The association between BMI self-selection, self-reported BMI and objectively measured BMI

Background : It is challenging to measure BMI accurately in population studies because it is an expensive and time-consuming task. As a result, subjectively measured BMI, calculated from self-reported height and weight, which we know to be inaccurate, is often used. The purpose of this study is to determine whether using a different, subjective method of measuring BMI, BMI self-selection, would be more accurate. BMI self-selection is determined by asking participants to place themselves in one of the BMI categories, underweight, normal weight, overweight or obese. Methods : A quantitative survey conducted in the outpatient departments of two hospitals in the South West of Ireland. Participants >18 years were included. Pregnant women were excluded. The final sample included 70 participants. Survey completion, including self-reported height and weight and BMI self-selection, preceded the objectively measured weight and height. Demographic and lifestyle factors were also recorded. Results : 59% of the sample were overweight or obese –63% of females and 54% of males. The sensitivity for correct BMI self-selection for normal weight, overweight and obese categories was 90%, 36% and 33% respectively. The sensitivity for BMI categories calculated from self-reported height and weight was 83%, 57% and 59% respectively. In analysis, age, regular alcohol consumption, unhealthy diet and lower levels of physical activity were all associated with underestimation of BMI. Conclusion:  BMI self-selection is an accurate tool when used with normal weight individuals but shows poorer ability to demonstrate accurate measurement in overweight and obese BMI categories. It is inferior to the traditional self-reported BMI method for measuring BMI levels in the general population. Presently, we recommend continuing with self-reported BMI for BMI measurement in large population studies. However, further testing of BMI classification in larger studies is suggested and investigation into alternative objective methods of measurement is warranted.