WASHINGTON (Reuters) - New York's mandate that fast-food restaurants post calorie information on their menus has changed consumer habits, the city said on Monday, contradicting a recent independent study showing no effect.

The city's Department of Health and Mental Hygiene released preliminary data showing evidence that people bought food with fewer calories at nine of the 13 fast-food and coffee chains included in a study on the effects of menu-labeling laws that went into effect in 2008.

Researchers surveyed more than 10,000 customers at 275 locations in early 2007 and another 12,000 this year.

They found statistically significant decreases at four chains — McDonald's, Au Bon Pain, KFC and Starbucks — and said diners who saw and acted on calorie information bought food containing 106 fewer calories on average than those who did not notice the postings.

All told, 56 percent of fast-food customers reported seeing the calorie information, researchers told the annual meeting of the Obesity Society in Washington.

The earlier study by researchers at New York University and Yale University, which included 1,156 adults who ate at Burger King, KFC, McDonald's and Wendy's immediately before and after the rule went into effect, found no change to consumer habits in low-income neighbourhoods.

The city's researchers said their study was more representative of dining habits because it included more people over a longer period of time and not limited to outlets in low-income neighborhoods.

In July 2008, New York became the first US city to require fast food restaurants to post calorie counts in large type on menu boards.

The system has since become a model for similar rules intended to combat obesity and promote good nutrition in California, other parts of New York state, the cities of Seattle and Portland, and elsewhere.

Health advocates see menu labeling as a tool for fighting obesity. About one-third of US adults are obese, a condition that increases the risk of heart disease, diabetes, cancer and other medical problems, and another one-third are overweight.

Both the city and New York University studies were funded by the non-profit Robert Wood Johnson Foundation.

Lynn Silver, assistant commissioner for New York's Bureau of Chronic Disease Prevention and Control, said government findings show diners are noticing and acting on the labels.

"Dietary change is likely to come gradually; it will start with consumers interested in making informed, healthy eating decisions and we hope industry will respond by offering more healthier choices and appropriate portion sizes," she said in a statement.

But city researchers also found that the labeling laws' influence can be overcome by restaurant marketing.

The privately held Subway restaurant chain, which has promoted its menu as a vehicle for weight loss and healthy eating, posted calorie information on some of its menus before the labeling laws went into effect in 2008.

The number of calories purchased at Subway more than doubled during the study period, which coincided with an advertising campaign to promote larger 12-inch sandwiches. The calorie gain at Subway was roughly the same as losses at seven other food chains, researchers said.