"A lot of people are looking at this and asking, 'What happened?'" Dr. Lori Minasian of the National Cancer Institute told USA Today.

Her study in Wednesday's Journal of the American Medical Association found that taking vitamin E or selenium does not ward off cancer.

Diet is one of medicine's most difficult study areas, researchers say.

"Cancer is complex and the interaction of nutrients is complex, so put the two together and you've got a lot of really tough scientific questions to answer," American Cancer Society Director of Nutrition and Physical Activity Colleen Doyle told the newspaper.

While researchers have noted that populations with healthy diets are less likely than others to get cancer, it is hard to determine which foods -- or even compounds -- keep cells from turning malignant, said Peter Gann of the University of Illinois-Chicago, who wrote an editorial accompanying Minasian's and another research study.

In addition, while people who get too little of a nutrient may increase their cancer risk, so may people who get too much, Minasian said.

Health-conscious consumers should instead focus on getting vitamins from plant foods, such as vegetables and whole grains, which contain precise mixtures of hundreds or even thousands of compounds, Gann said.

People should also exercise, maintain a healthy weight and avoid tobacco, Doyle said.