How much does it really cost to eat a healthy diet?

Economists, health researchers and consumers are struggling to answer that question as food prices rise and the economy slumps. The World Bank says nearly a billion people around the world live on a dollar a day, or even less; in the United States, the daily food-stamp allowance is typically just a few dollars per person, while the average American eats $7 worth of food per day.

Even middle-class people struggle to put healthful food on the table. Studies show that junk foods tend to cost less than fruits, vegetables and other healthful foods, whose prices continue to rise.

This fall a couple in Encinitas, Calif., conducted their own experiment to find out what it was like to live for a month on just a dollar a day for food. Overnight, their diets changed significantly. The budget forced them to give up many store-bought foods and dinners out. Even bread and canned refried beans were too expensive.

Instead, the couple  Christopher Greenslate, 28, and Kerri Leonard, 29, both high school social studies teachers  bought raw beans, rice, cornmeal and oatmeal in bulk, and made their own bread and tortillas. Fresh fruits and vegetables weren’t an option. Ms. Leonard’s mother was so worried about scurvy, a result of vitamin C deficiency, that they made room in their budget for Tang orange drink mix. (They don’t eat meat  not that they could have afforded it.)