Too Much of a Good Thing

Almost all essential nutrients can be toxic if you take in too much of them. That cautionary note likely applies to fruits and vegetables as well. Legendary biochemist Bruce Ames once pointed out that plants evolved to make chemicals that are toxic to insects and other animals that might eat them or to ward off infections by bacteria, yeast, and other organisms. Many of these chemicals are natural carcinogens when tested, but, as Ames pointed out, we have evolved multiple detoxification mechanisms to protect us. Some of these plant-made agents may slip through our defenses. And we have also altered the chemical content of the foods we eat, especially fruits and vegetables, by selective breeding for many characteristics, such as sweetness, that could increase the natural carcinogens.

Could eating too much of some kinds of fruits or vegetables be bad for you?

The answer is yes.

Brussels sprouts

Many people like the edgy bitterness of this cruciferous vegetable. But this bitterness is sometimes a signal of potentially cancer-causing chemicals. In a pooled analysis of cohort studies, showed a modest increase in pancreatic cancer among people consuming Brussels sprouts three times a week. In a separate analysis, the high consumption of Brussels sprouts was also linked to a higher risk of developing high blood pressure. If you think about the unusual shape of the Brussels sprout, the tight packages of leaves that we eat emerge from the stalk, which would usually be covered with bark or spines for protection. The fragile sprouts don’t have anything like that, and so turn to a different defense mechanism: chemical warfare. Given what we’ve found, it makes sense to eat this vegetable not more than once a week while we wait for more data.
 As we dig more deeply into the roles of specific fruits and vegetables, I expect to see more of the unexpected. Plants may seem like simple organisms compared to animals, but their biology is complicated!

Too much spinach

. This green leafy vegetable is a healthy, versatile plant. You can eat it raw in salads, use it as a bed for salmon, or sauté it as a side dish. But spinach is quite high in oxalates. The kidneys can turn these naturally occurring acids into kidney stones. The more oxalates consumed, the higher the risk of developing these painful stones. This doesn’t mean you should avoid spinach. But if you have had a kidney stone, it would make sense to limit spinach to a few times a week and rely on a wider variety of greens, most of which are lower in oxalates. You might also eat cheese or some other dairy food along with spinach because these foods reduce the absorption of oxalates.

Grapefruit juice

This popular juice contains potent compounds that alter the metabolism of many drugs. Depending on the drug, these changes can lead to too much or too little of the drug in the bloodstream. If you take medications and you like to drink grapefruit juice or eat grapefruit, talk with your health care provider about possible interactions.