US President Donald Trump speaks in front of fast food he purchased for a ceremony honoring the 2018 College Football Playoff National Champion Clemson Tigers, in the State Dining Room of the White House in Washington, DC. Photo: AFP

Labeling fast-food menu items with calorie counts only leads to a slight, short-term decrease in average calories purchased in the United States, a study published on Thursday found.

And after a year, that effect is mostly diminished.

Researchers gathered data on almost 50 million menu purchases at 104 locations of a fast-food franchise with three restaurant chains in Louisiana, Texas and Mississippi from April 2015 to 2018.

During the last year of the study period, the menu items included labels with calorie counts.

In the months following the addition of labels, customers bought on average 60 calories less than before the calorie counts were included. Notably, about 40 of those calories were attributed to side dishes (including desserts).

But after a year, the average calories per transaction had reduced to only 23 calories less than before labeling. 

The trend "could have occurred if customers initially responded to the novelty of calorie labels but stopped noticing them over time," the study suggested.

Two other possibilities are that customers decided to purchase healthier items with a similar calorie count; or that restaurants started adapting their recipes as time went on, offering lower calories.

The study did not examine either of these potential explanations, said author Joshua Petimar.

"What this shows us is that calorie labelling alone is not a solution in itself," said Amelia Lake, a registered dietitian and professor in public health nutrition at Teesside University. 

"Obesity is a complex condition and therefore needs a multifaceted approach to tackle it."

Authors of the study, published in the BMJ journal, said that although the calorie reduction was low, it could still help adults to lose about half a kilogram over three years. 

"Although the results of this study might be disappointing to some, small changes to calorie intake can have meaningful effects at the population level," wrote Asha Kaur of the Centre on Population Approaches for Non-Communicable Disease Prevention at the University of Oxford in an editorial. 

About 29 percent of adults in England were classified as obese in 2019, an increase of three percent since 2016, according to the NHS.