The price may not be right when it comes certain foods on sale in UK supermarkets, found a study published in the American Journal of Clinical Nutrition.

Researchers analyzed the purchase records of all foods and beverages purchased by 27,000 UK households. Each purchase was scored on health based on the UK Food Standards Agency (FSA) criteria. Until now, there’d only been anecdotal evidence regarding the impact supermarket price promotions have on people’s diets, Theresa Marteau, director of the behavior and health research unit at the University of Cambridge, said in a press release.

What Marteau and her team found were supermarkets no more frequently promote unhealthy foods than healthier foods. However, when price, discounts, and brand characteristics were taken into account, “the magnitude of the sales increase was larger in unhealthy foods than in healthier food categories.” Specifically, a 10 percent increase in price promotions led to a 35 percent sales increase for unhealthy foods and only a 20 percent increase for healthier foods.

If there isn’t much of a difference between how often unhealthy and healthy foods are promoted, why the sales boost? Researchers speculated it may have to do with the fact more of the foods in the unhealthy categories consist of non-perishable items, while healthier foods, such as fruits and vegetables, are perishable. In which case, consumers may be encouraged to stock up on non-perishable foods, so that they last long after the promotion.

Economic status also played a role in the study. Researchers found "a higher socioeconomic status tended to respond to price promotions more than those from disadvantaged backgrounds, for both healthier and less healthy food."

"It seems to be a widely held idea that supermarkets offer promotions on less healthy foods more often than promotions on healthier foods, but we did not find this to be the case, except within a minority of food categories," said study author Dr. Ryota Nakamura, the Centre for Health Economics at the University of York in the UK. "Yet, because price promotions lead to greater sales boosts when applied to less healthy foods, our results suggest that restricting price promotions on less healthy foods has the potential to make a difference to people's eating habits and encourage healthier, more nutritious diets."

Source: Nakamura, R et al. Price promotions on healthier vs. less healthy foods: a hierarchical regression analysis of the impact on sales and social patterning of responses to promotions in Great Britain. AJCN. 2015.