The plants we eat today are survivors of 3.5 billion years of competition. In the "survival of the fittest", the successful plants have developed a wide array of defenses against bacteria, fungi, wounds, insects and foraging animals. Humans have also evolved to benefit from some of these substances; these are the phytochemicals.
Our ancestors also learned to avoid plants that are poisonous to humans. Substances in edible plants that are harmless or beneficial in small quantities can be toxic in large amounts, so you should never eat huge amounts of any one plant, even if it contains beneficial substances. When you hear that a new phytochemical has been identified, don't rush out and eat huge amounts of that fruit or vegetable, or buy a pill that contains the new substance. Just continue to eat a wide variety of the fruits, vegetables, whole grains, beans and seeds and you'll benefit from ALL of the phytochemicals.
More on phytochemicals
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