"If you don't get enough protein, you might still get enough calories without getting enough nutrients," she said.
And although Americans may not realize it, the country has a contingency plan for ensuring most people meet the dietary guidelines for many essential nutrients: fortification.
"You don't see overt deficiencies in the U.S. as a population because there is so much fortification -- in cereals, bread, etc.," Slavin said.
Iodized salt, for example, has been available in the United States since 1924, and prevents mental retardation from iodine deficiency. Folic acid is added to flour; it can prevent neural tube defects. Niacin has been added to bread since 1938, which prevents a disease called pellagra (symptoms include dermatitis, dementia and diarrhea).
Even kids who eat a lot of fast food might be protected from malnutrition, Slavin said, because most fast food is high in animal protein.
The main "shortfall" nutrients, according to the U.S. Department of Agriculture, are calcium, fiber, potassium and vitamin D.