To a nation of dashboard diners and connoisseurs of cubicle cuisine, what could be better than getting your full day's worth of vitamins and minerals in just one serving of food?

But before you polish off that bowl of Total (100 percent of the Daily Value, or DV, of 10 vitamins and minerals), munch that PowerBar Essentials (20 vitamins and minerals) or down that Dilberito, the new frozen burrito from the "Dilbert" cartoon's creator (100 percent of the DV of 23 vitamins and minerals), assess what else you've consumed recently. Eaten alone, any of these is a sound (and tasty) choice. But, especially if you take vitamin/mineral supplements, a steady diet of such nutritional overachievers could be toxic.

For example, 1-2 grams daily of vitamin C (about 17 times the RDA) long term can cause gastric irritation and (rarely) kidney stones. Consistently getting 15,000 micrograms daily (also about 17 times the RDA) of retinol equivalents (vitamin A) can cause nausea and liver damage. Chronic niacin overload can also harm the liver.

Iron-fortified foods are risky if you're one of 1 million Americans with hemachromatosis, says Mark Kantor, Ph.D., associate professor of nutrition/food science at the University of Maryland in College Park. This inherited, potentially deadly liver condition causes the body to absorb too much iron from food. Symptoms (irritability, headache, fatigue, joint diseases, enlarged liver) don't appear until later in life.

Not enough of a good thing

While some people may be in danger of ingesting too many nutrients, most American women don't get enough of several, including iron, calcium, vitamins B6 and E, magnesium and zinc, says the National Center for Health Statistics.

Since the government mandated adding folic acid (a B vitamin that helps prevent neural tube defects in fetuses) to grain products like flour and bread, we seem to be coming closer to getting enough of it. Still, to be safe, women of childbearing age should take a supplement containing 400 micrograms, especially if they're eating a low-carbohydrate diet, says Paul Jacques, Sc.D., lead researcher for a recent folic-acid study at Tufts University in Boston.

Despite our shortcomings, nutritionists don't regard superfortified foods very highly because they don't provide the disease-fighting phytochemicals and other compounds you get from fruits, vegetables and whole grains. "It's OK to eat fortified foods as part of an overall healthy diet, but they're not a substitute for one," says Kantor. Rely on them occasionally, but make eating a balanced diet an everyday thing.