## 2 Bioactive Compounds Commonly Found in Green-Colored Fruits and Vegetables
### Flavonoids  
#### 2.1.1 Flavonols: Kaempfer, Quercetin, and Rutin  
General Characteristics and Food Sources  
Kaempfer, quercetin, and rutin are natural flavonoids, a type of flavonoid, found in a variety of plants and plant-derived foods (Figure 1). Kaempfer can be easily found in beans, broccoli, kale, spinach, and tea [33], whereas quercetin is found in onion, asparagus, and berries, as listed in Table 1[34]. Lastly, rutin is predominantly found in apples, buckwheat, citrus fruits, cranberries, mulberry, red wine, and tea [35; 36]. In addition, rutin derivatives have been tested as a common dietary flavonoid that is consumed in fruits, vegetables, and plant-derived beverages and is metabolized by gut microflora to a range of phenolic compounds [37].  
As listed in Table 2, kaempfer is a tetrahydrooxyflavone in which four hydroxy groups are located at positions 3, 5, 7, and 4\({}^{\prime}\), C\({}_{15}\)H\({}_{10}\)O\({}_{6}\) (MW 286.23 g/mol). Quercetin belongs to flavonoids, one of the six subclasses of flavonoid compounds with the molecular formula C\({}_{15}\)H\({}_{10}\)O\({}_{7}\) (MW 302.24 g/mol). Rutin, generally termed vitamin P, with the molecular formula of C\({}_{27}\)H\({}_{30}\)O\({}_{6}\), is considered to be one of the most therapeutically active phytochemicals. The difference between quercetin and kaempfer is that the latter lacks an OH group at the 3\({}^{\prime}\) position. Quercetin, an aglycone with no attached sugar, exists as a brilliant citron yellow needle crystal, poorly soluble in water but soluble in alcohol and lipids. Quercetin glycosides are formed by attaching a glycosyl group, such as glucose, rhamnose, or rutinose, by replacing one of the OH groups, usually at position 3. Thus, dietary quercetin is present mainly as various O-glycosic forms, including quercetin-3-O-rutinoside (rutin), quercetin-3-O-glucose (isoquercetin), and quercetin-3,4\({}^{\prime}\)-O-diglucoside.  
Bioavailability  
Pharmacokinetic studies of kaempfer, a highly polar glycoside, have shown that this polyphenol is poorly absorbed compared with the moderately polar aglycone [38]. The plasma concentrations of quercetin and kaempfer were quite similar. These results may be due to better clearance of plasma kaempfer conjugates from the kidney [39]. In general, kaempfer is metabolized to the methyl, sulfate, or glucuronide forms, but the oral bioavailability of kaempfer is extremely low and absorption is poor [40; 41]. The absorbed kaempfer undergoes metabolic transformation by intestinal conjugation enzymes to yield the glucurono/sulfoconjugates (methylated or nonmethylated) forms in the liver [42] and small intestine [39]. Kaempfer and its glycosides are metabolized by the bacterial microflora that releases aglycones and broken aglycone C3 ring to form compounds, such as 4-methylphenol, phloroglucinol, and 4-hydroxyphenylacetic acid, which are either absorbed and can reach systemic circulation and tissues or be excreted in feces and urine.  
Moreover, quercetin has low bioavailability. The expected absorption rate of quercetin glucoside, the naturally occurring form of quercetin, ranges between 3% and 17% of healthy individuals consuming 100 mg, and this low bioavailability may be due to low absorption, extensive metabolism and/or rapid clearance [43]. Quercetin normally exists asquercetin glycoside in plants and contains a quercetin aglycone conjugated to a sugar such as glucose or rutinose. Quercetin glycosides show different absorption rates depending on the type of attached sugar [44]. Furthermore, quercetin glucoside is absorbed more efficiently than rutin [43]. In human studies, the requirement of the colonic microbiota for the hydrolysis of rutinoside may explain the low bioavailability of rutin compared with quercetin-3-glucoside [45].