Shopping healthy and cheaply isn’t always easy. (AP Photo/Mel Evans)

Healthy eating really does cost more.

That’s what University of Washington researchers found when they compared the prices of 370 foods sold at supermarkets in the Seattle area. Calorie for calorie, junk foods not only cost less than fruits and vegetables, but junk food prices also are less likely to rise as a result of inflation. The findings, reported in the current issue of the Journal of the American Dietetic Association, may help explain why the highest rates of obesity are seen among people in lower-income groups.

The scientists took an unusual approach, essentially comparing the price of a calorie in a junk food to one consumed in a healthier meal. Although fruits and vegetables are rich in nutrients, they also contain relatively few calories. Foods with high energy density, meaning they pack the most calories per gram, included candy, pastries, baked goods and snacks.

The survey found that higher-calorie, energy-dense foods are the better bargain for cash-strapped shoppers. Energy-dense munchies cost on average $1.76 per 1,000 kcal, compared with $18.16 per 1,000 kcal for low-energy but nutritious foods.

The survey also showed that low-calorie foods were more likely to increase in price, surging 19.5 percent over the two-year study period. High-calorie foods remained a relative bargain, dropping in price by 1.8 percent.

Although people don’t knowingly shop for calories per se, the data show that it’s easier for low-income people to sustain themselves on junk food rather than fruits and vegetables, says the study’s lead author Adam Drewnowski, director of the center for public health nutrition at the University of Washington. Based on his findings, a 2,000-calorie diet would cost just $3.52 a day if it consisted of junk food, compared with $36.32 a day for a diet of low-energy dense foods. However, most people eat a mix of foods. The average American spends about $7 a day on food, although low-income people spend about $4, says Dr. Drewnowski.

But it’s easier to overeat junk food, Dr. Drewnowski adds, both because it tastes good and because eaters often must consume a greater volume in order to feel satisfied. Still, even those who consume twice as much in junk food calories are still spending far less than healthy eaters.

“If you have $3 to feed yourself, your choices gravitate toward foods which give you the most calories per dollar,’’ said Dr. Drewnowski. “Not only are the empty calories cheaper, but the healthy foods are becoming more and more expensive. Vegetables and fruits are rapidly becoming luxury goods.”