The Effect of Flavonols on Health and Exercise


What are flavonols?
Flavonols are a non-nutrient, bioactive compound within the flavonoid family (1, 2). Flavonoids are 3-ringed polyphenolic compounds that are studied for a link between their consumption and a reduced risk of chronic diseases (1). Flavonoids may help inhibit the oxidation of low-density lipoproteins (LDL) cholesterol which can be a precursor to atherosclerosis, the plaque buildup which can contribute to heart disease (3). Flavonoids continue to be examined by scientists due to their association with beneficial health effects including reduced risk of cardiovascular disease, protection against LDL cholesterol oxidation and a possible, but conflicting, link to cancer prevention (4).

Research surrounding flavonols typically focuses on three types — quercetin, myricetin and kaempferol. Quercetin is the most ubiquitous flavonol and has been inversely associated with some organ-specific cancers (4, 5). In addition, quercetin supplementation has been studied for its possible effects on physical performance (6, 7).
Where are flavonols found?
In the human diet, the major sources of flavonols are fruits, vegetables, tea and red wine (2). Flavonols are pale yellow in color and found in larger amounts in onions, broccoli, kale, apples, tea and buckwheat (5). In food, flavonols are the most widespread group of flavonoids with quecertin being the most prominent (8). In studies, quercetin intake makes up about 75% of the total flavonol intake of 13-64 mg/day (8).

Growing and processing may influence the concentrations of flavonoids in foods (1). Flavonoids are produced in direct response to environmental conditions such as carbon dioxide levels and ultraviolet light. This means there are differences in flavonoid concentrations in similar foods depending on the region and time of year in which they are grown (1). During the summer, leafy vegetables like lettuce, endive and leeks have been found to have three to five times more flavonoids than in other seasons (2). Tomatoes and berries have been found to have seasonal variation in flavonoid content, but foods such as red cabbage and pears are not influenced by the seasons (2). Flavonols are found in greater concentrations in foods with a high skin to volume ratio, like cherry tomatoes (2). High accumulations have been found in the leaves and skin of foods such as red grapes, Spanish cherry tomatoes and apples (2). Flavonol composition may change significantly when berries and grapes ripen (2). Flavonoids are resistant to heat but oxygen, moderate degrees of acidity and preparations, especially peeling, skinning and trimming, may cause some losses (2). Boiling results in the greatest reduction of quercetin likely due to the leaching action (2). As a result of the extraction process, fruit juice has been shown to have higher concentrations because flavonoids are released from the rind (2). The particle size’s influence on flavonoid concentration is demonstrated when the tea made from tea bags have more flavonoids than tea from loose tea leaves (2). In addition, increased brewing time allows for greater flavonoid extraction from black tea (2).

The U.S. Food and Drug Administration (FDA) has GRAS (generally recognized as safe) status criteria established for quercetin (7, 9). Quercetin may be incorporated into supplemental chews, food bars or beverages (8, 10, 11). Because it is difficult to ingest sufficient quantities of quercetin from the typical diet, supplements are used when studying its effects (7).
How do flavonols benefit health?
Flavonol intake has been associated with reduced risk of coronary heart disease (CHD) mortality, stroke, hypertension and reduced C-reactive protein (CRP) concentrations (a biomarker for chronic inflammation) (3, 12-14). A 2003 analysis of seven studies found that high intakes of dietary flavonols from fruits, vegetables, tea and red wine may be associated with a reduced risk of CHD mortality (3). Flavonol intake from food and beverage sources ranged from two to over 34 mg with tea being the primary source. In the study, the third of individuals with the highest flavonol intake had a 20% lower risk of CHD mortality versus those in the lowest third, independent of other dietary components or known CHD risk factors. Accuracy of flavonol intake was a limiting factor in this study due to self-reported food intake and potential variations of flavonol content in similar foods (3). A 2012 analysis of nine studies found no signification association between the highest flavonol intake and reduced risk of CHD (15). Men and women with the highest total flavonoid intake had an 18% lower risk of fatal CVD compared to the lowest intake (16).

When examining the association between flavonol intake and risk of stroke, a 2010 study found that compared with a low intake of flavonols, a high intake was inversely associated with fatal and nonfatal stroke (12). Although there were limitations to the study, those with a high intake compared with a low intake had a 20% lower risk of stroke incidence (12). Total flavonoid intake has been associated with a lower risk of fatal stroke in men while other studies have found no association between total flavonoid intake and stroke risk in women (13, 16). A 2008 study found that high intakes of flavonols were associated with lower risk of ischemic stroke but not CVD mortality (17).

