The study shows that consumer spending on food increases by 15 per cent over the holiday season (Thanksgiving to New Year), with most of the increase attributed to higher levels of junk food.
But shoppers buy the greatest amount of food after New Year - the equivalent of a nine per cent increase in calories above holiday levels, said Professor Lizzy Pope of the University of Vermont, who led the study as a post-doctoral researcher at Cornell University's Food and Brand Lab.
"They do pick out more healthy items, but they also keep buying higher levels of less-healthy holiday favourites. So their grocery baskets contain more calories than any other time of year we tracked," Pope added.
The findings are surprising given the holidays' reputation for overeating - and suggest that people need better strategies for shopping under the sway of "res-illusions," the research team said.
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The researchers recommend that consumers use written grocery lists to deter impulsive junk food purchases; substitute as much junk food as possible with fresh produce and nutrient-rich foods, and split grocery baskets visually to ensure nutritious foods represent at least half of purchases.
More than 200 households in New York State were recruited to participate in the seven-month study of grocery store spending behaviours, from July 2010 to March 2011.
"Despite New Year's resolutions to eat healthier, people tend to hang on to those unhealthy holiday favourites and keep buying them in the New Year," said co-author Drew Hanks, The Ohio State University, who worked on the study as a post-doctoral researcher at Cornell.