Americans are fat: 32.2% of U.S. men and 35.5% of women are obese, up from about 13% and 17%, respectively, in 1978 — when obesity rates were already higher than in other high-income nations. The NRC report predicts that other industrialized countries like Australia and the Netherlands aren't far behind; in general, obesity rates in the United States appear to be ahead of those in the other countries by 15 to 25 years.
It's no secret that excess weight contributes to a host of health problems, including Type 2 diabetes; high blood pressure; heart disease; gallstones; and certain cancers, such as colorectal, breast, endometrial and pancreatic cancer. The report finds that the relationship between obesity and rates of various diseases — especially heart disease and diabetes — is also apparent in country-to-country comparisons. In an analysis of 10 high-income countries, those with the highest rates of obesity were hardest hit by diabetes and heart disease in adults over 50.
But the direct impact of obesity on population-wide mortality was somewhat less clear-cut, the report found, influenced by factors such as age, level of overweight and other contributing circumstances like smoking. From the report:
According to one  international study, each increase of 5 units in BMI resulted in a 30% increase in overall mortality. In short, the authors concluded, obesity does increase mortality risk; the increase is relatively modest for Class 1 obesity (BMI between 30 and 35) but is significantly greater for those with a BMI above 40. The authors also found that the relationship between BMI and mortality risk changes with age. BMI has its largest effect on mortality for adults under 50, and the correlation between BMI and mortality decreases beyond that age. The older adults at greatest risk of dying are those at the extreme ends of the BMI spectrum — either extremely underweight or extremely overweight.
The current best estimates suggest that obesity may account for a fifth to a third of the shortfall in longevity in the U.S., compared with other countries. And if obesity trends in the U.S. were to continue, their consequences could offset the improvements in longevity expected from declines in smoking.
The good news is that U.S. obesity rates seem to be leveling off, particularly among women whose rate stayed steady at 32% for 10 years, until a small uptick in 2008. And some data suggest that mortality risk associated with obesity has declined.
On average Americans don’t live as long as people in many other wealthy nations, and they’re less healthy overall — this, despite the fact that the U.S. spends more on health care than any other nation in the world. A new report by the National Research Council lays out the reasons for America’s longevity problem, but also indicates that the U.S. is poised to gain ground.