Nearly three quarters of U.S. adults are overweight or obese, according to a sweeping new study. The findings have wide-reaching implications for the nation’s health and medical costs as it faces a growing burden of weight-related diseases.

The study, published on Thursday in The Lancet, reveals the striking rise of obesity rates nationwide since 1990 — when just over half of adults were overweight or obese — and shows how more people are becoming overweight or obese at younger ages than in the past. Both conditions can raise the risk of diabetes, high blood pressure and heart disease, and shorten life expectancy.

The study’s authors documented increases in the rates of overweight and obesity across ages. They were particularly alarmed by the steep rise among children, more than one in three of whom are now overweight or obese. Without aggressive intervention, they forecast, the number of overweight and obese people will continue to go up — reaching nearly 260 million people in 2050.

“I would consider it an epidemic,” said Marie Ng, who is an affiliate associate professor at the Institute for Health Metrics and Evaluation at the University of Washington School of Medicine and a co-author of the new paper.

Dr. Ng and her co-authors wrote that existing policies have failed to do enough to address the crisis, adding that “major reform” was needed to prevent it from worsening.

“It’s going to require a lot more attention and a lot more investment than we are currently giving the problem,” said Dr. Sarah Armstrong, a professor of pediatrics and population health sciences at Duke University who was not involved in the study.