The study of almost 27,000 UK households, published in American Journal of Clinical Nutrition, studied detailed data on purchase records of all foods and beverages – finding that unhealthy foods were more likely to see an increase in sales from price promotions.
"There's plenty of anecdotal evidence, but very little empirical evidence, about the impact of price promotions on people's diets," said senior researcher Professor Theresa Marteau from the University of Cambridge. "In this study, we examined whether less healthy foods are more likely to be promoted than healthier foods and how consumers respond to price promotions."
The research data showed - perhaps surprisingly - that on the whole less healthy items were no more frequently promoted than healthier ones. However, after accounting for price, price discount, and brand characteristics, the magnitude of the sales increase was larger in less healthy than in healthier food categories, said the team.
Indeed, a 10% increase in the frequency of promotions led to a 35% sales increase for less healthy foods and a just under 20% sales increase for healthier foods.
"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," commented first author Dr Ryota Nakamura from the University of York, who carried out the research whilst at the University of East Anglia.
"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."
The team suggested that their findings could potentially be explained by the fact that many products from less healthy food categories are often non-perishable, while those from healthier food categories - in particular fruit and vegetables - are perishable.
As a result, stockpiling during promotion may therefore be more likely to happen in less healthy food categories.
Nakamura and colleagues also found that households of a higher socioeconomic status tended to respond to price promotions more than those from disadvantaged backgrounds, for both healthier and less healthy foods.
The research, which was funded by the UK Department of Health, based their scores of healthiness on criteria set out by the UK Food Standards Agency (FSA).
Source: American Journal of Clinical Nutrition
Published online ahead of print, doi: 10.3945/ajcn.114.094227
“Price promotions on healthier compared with less healthy foods: a hierarchical regression analysis of the impact on sales and social patterning of responses to promotions in Great Britain”
Authors: Ryota Nakamura, et al