Plant foods contain important, non-nutrient substances that provide colour to attract insects (eg purple anthocyanins in blueberries), protect them from the sun (eg red lycopene in tomatoes) or have an antimicrobial action against fungi or plant viruses (eg flavonoids in apples) and other predators (eg allyl isothiocyanate in horseradish).
While these phytochemicals contribute to the flavour of plant foods, once they are absorbed they can have a variety of beneficial, medicinal actions. Some act as antioxidants to protect against premature ageing, while others influence your metabolism to lower the risk of developing a number of health conditions ranging from high blood pressure and raised cholesterol to diabetes, heart attack and even cancer.
Over 30,000 phytochemicals have been identified, which can be divided on the basis of their chemical structures into polyphenols, carotenoids, phytosterols and sulphur-containing compounds.
The average diet includes around one gram of phytochemicals per day, mostly in the form of polyphenols, but those who eat the most plant-based foods and drink the most tea obtain the greatest amounts.
Polyphenols are a group of plant chemicals with more than one phenol ring in their structure. They form a diverse group that includes tannins, lignans, stilbenes, phenolic acids and flavonoids, of which the latter are the best studied and understood. Examples include capsaicinoids in chilli peppers and limonenes found in citrus fruit and pine nuts.
Flavonoids (sometimes known as bioflavonoids) are antioxidants that protect plants from attack by microbes, insects and UV light. Based on their structure they are further divided into:
- Flavonols (eg quercetin, rutin) of which onions, apples and red wine are good sources
- Flavones (eg apigenin, luteolin) found in parsley, celery and chamomile tea
- Flavanones (eg naringenin, hesperidin) present in grapefruit and oranges
- Flavanols (eg catechin, epicatechin, gallocatechin) found in cocoa, chocolate, pomegranate, green and black teas
- Anthocyanins (eg cyanidin) of which black grapes, blueberries, blackberries and acai berries are rich sources
- Isoflavones (eg genistin, daidzin) found especially in soy bean products and red clover sprouts.
There is growing evidence that regular consumption of flavonoid polyphenols can have a marked effect on human health. Among the most important of these are the soy isoflavones, the green tea catechins and the anthocyanins.
The beneficial effects of some phytochemicals are activated by beneficial digestive bacteria in the intestines. Soy isoflavones, for example, are converted into a more powerful form known as equol.
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