Methodology

What is Obesity?

Obesity means an amount of body fat that exceeds the level generally considered healthy for a particular height.1,2 There are many methods of measuring body fat, some of which are expensive and time consuming. Body mass index, which is inexpensive and easy to calculate, is typically used as a proxy. Health officials recommend that individual health assessments should consider other factors as well. Research has demonstrated that a high BMI is strongly correlated with the same negative health consequences as high body fat, although the association between BMI does vary among ethnic groups.

BMI is a person's weight in kilograms divided by his or her height in meters squared. For measurements in pounds and inches, BMI is calculated using the following formula:

For adults, BMI levels are associated with the following weight classifications:

Methodology for Obesity and Other Rates Using BRFSS

Annual Data

Data for this analysis was obtained from the Behavioral Risk Factor Surveillance System dataset (publicly available on the web at www.cdc.gov/brfss). The data were reviewed and analyzed for TFAH and RWJF by Sarah Ketchen Lipson, PhD candidate and Daniel Eisenberg, PhD, Associate Professor, Health Management and Policy at the University of Michigan School of Public Health.

BRFSS is an annual cross-sectional survey designed to measure behavioral risk factors in the adult population (18 years of age or older) living in households. Data are collected from a random sample of adults (one per household) through a telephone survey. The BRFSS currently includes data from 50 states, the District of Columbia, Puerto Rico, Guam and the Virgin Islands. Variables of interest included BMI, physical inactivity, diabetes, hypertension and consumption of fruits and vegetables five or more times a day. BMI was calculated by dividing self-reported weight in kilograms by the square of selfreported height in meters. The variable 'obesity' is the percentage of all adults in a given state who were classified as obese (where obesity is defined as BMI greater than or equal to 30). Researchers also provide results broken down by race/ethnicity — researchers report results for Whites, Blacks and Latinos — and gender. Another variable, 'overweight' was created to capture the percentage of adults in a given state who were either overweight or obese. An overweight adult was defined as one with a BMI greater than or equal to 25 but less than 30. For the physical inactivity variable a binary indicator equal to one was created for adults who reported not engaging in physical activity or exercise during the previous thirty days other than their regular job. For diabetes, researchers created a binary variable equal to one if the respondent reported ever being told by a doctor that he/she had diabetes. Researchers excluded all cases of gestational and borderline diabetes as well as all cases where the individual was either unsure, or refused to answer.

To calculate prevalence rates for hypertension, researchers created a dummy variable equal to one if the respondent answered "Yes" to the following question: "Have you ever been told by a doctor, nurse or other health professional that you have high blood pressure?" This definition excludes respondents classified as borderline hypertensive and women who reported being diagnosed with hypertension while pregnant.

Why are Reported National Obesity Rates Higher Than State-by-State Rates?

How is it that only [x] states have obesity rates exceeding 35 percent, yet the national obesity rate is 37.7 percent? This paradox is explained by the fact that people do not always accurately report their height and weight. State obesity rates are collected by the BRFSS, which relies on selfreported height and weight. Research has demonstrated that respondents tend to overestimate their height and underestimate their weight. In fact, one study found that, due to this phenomenon, the BRFSS may underestimate obesity rates by nearly 10 percent. There is also research indicating that certain groups of people are more likely to misreport their weight and/or height. For example, one study found rural Blacks are more likely than rural Whites to misperceive their weight. Another study of adolescents found that females underreported their weight more than males, and overweight individuals underestimated their weight more than non-overweight individuals. The NHANES survey, from which the national obesity rate is derived, calculates its obesity rate based on physical examinations of respondents. Accordingly, the higher rates reflected by the NHANES survey are likely a more accurate reflection of the true extent of the obesity epidemic.

Notes

1 Centers for Disease Control and Prevention. About adult BMI. https://www.cdc.gov/healthyweight/assessing/bmi/adult_bmi/. Accessed April 26, 2017.

2 The National Institute of Diabetes and Digestive and Kidney Diseases. Overweight and obesity statistics. https://www.niddk.nih.gov/health-information/health-statistics/overweight-obesity. Accessed April 27, 2017.