Leaves and stems of plants, green vegetables or leafy greens, are widely consumed by humans. The protein contents are higher than in fruits, and they contain low amounts of sugar.
Some green vegetables produce secondary metabolites that have bitter or astringent properties and may produce toxic alkaloidal and other compounds such as hemoglutenens. Others produce intestinal enzyme inhibitors, such as lectins, which bind to mucosal surfaces and inhibit digestion, especially that of proteins.
Plant secondary metabolism produces a large number of specialized compounds (~ 200.000) that do not aid in the growth and development of plants but are required for the plant to survive in its environment. Specialized compounds from secondary metabolism are essential for communicating with other organisms in mutualistic (e.g. attraction of beneficial organisms such as pollinators) or antagonistic interactions (e.g. deterrent against herbivores and pathogens). They further assist in coping with abiotic stress such as increased UV-radiation.
The broad functional spectrum of specialized metabolism is still not fully understood.
Well known specialized compounds include alkaloids, polyphenols including flavonoids, and terpenoids. Humans use quite a lot of these compounds, or the plants from which they originate, for medicinal and nutraceutical purposes.
A sharp distinction between humans and “animals” is essential if we are to bend them to our will, make them work for us, wear them, eat them–without any disquieting tinges of guilt or regret. It is unseemly of us, who often behave so unfeeling toward other animals, to contend that only humans can suffer. The behavior of other animals renders such pretensions specious. They are just too much like us.
Carotenoids are a class of more than 750 pigments synthesized by plants, algae, and photosynthetic bacteria. These richly colored molecules are the sources of the yellow, orange, and red colors of many plants. Fruit and vegetables provide most of the 40 to 50 carotenoid phytonutrients found in the human diet.
The most common carotenoids in North American diets are α-carotene, β-carotene, β-cryptoxanthin, lutein, zeaxanthin, and lycopene.
Provitamin A carotenoids - α-carotene, β-carotene, and β-cryptoxanthin - can be converted by the body to retinol (vitamin A), but not lutein, zeaxanthin, and lycopene.
Dietary lutein and zeaxanthin help maintain optimal visual function - they absorb damaging blue light that enters the eye.
The results of observational studies suggest that diets high in carotenoid-rich fruit and vegetables are associated with reduced risks of cardiovascular disease and some cancers. But high-dose β-carotene supplements did not.