Chapter 164
Plants Fight Back
In the chapter “Calorie Restriction,” I will show how to harness the beneficial effects of limited food intake to extend life and prevent disease, but long-term dietary restriction is not a realistic health strategy for many people. Because of our powerful, evolution-driven urge to eat, most people find it hard to cut their food intake by even 10 or 20 percent. A more practical alternative would be to activate the stress response to limited food intake in another way. That would be possible with xenohormesis, from the Greek xenos, “stranger,” “foreigner,” “other.” This means that the stress resistance of stressed plants is transferred to the animals that eat them. In other words: Instead of exposing ourselves to the stressor in order to activate our body’s own defenses and strengthen protection against future stressors, we simply let plants take the hits.
Couch potatoes have nothing in common with real potatoes. Plants cultivate the ultimate motionless lifestyle. Since they are stuck in place, they had to develop a completely different method of responding to threats—biochemically. They produce—from scratch—a dizzying variety of chemical compounds to cope with whatever comes their way. If, for example, it gets too hot for us, we can go into the shade, but if it gets too hot for plants, things get dicey—they are the shade!
Plants have had almost a billion years to develop an entire chemical toolkit of protective substances, some of which can play a similar role in our bodies. After all, where do most vitamins ultimately come from? Plants produce them for themselves, and we hijack them for similar cellular tasks in our bodies. There is also a shared set of “vitagenes” that has been preserved over the course of evolution and that encodes a range of repair and maintenance processes—for example, heat-shock proteins that are useful for physical fitness and confer survival advantages. Many nature documentaries marvel at how closely related we are to chimpanzees, but about one fifth of our genes are the same as in the banana, even though it has been more than a billion years since our common ancestor. Nature has by no means reinvented the wheel over and over for important cellular processes—such as basal metabolism and maintaining DNA integrity. Plants and animals are even exposed to some of the same stress.
We are attacked by bacteria, and the same is true for plants and fungi. When bacteria attack a particular fungus, it produces a molecule called penicillin, which it provides to us for free—and when a fungus attacks a particular bacterium, it produces rapamycin to combat fungi by inhibiting the TOR signaling pathway in fungi, plants, and animals, including us. Remember, that is the same enzyme signaling pathway that drives aging and that can be modified to extend life.
When plants are infected, they produce the active ingredient aspirin, which suits us nicely when we are infected ourselves. Plants and humans have similar signaling systems to heal wounds. Plants have to protect their DNA from damage by free radicals, so they brew complex antioxidants that we can take for ourselves instead of reinventing the wheel. In a sense, the vegetable drawer in the refrigerator is something like nature’s pharmacy.
We can simply leave the stress to the plants, because their stress-response molecules astonishingly activate the same protective responses in us as well. A large share of the health-promoting effects of edible plants known so far is attributed to the pharmacologically active substances produced by plants’ sophisticated stress responses—substances we can latch onto. For example, I have often mentioned polyphenols, a class of phytonutrients for which there is a great deal of specialist literature on their health effects. Plants produce polyphenols for their own protection, but we can take them and use them for similar purposes.
Xenohormesis explains how plants stressed by environmental strain produce bioactive substances that can confer survival advantages on those who eat them. For example, strawberries stressed by drought contain more antioxidants and other phytonutrients. Have you ever eaten a wild strawberry? Its flavor cannot be compared with the flat veneer of flavor in cultivated strawberries. The healthiest grapes often grow in relatively dry, infertile soil in blazing sun. Studies show that the nutrient content of common fruits and vegetables can be increased by lack of light, water, or nutrients, by cold stress, or by nibbling vermin. That could explain why organic vegetables are estimated to have 10 to 50 percent more phytonutrients than conventionally grown vegetables. Organic grape juice contains more polyphenols and resveratrol than conventional grape juice. Correspondingly, soup made from organically grown vegetables contains almost six times as much salicylic acid as soup made from non-organically grown vegetables.
When you starve plants, they do the same thing as mammals: They activate self-preservation. So we can burden plants with the task of producing molecules that activate cells’ stress resistance, adjust metabolism, and strengthen resilience against disease. And then we can snatch them up and use them ourselves. The fact that many phytonutrients mimic the physiological effects of dietary restriction is no accident. Plants produce these substances to save their own green ass in times of scarcity. So instead of going hungry, thanks to xenohormesis we can let the plants do the work and use their efforts as a means of doing something for our own health.
Plants Fight Back
The flip side of xenohormesis is that plant compounds themselves can cause hormetic stress, which ultimately makes us stronger. You may remember from the chapter “Oxidation” that the strengthening of our antioxidant and DNA-repair defenses by green tea appears to be a consequence of its mildly pro-oxidative properties. Because it is a little harmful, it benefits us. The many small hits with every sip strengthen our defenses in case a real blow comes along. It is somewhat like the small irritations that become calluses on the hands, which make us less sensitive. The end result? Intervention studies in rodents showed that green tea extended their life span, and observational studies in human populations showed that tea drinkers live several years longer on average.
Do you remember the broccoli story in that same chapter? The one in which the compound sulforaphane in cruciferous vegetables is the most potent natural inducer of Nrf2, a “guardian of health span and gatekeeper of the species’ longevity”? Our body would not ramp up detoxification enzymes in the liver every time we eat broccoli if it did not perceive broccoli somewhere as a threat. It is roughly like applying capsaicin from hot peppers to the skin so that heat receptors are activated and thereby trick the body, which starts sweating and evaporating fluid in order to actually lower core body temperature. The body seems to imagine each little broccoli floret as a tiny medieval club and responds by battening down the hatches. We can benefit from this vigilance against plant compounds and ultimately enjoy a longer life.
It is not surprising that the body is precisely tuned to respond defensively to numerous plant compounds. After all, plants do not want to be eaten. It is assumed that plants form sulforaphane to deter predators through its bitter taste. Allicin, the garlicky compound in garlic, is probably produced for the same purpose. In a Petri dish, at higher concentrations than even garlic lovers can achieve, certain compounds in garlic can be toxic to mammalian cells (from human foreskin, so do not apply raw, crushed garlic to the skin), but at normal consumption levels—levels our bodies have learned to handle over the course of evolution—a subtoxic dose in pasta sauce can trigger the adaptive stress responses that make up garlic’s healthy effect. Whether some of our healthiest plants are in fact mildly poisonous or whether our bodies merely treat them that way, the end result is the same: health through hormesis.