Based on a large national dietary survey, the researchers found that without fortification, the diets of a large number of children and teens would be nutritionally inadequate. With fortification the picture is better, but not perfect.
"Foods with added nutrients (most notably breakfast cereals, enriched grain foods, fluid milks) supplied important amounts of many but not all vitamins and minerals in diets of U.S. children and adolescents," Louise Berner told Reuters Health in an email.
Berner is a food science and nutrition researcher at Cal Poly State University in San Luis Obispo, California.
Currently, the U.S. Food and Drug Administration requires some fortification of food, such as enriching refined flour with vitamins and iron and adding vitamin A to low- and non-fat milk.
Food manufacturers may also add nutrients to food voluntarily - some brands of orange juice, for example, are fortified with added calcium.
Berner and colleagues wanted to find out both how much of an impact fortification has on kids' nutrition and determine which foods were providing the added nutrients.
The researchers used data from the National Health and Nutrition Examination Survey to analyze the diets of 7,250 children and adolescents ages 2 to 18 years old.
Berner's team looked at the types of food eaten and any supplements taken and assessed the nutrient content of each food. Then they assessed how nutritionally adequate each kid's diet was by seeing whether it met Estimated Average Requirements (EAR).
The EAR is the average daily nutrient intake level estimated to meet the requirement of half the healthy individuals in a particular group based on age or gender.
On average, girls ages 14 to 18 years old were most likely to fall short of the EAR for their age, while boys and girls 2 to 8 years old had the lowest rates of inadequate nutrient intakes.
The study team found that fortified foods contributed half or more of the intakes of vitamin D, thiamin, and folate to children's diets; 20 to 47 percent of the intakes of vitamin A, vitamin C, riboflavin, niacin, B-6, B-12, and iron; 12 to 18 percent of the intake of zinc; but only 4.5 to 6.6 percent of calcium.
Even with the increased nutrients from fortified sources, a substantial percentage of kids still had intakes of vitamins A, C and D that were less than the EAR for their age and sex.
The fortified foods also did not appear to lead to excessive intakes of any nutrients, which is a concern others have expressed in the past, Berner and her colleagues note in the Journal of the Academy of Nutrition and Dietetics.
The fortified foods that provided the most nutrients were breakfast cereals, milk and milk drinks, breads, rolls and other products made with enriched grains.
"This research study provides a good picture of the contribution of fortified foods to kids' diets in the U.S.," Berner said, "but, it should not be misinterpreted as a dietary recommendation to consumers - that was not the intent of the research."
So many unfortified foods, including fruits, vegetables, meats, fish and so forth, are critically important parts of healthful diets yet are often under-consumed, Berner said.
"But, selectively, I think it makes sense - for example, choosing a fortified breakfast cereal instead of an unfortified one," she said.
Berner and her coauthors advise consumers to obtain nutrients primarily from foods that are naturally nutrient-dense. And, they point out, not all fortified foods are healthy foods.
"The trouble with fortification is that while it can increase the 'good,' it doesn't necessarily do anything to decrease the 'bad'," Dr. David Katz told Reuters Health in an email.
Katz is the founding director of the Prevention Research Center at Yale University School of Medicine and medical director for the Integrative Medicine Center at Griffin Hospital in Derby, CT. He was not involved in the new study.
Katz said there are examples of fortification that are very important no matter what the diet is like. Vitamin D in a population that doesn't get a lot of sun exposure is perhaps the best example.
Katz said the paper demonstrates that in a culture that eats very poorly, we need fortification to have adequate nutrient intake.
"But what this paper does not address at all is: what would happen if we actually ate well," he added.
Katz said it's a mistake to think that preventing nutrient deficiencies with fortified "junk" foods is in any way the same as eating truly good foods.
"Eating a variety of wholesome foods would provide those same nutrients, along with many others, and without the sugar, salt, refined starch, unhealthy oils, excess calories and so on," Katz said.