Consumers are adopting more sustainable diets. Or so they say. A new study found both obese and thin people across the UK lie about food to the same level, with everyone misreporting how many calories they eat by an average of 900 calories. This false data could be affecting national health advice, according to the researchers.

For their study, researchers at the University of Essex looked at 221 adults in the UK with an average age of 54 and diverse body shapes. They asked them to keep track of what they ate by using a food diary and then checking the amount of energy consumed, using radioactive water and also testing the urine of the participants.

Obese people misreported how much food and drinks they ate by an average of 1200 calories and slimmer participants by 800 calories. However, obese participants burnt 13% more energy doing day-to-day tasks, which is equal to around 400 calories. This means they don’t lie about food more than slimmer people, the researchers argued.

On average, everyone misreported the number of calories they eat by an average of 900 calories. This is equivalent to five pints of beer, three McDonald’s Cheeseburgers, seven packets of ready salted chips, 18 apples, or 300 cherry tomatoes, the researchers estimated.

“When we considered the different body sizes and the different energy needs they have there was no difference in how much they underreported their food intake,” the researchers wrote in a statement. “As energy requirements increase with a larger body size there is more error between what people report and what they actually eat.”

This changes the narrative about obese people lying more about their energy intake compared to slimmer people, said Professor Gavin Sandercock. He called the UK government to overhaul its health advice, which has historically relied on self-reported energy intake values – which the study showed could be largely incorrect.

The reasons behind the phenomenon

While the study didn’t address the reasons for the phenomenon, separate research published a few days earlier looked into food choices and social stigma. The team at City University London’s Bayes Business School found that people tend to choose healthier food when they are with outsiders for fear of being negatively judged.

Researchers surveyed about 1,000 individuals and did experiments with several hundred adults in a large US city and university. One experiment offered the choice between M&Ms and raisins as a snack. When participants were in the presence of a fellow student from their university, only 12% chose the healthier raisins. However, when participants were in the presence of an unknown student from another university, this figure almost doubled to 31%.

Why was this the case? The researchers argued people feel judged to a larger extent by “outgroup” members. As a result, they pick healthier food choices to make a more positive impression on them.

“Our research shows that we can use this important role of food for consumer welfare if we highlight that healthy food is not only good for consumers, but also helps them to impress others. These findings could be very significant to those hoping to improve healthy eating practices in the UK,” Janina Steinmetz, study author, said in a statement.

The first study was published in the American Journal of Human Biology and the second one in the journal Psychology & Marketing.