Confirming a gastronomical trend that nutritionists have long suspected, a new study demonstrates that food portion sizes have grown dramatically over the last two decades -- a finding that may help to explain the growing obesity levels in the U.S.
Today's average hamburger is 23% larger, an order of fries is 16% bigger and the size of a soft drink has jumped 50%. And that's true whether you eat in a fast-food joint, dine in a classy restaurant or prepare supper at home, according to researchers from the University of North Carolina at Chapel Hill.
Although the researchers could not establish a direct link between increased portion sizes and weight gain, experts say the results provide powerful insight into why the incidence of obesity has more than doubled since 1971, climbing from 14.5% of the population to 30.9%.
"Many people have thought that portion sizes might be on the rise, but until now, there have been no empirical data to document actual increases," said Barry Popkin, a professor of nutrition and co-author of the study with graduate student Samara Joy Nielsen. "We think this is important information, not only because it documents this trend, but also because obesity presents a growing health threat both in the United States and abroad."
The report in today's Journal of the American Medical Assn. probably underestimates the magnitude of the problem, added nutritionist Megan McCrory of Tufts University, because the data are current only to 1996. "It's very likely that portion sizes being consumed now are actually larger, and underreporting has gotten worse over time."
Several studies, she noted, show that people tend to underestimate how much they eat -- and that the under-reporting is especially severe among the overweight.
Popkin and Nielsen used data from three government eating surveys that involved more than 63,000 people over age 2. They focused primarily on selected popular foods that are known to be high in fat and calories, including pizza, hamburgers, soft drinks, French fries and Mexican food.
These food groups accounted for 18.1% of all calories in the average diet in 1977, but 27.7% of calories in 1996. Portion sizes of all groups increased over the period studied with one exception: Pizza slices, after a brief flirtation with a larger size, are slightly smaller.
The study also confirmed two other notable trends: the growing incidence of eating out and the increasing role of snack foods in our diets. While 77% of all calories were consumed at home at the beginning of the study, only 65% were consumed at home in 1996. Snacks accounted for 11% of the calories in our diet in 1977, but 19% in 1996.
The largest portions were served at fast-food restaurants -- no surprise because of the growing practice of "super-sizing" meals. Chains such as McDonald's and Burger King have found that they can increase profit margins by offering larger sizes of inexpensive foods such as fries and soft drinks.
"People think that they are getting more value for their money, but they don't stop to think that they will be eating more and gaining weight," McCrory said.
Sit-down restaurants served the smallest portions overall, but those had grown as well. Perhaps more surprising was that home portions were also growing. "That indicates marked changes in eating behavior in general," Popkin said.
"We're getting so used to these big portion sizes when we eat out that, when we go home, we forget what a normal portion size is," said nutritionist Margo Wootan of the Center for Science in the Public Interest.
In practical terms, the average hamburger now contains 97 more calories than it did 20 years earlier, the average order of fries 68 more calories, and the average soft drink 49 more calories.
Federal agencies recommend an average daily intake of only 1,600 to 2,800 calories per day, depending on a person's age, gender and activity level. Eating an extra 10 calories per day for a year translates into a weight gain of one pound.