WASHINGTON — Snacking has become a habit for many American children, the U.S. Department of Agriculture said, with the average number of snacks consumed now totals three a day, adding almost 200 calories per day.

“We are a nation of grazers,” said Kara Nielsen, a consumer researcher with CCDInnovation in San Francisco, who follows the eating habits of Americans.

She said the busy middle-class lifestyle of children spending time in cars moving from school on to sports or music lessons without time at home has led to the strong growth of packaged foods to eat on the run. Even healthy fruits such as blueberries are being packaged in individual servings to increase the convenience factor.

The current pattern of children snacking represents a three-fold increase from the single daily snack that was the average in the 1970s. It has been an uphill battle to get children to consume fruits and vegetables when they snack, rather than the high-calorie, less nutritious foods many currently favor.

In a 2012 survey for the Produce for Better Health Foundation, nearly two-thirds of mothers with children aged 10 and younger said cost was the key factor in their decisions whether to buy fruits and vegetables. Concerns about cost were especially important to lower-income households.

The U.S.D.A. offered some different insights.

“Swapping common snack foods with a ½-cup serving of fruits or vegetables can be done without compromising a household’s food business,” the E.R.S. said.

Studies conducted by the E.R.S. show that comparing foods high in saturated fats and sugars with fruits and vegetables by a relevant measure — price per average amount consumed — indicated that fruits and vegetables were less expensive than the less nutritious alternatives.

Another E.R.S. study found Americans on a 2,000-calorie per day diet may meet the daily requirement for multiple servings of fruits and vegetables recommended in the “2010 Dietary Guidelines for Americans” for between $2 and $2.50 a day.

What would be the significance if caregivers offered children one snack serving a day of fruits or vegetables in place of one snack of cookies or chips? The U.S.D.A. said the effects would be both a beneficial upgrade in vitamins and fiber but also a reduction in calories, which is especially helpful in reducing the incidence of obesity.

“Replacing one energy-dense snack each day with a fruit or vegetable could reduce caloric intake and decrease the prevalence of overweight and obesity,” the E.R.S. said. “For example, a child replacing 1 oz of potato chips (150 calories) with a cup of grapes (104 calories) or a medium-size apple (95 calories) would consume 46 to 55 fewer snack calories. If done on a daily basis, all else equal, this simple behavior could result in about half a pound less of body weight at the end of a month.”

To further study the effects of children snacking on fruits and vegetables rather than less nutritious foodstuffs, the E.R.S. compared the cost per portion of 20 different fruits and vegetables and 20 other snack foods. The results showed that 9 of the 20 fruits and vegetables and 8 of the other snack foods cost 25c a portion or less. Eight more fruits and vegetables and 10 other snack foods cost between 26c and 50c per ½ cup portion.

“On average, the 20 fruits and vegetables cost 31c per portion, while the 20 other snack foods cost 33c per portion,” the E.R.S. said.

By far the most expensive per portion were muffins, at more than 80c per serving. The most expensive fruit or vegetable were red peppers, at 60c per portion. Among the cheapest healthy snacks per portion were celery, bananas, raw broccoli florets, raisins and baby carrots, all less than 20c per portion. The least expensive snack surveyed were graham crackers at 14c a portion.

Specific examples were cited to show the cost of substituting several popular energy dense, low-nutrient snacks with widely available fruits and vegetables.

The E.R.S. said swapping a 1-oz chocolate chip cookie with ¼ cup of raisins would save 14 calories and cost 3c more; switching to ½ cup of baby carrots from a 4-oz serving of ready-to-eat pudding would save 130 calories and 19c, and substituting 1.1-oz of potato chips with ½ cup of strawberries would save 142 calories and 14c.

“Making these three substitutions could reduce caloric intake by 286 calories with little change in cost, as higher costs for some substitutions are offset by lower cost for others,” the E.R.S. said.

Most mothers involved in the studies were well aware that their families fail to consume the recommended number of servings of fruits and vegetables. They said they would be open to options that incorporate the foods into daily diets.

The conclusion of the E.R.S. investigation into snacking choices was good news for families seeking healthier snacks.

“The results of this analysis … show that replacing less healthy snack foods with fruits and vegetables does not have to compromise a household’s food budget,” the E.R.S. said.