The difference between the healthiest diets and the least healthy diets is $1.50 a day, U.S. researchers found. Lead author Mayuree Rao, a junior research fellow in the Department of Epidemiology at the Harvard School of Public Health, and colleagues conducted a meta-analysis of 27 existing studies from 10 high-income countries that included price data for individual foods and for healthier vs. less healthy diets. The researchers evaluated the differences in prices per serving and per 200 calories for particular types of foods, and prices per day and per 2,000 calories -- the U.S. Department of Agriculture's recommended average daily calorie intake for adults -- for an overall diet. Both prices per serving and per calorie were assessed because prices can vary depending on the unit of comparison Rao said. The meta-analysis found healthier diet patterns -- for example, diets rich in fruit, vegetables, fish and nuts -- cost significantly more than unhealthy diets that include a lot of processed food, meat and refined grains. On average, a day's worth of the most healthy diet patterns cost about $1.50 more per day than the least healthy ones, Rao said. The researchers suggested unhealthy diets might cost less because food policies have focused on the production of "inexpensive, high volume" commodities, which has led to "a complex network of farming, storage, transportation, processing, manufacturing and marketing capabilities that favor sales of highly processed food products for maximal industry profit." "While healthier diets did cost more, the difference was smaller than many people might have expected. Over the course of a year, $1.50/day more for eating a healthy diet would increase food costs for one person by about $550 per year," said senior author Dariush Mozaffarian, an associate professor at Harvard School of Public Health and Harvard Medical School. "On the other hand, this price difference is very small in comparison to the economic costs of diet-related chronic diseases, which would be dramatically reduced by healthy diets." The findings were published in the British Medical Journal.