ROCHESTER, Minn. — For years, researchers have reported that people tend to underestimate how many calories they eat, and that people with obesity tend to under-report their calories consumed the most.
But a new study by researchers at the University of Essex has shed light on what really may be happening when people with large bodies underestimate how much they eat. It finds that the error is not specific to anything unique about people with obesity at all.
Under-counting calories is a problem for the dietary status quo because both health workers and nutritionists believe their work requires that they ask people how much they eat — data generally considered unreliable.
As recently as 2019, a paper in the Journal of Health Psychology identified 34 studies showing that "having a body mass index greater than 30 is associated with significant under-reporting of food intake."
That paper listed variables associated with inaccurate calorie counting among people with obesity, including gender (female), a desire to lose weight, less realistic weight loss goals and low education.
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But others have stated for years that "the mechanisms responsible for the phenomenon are not well understood," as Columbia University researchers wrote in the New England Journal of Medicine in 1992.
Those scientists said the problem of under-counting calories among people with high BMI did not appear to be one of deception, as the persons they had studied "were distressed" when informed of their error.
According to the authors of the new research, as reported in the American Journal of Human Biology , underestimating your calories is a common process of being off by a standard percentage. In the context of high caloric needs for people who are active, younger or have a high body mass index, it's an error that gets bigger in absolute terms.
"A widely accepted narrative is that obese people are untruthful about their food intake because they are likely to have a negative perception of their body image ... and are more likely to exhibit periodic dietary restraint," Sally Waterworth, coauthor and lecturer in the School of Sport, Rehabilitation and Exercise Sciences at the University of Essex, told Forum News Service.
"We wondered if this narrative was correct, or whether the greater under-reporting in obese people is simply because, as energy requirements increase with a larger body size, there is more error between what people report and what they actually eat."
Waterworth and colleagues recruited 221 United Kingdom participants in a government food intake survey, then sorted them according to age and physical activity level.
Thanks to a sophisticated energy expenditure measurement method, they were able to compare the calories reported as consumed to the actual calories burned while remaining weight stable — actual caloric intake.
They learned that persons with high BMI had indeed underestimated how much they ate — by 1,300 calories on average, compared to just under 800 calories for people without obesity. But they also found that athletes and younger people had underestimated their intake by the same amount.
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"Everybody under-reports, let's be clear on that," Waterworth says of the finding. "But when you control for body weight, we all under-report the same.'
Caloric goals are to blame
The authors believe the problem starts with having universal recommendations for daily caloric intake, goals that are unrealistic for everyone, but counter-productive for those with obesity.
"Simply having recommendations for energy intake for everybody doesn't make sense," Gavin Sandercock, co-author and professor of exercise sciences in the school of Sport, Rehabilitation and Exercise Sciences at the University of Essex, said in an interview. "A very simple thing would be to adjust those for your size as well as how active you are. But it is how big you are that is more important."
The authors learned that the typical person in the United Kingdom reports eating roughly 1,800 calories a day, no matter their size. Although they did not study the question, the authors believed the figure was likely anchored around government caloric targets of 2,000 calories daily for women, and 2,500 for men.
In the U.S., the caloric targets are similar, but presented in ranges.
"I couldn't live on 1,800 calories," Sandercock said. "I couldn't even sleep all day on 1,800 calories. No man my size could ... It's certainly not enough for them to live and meet the activity guidelines."
"We're really just questioning the validity of the tool," said Waterworth, "whether it should be used, and whether it really matters, because it's so far out from reality."
Her advice is for health officials to ditch the numbers and "focus on food."
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"Obese people get wrongly labeled in a number of ways," Sandercock said. "But this is the first time we've seen them labeled as liars."
"It's just not helpful. Shaming is no way to get people to be more healthy. From the scientific point of view, trying to blame a methodological issue on a subgroup ... it's irresponsible."
"They would have been much better blaming it on athletes and people who are active," Sandercock said. "I wouldn't have minded being blamed. I know that I eat a lot."
