Almost half of all adult Americans will be obese, and around a quarter will be severely obese, by 2030, according to a new study.

The paper, led by researchers at the Harvard T.H. Chan School of Public Health, predicts that more than half of the adult population of 29 US states will be obese by 2030 and that, among adults, all states will have an obesity prevalence of over 35% by the same year. They further estimate that the current rates of adult obesity and severe adult obesity in the US are around 40% and 18%, respectively.

Too big to fall

“The high projected prevalence of severe obesity among low-income adults has substantial implications for future Medicaid costs,” said lead author Zachary Ward, programmer/analyst at Harvard Chan School’s Center for Health Decision Science and lead author of the study.

“In addition, the effect of weight stigma could have far-reaching implications for socioeconomic disparities as severe obesity becomes the most common BMI category among low-income adults in nearly every state.”

The prediction is quite troubling as obesity comes associated with several health and economic impacts, on both an individual and social scale. Severe obesity is especially linked to increased rates of chronic disease and medical spending, the team explains, and with drastic reductions in life expectancy.

The team drew on self-reported body mass index (BMI) data from adults who participated in the Behavioral Risk Factor Surveillance System Survey (BRFSS) between 1993 and 2016 (for a total of 6.2 million data points). The BMI is calculated by dividing a person’s weight in kilograms by the square of their height in meters. A BMI of over 30 is considered indicative of obesity, while one of 35 or higher corresponds to severe obesity.

Since self-reported data in general and self-reported BMIs, in particular, tend to be unreliable (as people conform to their own biases), the team developed and used novel statistical methods to correct the data. Furthermore, using the wealth of information collected by the BRFSS, they looked at obesity rates for specific states, income levels, and subpopulations.

Several US states will have adult obesity rates close to 60% by 2030, they report, while the least-affected states will still record rates close to 40%. On a national average, they report, severe obesity will become the most common BMI category for women, non-Hispanic black adults, and those with annual incomes below $50,000 per year.

The team hopes that their study will help guide policy meant to prevent such a situation. For example, they cite previous research showing that a tax on sugar-sweetened beverages is an efficient and cost-effective prevention method for obesity.

“Prevention is going to be key to better managing this epidemic,” said Ward.

The paper “Projected U.S. State-Level Prevalence of Adult Obesity and Severe Obesity” has been published in the New England Journal of Medicine.