Flavonol intake has been inversely associated with serum C-reactive protein concentrations (14). Quercetin and kaempferol were two of the flavonoids with the strongest significant inverse association with CRP (14). Suggested beneficial qualitites of quercetin include cardioprotective, anticarcinogenic, antiapoptotic and ergogenic characteristics (7). Since quercetin is found in small quantities in the typical diet, quercetin supplements have been of interest to military and athletic populations for their ergogenic potential (7).
How do flavonols affect exercise and physical performance?
Quercetin has been linked to increasing physical endurance in mice and humans although results in humans have been mixed. Quercetin has a caffeine-like psychostimulant effect which may enhance mental and physical performance (9). In opposition, a study examining this psychostimulant effect found no improved endurance with a 2 g single-dose of quercetin in the heat (9, 18). A study examining the effect of quecetin supplementation (one gram per day for three weeks) on ultramarathon runners found no effect on ratings of perceived exertion (RPE) or endurance performance during a self-paced, 160-km Western States Endurance Run (WSER) compared to the group receiving the placebo supplement (8). A possible reason for the lack of effect is that the race was self-paced whereas in laboratory trials, participants are cycling or running at the fastest speeds possible. Three weeks of 1000 mg/day quercetin supplementation had no impact on cycling efficiency or fuel utilization in well-trained, but not elite, cyclists during three consecutive days of exercise (19). A study found young, untrained adult males achieved a significantly greater distance during a 12-minute time trial after consuming a quercetin supplementation beverage containing 1000 mg/day quercetin for two weeks versus a placebo (11). During this trial, oxygen consumption, heart rate, respiratory rate and RPE did not differ between the quercetin and placebo groups (11). Compared with studies in trained athletes indicating no improvement with quercetin supplementation, the untrained subjects demonstrated a performance benefit (11). Trained male cyclists that supplemented 1000 mg/day quercetin for three weeks before and during a three day period of intensified exercise had no significant differences in oxidative stress, inflammation or plasma antioxidant capacity compared to the placebo group (20). In a study evaluating the effects of quercetin supplementation on soldier performance, soldiers consumed 1000 mg/day of quercetin or a placebo with energy bars for 8.5 days (6). This amount of quercetin for a little over one week had no positive effect on aerobic performance (6). Young female swimmers supplemented with 1000 mg/day of quercetin over 8 weeks demonstrated no improvement in exercise performance compared to the placebo group (21).

In a small study examining the effect of five days of 1000 mg/day quercetin supplementation compared to a placebo on VO2max in healthy, untrained, sedentary men and women, there was no significant interaction with VO2max, RPE at VO2max, or ratings of delayed-onset muscle soreness (10). In athletic male students, an increase in VO2max was seen after supplementation but the improvement was not significant (22). An analysis examining several studies on quercetin supplementation and endurance found a trivial to small benefit on human endurance exercise capacity (7). The analysis showed a 3% improvement in both VO2max and endurance exercise capacity with quercetin supplementation.

Most studies examined a 1000 mg/day quercetin supplementation for a duration between one and eight weeks before testing without demonstrating performance improvements (6, 8-10,19-22). While some studies have found very small but significant improvements in endurance exercise capacity, others have failed to find significant improvements (7, 2011; 6-11, 19-22). Some studies looking at trained athletes have not found a significant improvement with quercetin supplementation while studies among sedentary non-athletes have demonstrated a performance benefit (6, 8-11, 19-22). It is possible that longer supplementation periods or higher doses may yield different results but recommendations cannot be developed without further research. At this time, there is not enough evidence to support quercetin supplementation for performance enhancement. As a flavonol, quercetin may provide other health benefits and should be consumed as part of a well-balanced diet that incorporates flavonol-containing foods (Table 1) such as onions, broccoli, kale, apple and tea.
Table 1. Flavonol content of popular foods

Food

Flavonol

Mean mg/100g, edible portion

Apple, skin only

Quercetin

19.36

Apple, raw, with skin (Malusdomestica)

Kaempferol

0.14

Myricetin

0.00

Quercetin

4.01

Apple, Red Delicious, raw, with skin

Myricetin

0.01

Quercetin

3.86

Blackberries, raw

Kaempferol

0.27

Myricetin

0.67

Quercetin

3.58

Blueberries, rabbiteye, raw

Kaempferol

2.36

Myricetin

2.92

Quercetin

14.42

Cranberries, dried, sweetened

Kaempferol

0.01

Myricetin

2.40

Quercetin

4.50

Cranberries, raw

Kaempferol

0.12

Myricetin

6.63

Quercetin

14.82

Elderberries, raw

Kaempferol

0.58

Quercetin

26.77

Goji berry, dried

Kaempferol

6.20

Myricetin

11.40

Quercetin

13.60

Plum, black diamond with peel, raw

Kaempferol

0.01

Myricetin

0.01

Quercetin

12.45

Plums, purple, raw

Quercetin

2.19

Raisins, golden seedless

Kaempferol

2.71

Quercetin

2.40

Raspberries, raw

Kaempferol

0.06

Myricetin

0.00

Quercetin

1.05

Strawberries, raw

Kaempferol

0.50

Myricetin

0.04

Quercetin

1.11

Arugula, raw

Kaempferol

34.89

Quercetin

7.92

Asparagus, cooked, boiled, drained

Quercetin

15.16

Broccoli, raw

Kaempferol

7.84

Myricetin

0.06

Quercetin

3.26

Kale, raw

Kaempferol

46.80

Myricetin

0.00

Quercetin

22.58

Onions, raw

Kaempferol

0.65

Myricetin

0.03

Quercetin

20.30

Onions, red, raw

Kaempferol

0.70

Myricetin

2.70

Quercetin

39.21

Chia seeds, raw

Kaempferol

12.30

Quercetin

18.42

Wine, table, red, Syrah or Shiraz

Quercetin

2.11

Cocoa mix, powder

Quercetin

2.03

Tea, black, brewed, prepared with tap water

Kaempferol

1.41

Myricetin

0.45

Quercetin

2.19

Tea, green, brewed

Kaempferol

1.31

Myricetin

1.02

Quercetin

2.49

Table adapted from: U.S. Department of Agriculture, Agricultural Research Service. 2013. USDA Database for the Flavonoid Content of Selected Foods, Release 3.1. Nutrient Data Laboratory Home Page: http://www.ars.usda.gov/nutrientdata/flav


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