Flavonoids in fruits and vegetables

Hertog, M.G.L., Holhnan, P.C.H. and Venema, D.P. (1992) Optimization of quantitative HPLC determination of potentially anticarcinogenic flavonoids in fruit and vegetables. /. Agric. Food. Chem., 40, 1591-1598. 

Figure 5.2. Typical flavonoids in the subgroups commonly found in fruits and vegetables.

In addition to the flavonoids with ring B attached to the C2 position of ring C, other types of flavonoids, particularly those with ring B attached to C3 (isoflavones), C4 (neoflavonoids), and open C ring (chalcones), have also been commonly found in fruits and vegetables. 

Different fruits and vegetables vary significantly in their structural constituents, macronutrients (proteins, oils, and carbohydrates), and micronutrients such as flavonoid profiles. It is almost impossible to develop one optimal method for extraction, separation, and analysis for each and every different fruit or vegetable. However, because of the relatively similar chemistry and biochemistry of flavonoids, some general statements can be abstracted from the existing literature. Flavonoids of fruits and vegetables... 

Erlund, L, Silaste, M.L., Alfthan, G., Rantala, M., Kesaniemi, Y.A., and Aro, A., Plasma concentrations of the flavonoids hesperetin, naringenin and quercetin in human subjects following their habitual diets, and diets high or low in fruit and vegetables, Eur. J. Clin. Nutr., 56, 891, 2002. 

The supplementation with an oral dose of 1 g of quercetin for 1 month to healthy normotensive subjects has been recently compared to placebo, showing no differences in selected cardiovascular risk factors including blood pressure [102]. However, these data do not exclude an antihypertensive effect of quercetin in patients with essential hypertension. In fact, a diet rich in fruit and vegetables (and presumably rich in flavonoids) lowered blood pressure in hypertensive but not in normotensive subjects [156,157]. 

Flavonoids constitute a large class of polyphenols found in fruits and vegetables that share a common skeleton of phenylchromane. This basic structure allows a large number of substitution patterns leading to several subclasses of flavonoids, such as flavonols, flavones, flavanones, flavanols, anthocyanidins, isoflavones, dihydroflavonols, and chalcones. Among the diverse flavonoid subclasses, flavonols (especially quercetin) and flavanols (catechins) are the most abundant in our food. Flavonols are present in foods as diverse glycosides, whereas flavanols are usually found as aglycones. 

Flavonoids are a family of antioxidants found in fruits and vegetables as well as in popular beverages such as red wine and tea. Although the physiological benefits of flavonoids have been largely attributed to their antioxidant properties in plasma, flavonoids may also protect cells from various insults. Nerve cell death from oxidative stress has been implicated in a variety of pathologies, including stroke, trauma, and diseases such as Alzheimer s disease and Parkinson s disease. 

The observed associations between folate, antioxidant vitamins, and cardiovascular disease may be confounded by other substances in fruits and vegetables, as the following examples of studies show Flavonoids (see Chapter 31) are naturally occurring, water-soluble antioxidants found widely distributed in vegetables, fruits, tea, and wine. There is an inverse relationship between flavonoids and decreased risk of coronary heart disease. Lycopene, the key antioxidant in tomatoes, shows an inverse association with myocardial infarctions. There is an inverse association between folate and cardiovascular disease. 

There are abundant and diverse flavonoids with carbohydrates and lipids, alkaloids (betalain alkaloids and other alkaloids), phenols (chromones, cou-marins, lignans, quinines, and other phenolics), terpenoids (monoterpenoids, sesquiterpene lactones, triperpenoid saponins, carotenoids, and other terpenoids), and minerals as micronutritional phytochemicals in fruits and vegetables of our daily diets. Among these phytochemicals, the flavonoids have specific functionality in relation to age-related diseases such as hypertension, diabetes, cardiac infarction, cataracts, and cancer. The authors of each chapter in the first section have presented their evidence in relation to the mechanism of the preventative and therapeutic ability of the compounds. 

Higher intakes of fruits, specifically superfruits and certain vegetables, are also associated with improved bone mineral density and reduced bone loss in both men and women. Clinical trial results indicate that a diet rich in fruits and vegetables preserves bone health. Other studies link bone health to superfruit phytochemicals such as carotenoids, flavonoids, quercetin, and vitamin C. For example, in the Framingham Osteoporosis Study conducted over four years at Tufts University, in Boston, loss of bone mineral density in hundreds of elderly women and men was inversely related to intake of dietary carotenoids—a phytochemical found in superfruits ... 

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