Just because plants produce toxins doesn't mean humans don't benefit from them. Humans and plants coevolved over millions of years, and it's not unreasonable for the human body to "expect" certain chemical triggers in the environment, and to go awry without them. It could be that plant toxins turn on specific cellular repair machinery which the body needs. This theory of hormesis is young but promising, and attempts to explain the health promoting effects of vigorous exercise, modest stress, resveratrol/sirtuin activation, and mTOR activation.
For example, restricting calories improves life span, but in high doses (zero calories) is fatal. But just because this strategy is bad at the limit doesn't mean it's bad in small does. Similarly, exposure to the sun results in skin cancer in large doses, but produces cancer fighting vitamin D in small doses.
What is the correct dose of plant toxins? We don't know, but it could well be higher than in bred plants, at least for adults. Children generally hate bitter foods and the reason could well be to protect their small livers from overdose.
I suspect that your knowledge on this topic has hit its expiry date.
You are probably thinking of the various studies relating to rats and mice. However the correlation failed when we went to monkeys, and there is no reason to believe that it works in humans. See http://www.nature.com/news/calorie-restriction-falters-in-th... for verification of that.
For example, restricting calories improves life span, but in high doses (zero calories) is fatal. But just because this strategy is bad at the limit doesn't mean it's bad in small does. Similarly, exposure to the sun results in skin cancer in large doses, but produces cancer fighting vitamin D in small doses.
What is the correct dose of plant toxins? We don't know, but it could well be higher than in bred plants, at least for adults. Children generally hate bitter foods and the reason could well be to protect their small livers from overdose.