THURSDAY, Sept. 24, 2015 (HealthDay News) -- When Mom told you to "eat your vegetables," she probably didn't mean eat the same two vegetables -- potatoes and tomatoes -- most of the time.
But according to a new government report, that's exactly what many Americans are doing.
The report, from the U.S. Department of Agriculture, finds that potatoes now make up 30 percent of the vegetables grown and processed for Americans each year, while tomatoes make up 22 percent.
Toss in lettuce (7 percent), and these three vegetables comprise 59 percent of the vegetables grown for and distributed to Americans, according to 2013 USDA data.
This could spell problems in terms of nutrition, the agency said, because people may not be getting the full range of nutrients offered by a wide variety of vegetables.
One nutrition expert agreed.
"Variety is so important in a healthy diet across the board," said registered dietitian Dana Angelo White. "While focusing on only a small number of fruits and vegetables is better than nothing, it still makes it difficult to meet your needs for essential nutrients."
A diet highly focused on potatoes, especially, can be harmful, said White, who is assistant clinical professor of athletic training at Quinnipiac University in Hamden, Conn.
"If the majority of potato consumption comes from processed sources like French fries and potato chips, they may be doing more harm than good," she said.
The new data, from the USDA's Economic Research Service, seems to bear that out.
White potatoes accounted for about 115 pounds of the 384 pounds per person of vegetables and legumes available to the average American in 2013, the agency found. However, two-thirds of those potatoes were used for French fries, potato chips and other processed or frozen potato products, the agency said.
The same trends were seen for tomatoes. Tomatoes accounted for 22 percent of vegetables made for Americans, the report found. However, of the 66 pounds of tomatoes available to each person in 2013, only 20 pounds were for fresh tomatoes. The other 46 pounds were processed in a number of ways, including canned tomatoes, tomato sauces (such as ketchup), or as ingredients in processed stews, soups and other items.
Still, the USDA noted that the numbers are getting a bit better over time. The agency said that in 1970, white potatoes, tomatoes and lettuce accounted for two-thirds (67 percent) of the vegetables grown for Americans. Since then, Americans have tended to eat less potatoes, but their intake of tomatoes has grown.
But even tomatoes have their nutritional limits, White said.
"Tomato and tomato products will offer valuable nutrients as well, but still fall short compared to a diet with more variety," she said.
The USDA noted that the nutrient content of fruits and vegetables varies widely. For example, the agency said that dark greens such as spinach and kale are especially rich in vitamin A and folate, while legumes such as dry beans and peas, kidney beans, split peas and lentils are high in protein and fiber.
Another nutrition expert suggested that consumers vary the "color" in their diet.
"All fruits and vegetables are designated their own beneficial properties of various nutrients, vitamins and minerals," said Sharon Zarabi, a nutritionist at Lenox Hill Hospital in New York City.
"As a dietitian, I try to teach my patients that food should be used as medicine," she said. "The colors that fruits and vegetables carry provide the variety of flavors and health benefits they offer."
There's more on healthy diets at the U.S. Centers for Disease Control and Prevention.