Is there more to a carrot than beta carotene? Is lycopene the best we get from tomatoes? And when we heap our plates with salmon, are we serving up something other than omega-3s?

For years the scientific community has viewed individual vitamins and nutrients as the best that food has to offer. Nutrition studies have isolated beta carotene, calcium, vitamin E and lycopene, among other nutrients, in order to study their health benefits in the body.

But now, after several vitamin studies have produced disappointing results, there’s a growing belief that food is more than just a sum of its nutrient parts. In a recent commentary for the journal Nutrition Reviews, University of Minnesota professor of epidemiology David R. Jacobs argues that nutrition researchers should focus on whole foods rather than only on single nutrients. “We argue for a need to return to food as the source of nutrition knowledge,” writes Dr. Jacobs with co-author Linda C. Tapsell, a nutrition researcher at the University of Wollongong in Australia.

Dr. Jacobs believes that nutrition science needs to consider the effects of “food synergy,” the notion that the health benefits of certain foods aren’t likely to come from a single nutrient but rather combinations of compounds that work better together than apart. “Every food is much more complicated than any drug,’’ said Dr. Jacobs. “It makes sense to want to break it down. But you get a lot of people talking in the popular press about carbohydrates and fats in particular as if they were unified entities. They’re not. They’re extremely complicated.’’

The narrow focus on the health effects of single nutrients stems from the earliest days of nutrition research. In 1937, two scientists won a Nobel Prize for identifying vitamin C as the essential component in citrus fruit that prevents scurvy. The finding spurred interest by the scientific community to study other biologically active nutrients in foods.

For as long as observational studies have shown that diets rich in fruits and vegetables, unsaturated fat and fish, among other things, are associated with better health, nutrition researchers have been busily deconstructing these foods to identify the most potent nutrients. For example, vitamin E has been widely studied as a heart protector.

But attributing the broad health benefits of a diet to a single compound has proven to be misguided. Several studies have suggested an association between diets rich in beta carotene and vitamin A, for instance, and lower risk for many types of cancer. But in a well-known 1994 Finnish study, smokers who took beta carotene were found to have an 18 percent higher incidence of lung cancer. In 1996, researchers gave beta carotene and vitamin A to smokers and workers exposed to asbestos. But the trial had to be stopped because the people taking the combined therapy showed markedly higher risks for lung cancer and heart attacks.

Since then, studies of other vitamins, notably vitamins E and B, have also failed to show a benefit. Manufacturers say the problem is that vitamins are too often examined in sick people while the real benefit may be in preventing disease. But Dr. Jacobs notes that the better explanation may simply be that food synergy, rather than the biological activity of a few key nutrients, is the real reason that certain diets, like those consumed in the parts of the Mediterranean and Japan, appear to lower the risks of heart disease and other health problems.

“People ask me what vitamins they should take,’’ said Dr. Jacobs. “I say ‘Don’t take any. Just make sure you have a nutrient-rich diet.’ ’’