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BMI underestimated

Self-reporting weight bias increasing

Bias is most notable in patients who are obese
Results revealed that self-reported height bias was evident in all age groups and for both genders

Patients are continuing to underestimate their weight, according to investigators from the University College Dublin and University of Limerick, Ireland. The research, which is published in the open access journal Plos One, also found that self-reported weight bias has increased for men and women, and in all age groups. The bias is most notable in patients who are obese.

The study team has previously reported that a patient’s ability to correctly identify their BMI had declined over a 10-year period.

The purpose of this study was to determine whether self-reported height bias or weight bias, or both, is responsible for the decline between self-reported and clinically measured BMI. Self-report height and weight bias is calculated by subtracting the clinically measured height or weight from the self-reported height or weight.

The research used self-reported and clinically measured height and weight data from the Surveys of Lifestyle Attitudes and Nutrition (SLÁN) involving a nationally representative sample of Irish adults. Data were available from 66 men and 142 women in 1998, 147 men and 184 women in 2002 and 909 men and 1,128 women in 2007. Respondents were classified into three BMI categories; normal (<25), overweight (25–<30) and obese (≥30).


The results revealed that self-reported height bias was evident in all age groups and for both genders at each time period (indicating that height is over reported), leading to an underestimation of BMI. However, the magnitude of the self-reported height bias was quite small for both males and females, and was systematic across all BMI categories and across each time period.

For weight, the results showed that self-reported weight bias was negative (indicating that weight is underreported), leading to an underestimation of BMI. Although this was evident in normal, overweight and obese BMI categories, the self-report weight bias increased most dramatically in each time period in the obese category, for both males and females.

For females in the overweight category, the increased was most dramatic in the latter two time periods. For males, the self-reported weight bias in the overweight and normal categories was stable at each time period. This was also the case in the normal category for men and women.

The authors claim that this study questions the validity of previous studies that have concluded both self-reported height bias and self-reported weight bias contribute equally to BMI underestimation in large population surveys.

They also claim that gathering sufficient data on why self-reported weight bias is greatest in the obese category (and is increasing across time) could have important implications for future studies.

“Accurate monitoring of height and weight are important in the diagnosis, prevention and reduction of overweight and obesity,” the authors write. “The implications for future studies are clear, in large population surveys, project investigators should at a minimum obtain clinical measurements from a subsample of participants to monitor underestimation and overestimation trends.”

The researchers do acknowledge that the results “must be cautiously interpreted”, accepting that the numbers are small (in both 1998 and 2002) and therefore limit the precision and the ability to generalise their findings across the wider population.


The authors conclude that the findings from the study are “significant for shaping future public policy around overweight and obesity awareness”.

They claim that the study suggests that self-report weight bias contributes to the increasing underestimation of BMI. With obesity levels rising, increasing self-reported weight bias in the obese category for both males and females is evidence, they claim, of the normalisation of obesity in the Irish population.

“Knowledge that the widening gap between self-reported BMI and measured BMI is attributable to an increased weight bias brings us one step closer to accurately estimating true obesity levels in the population using self-reported data,” they conclude.

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