Eat your vegetables!

From early childhood, the connection between good health and eating vegetables is drilled into us at home, at school and in just about any discussion of good eating habits.

While the positive benefits of eating multiple servings of vegetables each day may be beyond question in theory, what happens in the real world appears far more complicated, according to research published recently by economists at the U.S. Department of Agriculture. Specifically, aggregate data indicate higher intake of vegetables is associated with greater caloric intake.

The researchers’ findings were discussed in an article published in the May edition of Amber Waves, a journal of the Economic Research Service of the U.S.D.A. The article, “Healthy vegetables undermined by the company they keep,” was written by Joanne Guthrie, a nutritionist, and Biing-Hwan Lin, a senior economist, both with the E.R.S.

Specifically, the research focuses principally on potatoes and tomatoes, the two most popular vegetables as measured by total consumption. In both cases, higher consumption is associated with higher overall caloric intake. While the results vary depending on whether the products are eaten at home or away from home, in both cases the effects reflect how the vegetable is most often prepared.

The research study followed another study from 2002 finding that while higher fruit intake was associated with a lower B.M.I., higher vegetable intake was not.

“Total vegetable consumption had no association with body weight,” Ms. Guthrie and Mr. Lin said. “When vegetables were separated into two groups — white potatoes only, and all other vegetables — white potato intake was associated with higher B.M.I. for both adult men and women. Intake of vegetables other than potatoes was associated with lower B.M.I. among women but not among any other age-sex groups.”

This finding suggested that the healthfulness of vegetables was not undermined by french fries alone, a popular food frequently fingered by nutritionists as calorically dense.

Overall intake of vegetables continues to fall shy of government recommendations, the authors said, citing research estimating Americans eat 1.5 cups of vegetables per day, or 50% to 60% of recommended levels. Of the 1.5 cups consumed, half (51%) come from potatoes and tomatoes, and only 10% come from dark green or orange vegetables.

“Some vegetables were eaten in their unadorned state — raw carrot sticks or sliced tomatoes — but most were consumed in prepared forms or as part of mixed dishes,” they said.

In their more recent study, the E.R.S. researchers analyzed how adding more vegetables — in forms that actually are consumed by Americans — affects calorie intake, sodium intake and fiber intake.

“Researchers held the total amount of food consumed as measured in weight (grams) constant; as more vegetables were eaten, the amount of other foods consumed was lowered,” the researchers said. “For the analysis, they examined four categories of vegetables: potatoes, tomatoes, dark green and orange vegetables, and all other vegetables.”

 “Contradicting what might have been expected from a shift to a higher vegetable diet, researchers found that calorie intake increased, with effects varying by type of vegetable and where the food was prepared,” the researchers said. “Eating an additional cup of potatoes in the forms prepared at home resulted in an increase of 88 calories, whereas eating an additional cup of potatoes prepared away from home increased calorie intake almost twice as much. This reflects the predominance of fried potatoes when eating out.”

The researchers noted potato chips are the most popular form of eating potatoes at home.

More surprising to the researchers was the impact of tomatoes on caloric intake.

“For tomatoes, the difference in the calorie increase from foods prepared at home versus away from home was even larger (than for potatoes),” they said. “An additional cup of tomatoes from home foods increased calories by 59 versus a 364-calorie increase from away-from-home foods. This may be attributable to the popularity of calorie laden combination foods like pizza as a tomato source. According to U.S.D.A.’s MyPlate guidance, a piece of pepperoni pizza provides ¼ cup of vegetable, but also contains approximately 400 calories, making it a high-calorie source of vegetables.”

The researchers suggested different thinking about vegetables and behavioral changes will be required.

“When considering how increased vegetable consumption can support efforts to prevent obesity and reduce sodium intake, thinking about the typical forms in which Americans eat vegetables is important. Rather than simply eating more of their current favorite forms of vegetable-containing foods, Americans will need to add vegetables in forms that come with fewer added calories and sodium,” the researchers said.