While checking out the fresh produce at our community’s marvelous Outdoor Market, I overheard a conversation between two mothers. They wondered why their children refuse to eat some vegetables that most adults like—and they both admitted that they had similarly disliked the same foods when they were children.

I didn’t consider it appropriate to intrude into their conversation, but since they raised a common parental concern, I’ll respond to it here.

Animals have legs, wings or fins that allow them to seek food and escape predators. Conversely, immobile plants have roots that anchor them to one location. Their survival thus depends on leaves and roots that absorb the available sunlight, water, and nutrients they need—and on biological strategies that discourage the predatory nibbling of herbivores.

Herbivores provide some beneficial services for plants, such as when they eat the fruit that encapsulates a seed, and then later excrete the seed at a distance where it can more easily germinate. To protect themselves from destructive nibbling, plants have had to develop defenses, such as bark and hard coverings around fruit, or by producing an excess of what the animals eat so that enough remains to maintain the plant.

Perhaps the most intriguing plant defense is one that helps to explain why children often avoid vegetables that they later enjoy as adults. Many plants (and especially wild plants) contain toxins that vary among plant species. The toxic levels are generally low enough so that they don’t kill the animal, but nibbling herbivores will avoid that plant in the future if the taste is obnoxious and digestion is difficult. Animals typically diversify their diet to get all the nutrients they need, and so this limits the intake of any one toxin, which is to the advantage of both plants and animals.

Toxin production is biologically expensive and somewhat dangerous, so plant tissues generally contain either a high level of toxins or else they grow rapidly. Rapidly growing and easily replaced tissue, such as leaves, would thus be less toxic than the more indispensable plant parts, such as stems and roots. If the plant has to give up something to herbivores in the ecological battle, give them easily replaced leaves. Although we humans eat a variety of leaves, we also eat the more toxic roots and stems (rather than the leaves) of some vegetables, such as carrots, onions, asparagus, broccoli, and cauliflower.

Organisms differ in their susceptibility to toxins. Immature rapidly growing organisms are generally less able to tolerate toxins than mature organisms. Many children thus consider the strong toxin-related odors and flavors of onions, broccoli, and similar vegetables to be obnoxious, while adults consider them spicy, piquant.

While children can often successfully (and perhaps honestly) complain to their parents about the bad taste of certain vegetables, an embryo has a more serious problem. Embryos absorb whatever the mother eats, and a rapidly growing embryo is especially vulnerable to toxins. The embryonic solution apparently is to send a complaining chemical message to the mother that results in what is commonly called morning sickness. The result is that pregnant women are often nauseated by spicy foods and avoid them, to the benefit of the embryo. Profet (1992) reported that women who don’t suffer from pregnancy nausea are more likely to miscarry or bear children with birth defects. Similarly, pregnant women who use high levels of alcohol or other toxic drugs risk disorders in their children, such as fetal alcohol syndrome.

I suspect that most parents don’t realize that some vegetables have low levels of toxicity that their children can detect, but they often intuitively make such vegetables more palatable by masking the sharp flavors, such as by placing sugar on carrots, and cheese sauce on broccoli and cauliflower.

This raises an interesting issue. Do children who most dislike such vegetables have a lower tolerance for the toxins than children who like the vegetables? Can forcing children to eat something that nauseates them result in learned strong negative adult rejection? Should children be taught to put up with a little discomfort if the disliked food provides important nutrient values? Good questions. No simple answers.

Food preferences are obviously more complicated than toxin levels. For example, an earlier column reported that smell is more important than taste in the recognition and selection of many foods, and the common use of herbs and spices enhances this process. Cultural differences in preferences also exist. Many Asians consider the smell of cheese to be obnoxious, and the odors of sauerkraut and pickles similarly attract and repulse people. On the other hand, everyone seems to like the odor of vanilla (which is somewhat similar to breast milk).

Getting children involved in food production and preparation can help reduce their aversion to vegetables. Teachers who develop classroom gardens that include vegetables commonly disliked by students discover that students will sample things that they grew. Involving children in the selection and preparation of home meals similarly encourages the exploration of new tastes.

I had the common childhood aversion to parsley, carefully removing each miniscule piece from the mashed potatoes or rice my mother had made. One day she simply asked me to get some sprigs of parsley out of the garden, but to kneel down and smell them before picking them and to select only the ones with the most pleasant smell. When I brought them to the kitchen, she showed me how to use a knife to mince them, and told me that she wouldn’t put any parsley in the potatoes I ate if I didn’t want any. For the first time in my life I nibbled on a sprig of parsley, and then said she didn’t have to do that. I would eat the parslied potatoes.

Many decades later, we have a large pot of parsley growing on our deck, and I understand how artfully my mother had eliminated my aversion to parsley.