The American diet is full of holes. The vast majority of us don't get enough vitamin E. About 50 percent of Americans don't consume the recommended amount of magnesium. And vitamin D shortages are so common that some doctors now recommend testing many people for possible deficiencies.

Those shortages can damage DNA and the structures that support cell growth, possibly leading to premature aging and degenerative diseases including cancer and dementia.

Swallowing a lot of vitamin and mineral pills might seem like the best solution for nutritional shortfalls, but that's usually not the case. Unlike foods, pills can provide toxic quantities of certain nutrients. Moreover, they are turning out to be poor surrogates for a healthy diet.

Observational studies that have linked higher intakes of a nutrient to reduced disease risk are being discredited by more definitive clinical trials that either find no benefit or increased risk in people who take the nutrient in supplemental form. The latest disappointments include certain B vitamins, which a recent study found did not decrease the risk of heart attack in high-risk women, and some antioxidants, which a large review found might actually increase the risk of premature death.

Those results underscore how little we know about how the various components of food interact. "The observational evidence is overwhelming that richly colored foods reduce the risk of cardiovascular disease and cancer," says Irwin Rosenberg, M.D., professor of medicine and nutrition at Tufts University in Boston. "But the idea that we can reproduce that in pills is flawed. It's becoming increasingly apparent that there are a lot-probably thousands-of substances in food that we haven't identified yet."

The ideal diet is varied, rich in fruits and vegetables, whole grains, and low-fat dairy products plus modest amounts of fish and low-fat meat and chicken. That meal plan provides nearly all the nutrients that most people need. For those who have trouble regularly consuming healthful foods-such as dieters who avoid carbohydrates or consume less than 1,200 calories daily or strict vegetarians-a daily multivitamin can help fill the gaps. And older people and those with chronic diseases might eat healthfully but still need higher amounts of certain nutrients.

Here's a rundown of some of the latest research on vitamins and minerals and our experts' take on areas where even conscientious eaters might need help.