The question looms large as California leads the nation in imposing the first statewide calorie-posting law.
By next Jan. 1, more than 17,000 restaurants in the state will be required to print calorie data directly on menus.
Researchers at Stanford Business School studied the effect of calorie information on consumer behavior by analyzing more than 100 million Starbucks transactions both before and after a similar calorie-posting law took effect in New York City in 2008.
They found that calorie postings led to a 6 percent calorie drop in the average purchase, with no statistically significant impact on Starbucks revenue.
The calorie reduction was even larger — 26 percent — for people who previously had been in the habit of making higher-calorie purchases, defined in the study as more than 250 calories.
The drop in calories per purchase was more significant in ZIP codes with higher income and higher education.
Though a 6 percent calorie drop at Starbucks scarcely makes a dent in the nation's collective waistline, the cost of posting calories is so low that even that small benefit could outweigh the price, the Stanford team concluded.
"Moreover, the long-run effects of calorie postings are potentially more dramatic (than 6 percent)," encouraging restaurants to innovate and offer low-calorie items, the researchers said.
The Starbucks study offers another drop of data in the national debate over how to combat obesity.
The percentage of Americans considered obese, with a body mass index greater than 30, rose from 15.9 percent to 26.6 percent from 1995 to 2008. That makes the United States the most obese country in the world, according to the Organization for Economic Cooperation and Development.
In California, 18.6 percent of adults — more than 4.5 million people — are classified as obese, according to the state's Department of Public Health. The figure was somewhat lower, just above 15 percent, in Santa Clara County.
Under California's new law, restaurants with 20 or more facilities in the state will have to display clearly the calorie and fat content information on menus and menu boards by Jan. 1, 2011. Starting last July 1, they were required to provide the information in brochure form.
The calorie-posting law, authored by State Senator Alex Padilla, D-Pacoima, was signed by Governor Arnold Schwarzenegger in 2008.
Obesity now costs the state $28.5 billion in health care costs, lost productivity and workers' compensation, the governor said.
New York City's calorie-posting law took effect April 1, 2008. The Stanford team analyzed every Starbucks transaction in the city from Jan. 1, 2008, to Feb. 28, 2009.
To control for other factors, they also looked at every Starbucks transaction in Boston and Philadelphia, where calorie postings were not in effect, during the same time period.
In another part of the study, Stanford tracked 2.7 million anonymous Starbucks cardholders, both inside and outside of New York City, to examine the impact of calorie-posting on an individual's behavior.
The individual data showed that female cardholders were more responsive to the calorie postings than males.
The researchers also found that calorie postings had the greatest effect on food, not beverage, choices. Average food calories per transaction fell by 14 percent, while average beverage calories per transaction did not substantially change.
Starbucks beverages ranged from brewed coffee, at 5 calories a cup, to the 24-ounce Hazelnut Signature Hot Chocolate with whipped cream, at 860 calories. Food items ranged from small cookies, at about 100 calories each, to muffins that were as much as 500 calories.
The Stanford team also found that calorie postings actually led to increased revenue in Starbucks stores located within 100 meters of rival Dunkin' Donuts, perhaps because customers were reminded of doughnuts' high calorie content.
"It is perhaps worth noting that some chains like Starbucks are in fact fully supportive of mandatory calorie posting — this is not necessarily bad for chain restaurants, as our study shows in the case of Starbucks," said Phillip Leslie, an associate professor of strategic management in Stanford's Graduate School of Business.
Leslie said calorie postings are a good idea but that much remains to be learned about their effects.
Still to be studied, he said, are questions like whether calorie postings cause chains to come up with more low-calorie offerings, or whether people offset their lower-calorie purchases by eating more at home.
In general, Leslie said, "information provision" such as calorie posting is at the forefront of policy interventions to combat obesity.
That strategy has worked in other areas, he said, noting his research showing that "restaurant-hygiene grade cards" led to a 20 percent decrease in hospitalizations for food-related illnesses.