavonoids include quercetin, kaempferol, catechins, and anthocyanidins. This nutrient group is most famous for its antioxidant and anti-inflammatory health benefits, as well as its contribution of vibrant color to the foods we eat. As an especially delicate group of nutrients with respect to cooking heats, flavonoids are often front and center in development of our cooking methods at WHFoods, where we always look for cooking methods best able to preserve nutrients. Due to unusually incomplete research information, we do not list values for specific flavonoids in our in-depth nutritional profiles for our foods, nor do we rank our foods based on their flavonoid content. However, in the Summary of Food Sources section of this article, you can find best food sources for five basic categories of flavonoids (including flavonols, flavan-3-ols, flavones, flavonones, and anthocyanidins).
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 one of the largest nutrient families 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 tangeretin found in tangerines (as well as other citrus fruits).
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.
We will also be providing you with more detailed information about flavonoids and specific foods in our Summary of Food Sources section.
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 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 is 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.
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.
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.
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 where 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 seems 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.
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, epicatechins, gallocatechins, and theaflavins); (3) flavones (which include apigenin and luteolin); (4) flavonones (which include hesperetin, naringenin, and eriodictyol); and (5) anthocyanidins (which include cyanidin, delphinidin, malvidin, pelargonidin, peonidin, and petunidin).
| flavonols | flavan-3-ols* | flavones | flavonones | anthocyanidins |
|---|---|---|---|---|
| onions | apples | parsley | oranges | blueberries |
| apples | bananas | bell peppers | grapefruit | bananas |
| romaine lettuce | blueberries | celery | lemons | strawberries |
| tomatoes | peaches | apples | tomatoes | cherries |
| garbanzo beans | pears | oranges | pears | |
| almonds | strawberries | watermelon | cabbage | |
| turnip greens | chili peppers | cranberries | ||
| sweet potatoes | cantaloupe | plums | ||
| quinoa | lettuce | raspberries | ||
| garbanzo beans |
*It's important to note that in the U.S. the largest single source of flavonoids is black and green tea, and that over half of all flavonoid intake comes from the flavan-3-ol subgroup that is so concentrated in tea; this subgroup includes catechins, epicatechins, gallocatechins, and theaflavins.
As you can see, it takes a variety of foods from a variety of different food groups to give you a good cross-section of flavonoid subcategories. The USDA estimates that in the U.S., daily total flavonoid consumption by the average adult is approximately 250-275 milligrams, with about half of total consumption coming in the form of flavan-3-ols from black and green tea.
The colorful reds, blues, and purples in berries are provided by their anthocyanidins, and that is why you find so many of these fruits listed in the anthocyanidin column.
As a group of phytonutrients, flavonoids emphasize—in a way that is not as well emphasized by perhaps any other nutrient—how valuable fruits and vegetables are to our nourishment and everyday health.
You will lose some flavonoids from plant foods during prolonged storage. For example, onions stored at room temperature will lose about one quarter to one third of their original flavonoid content over six months, with most of the loss occurring in the first two weeks.
As water-soluble nutrients, flavonoids can be lost through water contact, and in some cases, up to 80% of specific flavonoids can be lost into cooking water during the boiling of foods. Because many flavonoids provide visible colors in a food, loss of flavonoids during boiling can often be seen in a dulling of the food's colors. Color changes of this kind are one of the indicators we use for overcooking; if you boil or steam a food long enough to see its vibrant colors start to dull or disappear, you can be sure that you are losing too many valuable nutrients from the food, including its health-supportive flavonoids.
Flavonoids are susceptible to damage by heat, and as mentioned earlier, they are also susceptible to damage over prolonged periods of time. This issue of time brings us to the benefits of fresh fruits and vegetables, which are likely to be more flavonoid-rich the fresher they are at the time of purchase. The issue of heat is one of the reasons we caution against frying or lengthy cooking even in medium heats. (Our Healthy Sauté method, for example, typically calls for cooking times of 5-8 minutes or less.)
Finally, we would note that flavonoids are often concentrated in the skins and outer portions of fruits and vegetables, and that these portions of the foods are excellent to consume. Due to risk of contamination on these outermost surfaces, you always want to wash the foods and gently scrub them with a vegetable brush. Of course, you can also reduce risk of contamination by purchasing certified organic foods. When you are storing flavonoid-rich foods, it is best not to damage their skins prior to storage, for example, by pre-cutting, pre-slicing, or pre-peeling and then placing in the refrigerator. They are best kept in whole, natural form until you are ready to consume them or prepare them for inclusion in a recipe.
