Veggie Virtues

The way they work is varied. There are hundreds of phytochemicals and some are known to be anti-oxidants, which rid our bodies of harmful free radicals and thereby promote health by preventing disease or perhaps even slowing the aging process. "Phytochemicals are like pollution control," says Wahida Karmally, director of nutrition at the Irving Center for Clinical Research at Columbia University and a spokesperson for the American Dietetic Association. "They can neutralize or act like scavengers and remove these substances from harming us, so they can provide help in preventing cell damage or preserving body tissues." Others can prevent cancer by inhibiting the activation of carcinogens, neutralizing their toxic properties and helping the body get rid of them. Some of them are even tissue specific, protecting specific organs in the body.

But they’re also part of the plant’s defense system, which explains their bitter taste. "Because plants don’t like to be eaten, they protect themselves from being eaten. They protect themselves from predators, not necessarily us—rodents, bacteria and so on—by secreting toxic compounds, and this is what the anti-oxidant phytochemicals really are," says Drewnowski. "Their role is to protect the plant from being consumed." In fact, Drewnowski calls them "natural insecticides."

Taste matters

Scientists have long known about the toxicity of these chemicals but the discovery that they’re also beneficial is more recent. Now there’s controversy over how much of them should be in vegetables. For years, the food industry has been breeding them out of veggies to reduce toxicity and bitter taste. But Drewnowski says that this trend may be turning.

Scientists are now experimenting with putting more functional ingredients into foods. For example, researchers have found ways to add more lycopene —the red pigment that helps prevent cancer—to tomatoes. They’ve also found that factors such as the soil and how fruits and vegetables are stored after they’re harvested can make a difference in phytochemical levels. Drewnowski says that broccoli can be genetically modified so that it has more sulforaphane, an anti-oxidant.

But increasing some of these compounds—which are also found in foods other than vegetables—could also increase the bitter flavor. "If we supplement soy flour with extra isoflavones, if we add more polyphenols to chocolate, if we brew coffees with more bitter catechins, will the consumer accept it?" Drewnowski wonders.

"The compounds that the food industry has been taking out are the same compounds that the cancer researchers want put back in," he says. "So this poses a slight dilemma from the view of the consumer, because those things may be more healthy but we know that most choices are made on the basis of taste. Taste alone rules." For instance, glucosinolates are a family of chemicals (sulforaphane falls in this group) which help fight cancer, but they are usually removed from brussel sprouts to make them taste better. So it may not do much good to put them back in if people won’t eat them.

Humans are sensitive to bitter taste much more so than sweet or sour tastes, and this aversion goes back a long way. "Sensitivity to bitter exists not only in humans, but there is very interesting research done by French anthropologists showing that it’s common to all primates," says Drewnowski. At some point in our evolution, being able to detect bitterness may have alerted us to potentially toxic plants.

In addition, some people are more sensitive than others to bitter tastes. Researchers use a substance called PROP (6-n-propylthiouracil) to study taste sensitivity. "It turns out that there are some people who cannot taste it at all, some people who can taste it and find it bitter, and some people who can taste it and find it so bitter that they’re ready to vomit on the spot," says Pam Dalton, a research scientist at the Monell Chemical Senses Center, in Philadelphia, PA. "Obviously if things taste very bitter to you, like vegetables, you’re probably less likely to prefer them than someone for whom the other flavors sort of blend with that and it’s not just the bitter taste that comes through."