As mentioned earlier, average consumption of flavonoids in the U.S. is far less than 1 gram per day (at about 250-275 milligrams), with black and green tea serving as the number one source of these phytonutrients. While we would classify this level of intake to be inadequate from a health standpoint, we don't have a good standard to use in determining optimal flavonoid amounts. There is no Dietary Reference Intake (DRI) for flavonoids and no Daily Value (DV); in addition, nutrient databases do no provide anywhere close to comprehensive information about the overall flavonoid content of foods. All of these limitations make statements about dietary deficiency somewhat tentative. However, from a common sense standpoint, let's say that a person consumed six vegetable and four fruit servings in a day, for a total of 10 vegetable-plus-fruit servings. Furthermore, let's say that all of these servings came from whole, natural foods. In this situation, a person's total flavonoid intake would be likely to fall somewhere near 1 gram (1,000 milligrams) or more. It is within this context that we consider average flavonoid intake in the U.S. to be inadequate. In addition, since a disproportional amount of U.S. flavonoids come from a single flavonoid subgroup (flavan-3-ols provided from black and green tea), there is likely to be flavonoid deficiency from the other subgroups given the pattern of flavonoid consumption in the U.S.
Risk of dietary deficiency for flavonoids is basically synonymous with low dietary intake of whole, natural foods, and in particular, low intake of vegetables and fruits. By far your best way to ensure ample flavonoid intake is to maximize your intake of whole natural foods, including fresh, brightly colored vegetables and fruits whose flavonoid pigments provide them with their vibrant colors. This approach sounds simple, and it is a great method for increasing your flavonoid intake.
Most documented risks for flavonoid deficiency have already been discussed since they involve poor dietary intake. Overconsumption of processed foods, overcooking of foods, and underconsumption of fresh vegetables and fruits are the primary circumstances related to deficiency. Problems with the chewing of fresh foods can increase a person's flavonoid deficiency risk, especially if these foods are avoided in a meal plan due to chewing problems. Lack of appetite can also put a person at risk of deficiency, simply due to overall low intake. In studies of the overall U.S. population, inadequate intake of nutrients—including flavonoids—can be associated with poverty and general lack of access to fresh foods.
As described earlier, a unique relationship exists between flavonoids and vitamin C. Flavonoids affect the transport of vitamin C around the body, and they also help regulate the 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 these nutrients have a special and unique relationship. The uniqueness of their relationship makes sense to us since so many foods are high in both flavonoids and vitamin C. Our top five WHFoods for vitamin C—namely papaya, bell peppers, broccoli, Brussels sprouts, and strawberries—are great examples since each of these foods is rich in flavonoids as well.
We are not aware of any evidence that dietary flavonoids can be directly toxic, even in meal plans that contain an abundance of fresh vegetables and fruits as well as an abundance of nuts, seeds, beans, legumes, and whole grains. When consumption of the foods above is very high, the total fiber content of the diet usually goes up dramatically. (In comparison to average fiber intake in the U.S., which averages about 16 grams per day, fiber intake in countries with high consumption of the foods above often exceeds 100 grams. (Even our Healthiest Way of Eating Plan averages over 50 grams of daily fiber.) So we would expect high flavonoid intake from whole natural foods to accompany diets high in dietary fiber, and we just do not see toxicity risks being associated with this type of dietary intake.
In addition, since flavonoids are water-soluble, we would expect them to follow a pattern associated with other water-soluble nutrients. That pattern involves lower risk of toxicity than is associated with fat-soluble nutrients, and in many cases, a decision by the National Academy of Sciences (NAS) not to establish a Tolerable Upper Intake Level (UL) for water-soluble vitamins like vitamin B1 or vitamin B2 when obtained from food. We suspect that a similar decision might end up holding true for flavonoids as well, although it's important to remember that the NAS has yet to even establish flavonoids as a required human nutrient or to set Dietary Reference Intake (DRI) amounts for flavonoids as a group or for any specific flavonoid.
As described earlier, there are no specific public health recommendations for flavonoid intake. There are currently no Dietary Reference Intakes (DRIs) from the National Academy of Sciences and there is no Daily Value (DV) from the U.S. Food and Drug Administration. However, as described earlier in our Risk of Dietary Toxicity section, our recommendation for optimal flavonoid intake is to focus on a whole, natural, fresh foods diet that provides ample servings of vegetables and fruits. In many of our sample daily meal plans, the total vegetable-plus-fruit servings add up to 5-8 servings or more. When coupled with other flavonoid-rich foods—including nuts, seeds, beans, legumes, and whole grains—your flavonoid intake is likely to far surpass the current U.S. average level of approximately 250-275 milligrams, and may in fact get closer to a level of approaching 1 gram (1,000 milligrams).