It was the first city in America to ban trans-fats from food outlets. Now New York has set another US first - from today, larger food chains such as McDonald's, Dunkin' Donuts and Starbucks must display the calories of their products on menus.

The new rules apply to all chains with at least 15 outlets across the country - which in the case of New York accounts for 2,000 restaurants.

The city authorities say the reform will help New Yorkers get a better sense of what they are eating as part of the ongoing battle against obesity and diabetes.

Over the next five years, the city says, the labelling will help prevent 130,000 New Yorkers from becoming obese and 30,000 from developing diabetes.

Some New York outlets had already begun to show calorie counts on their menu boards by the end of last week. At Chipotle, a Mexican food chain, in midtown Manhattan, chips were showing an alarming 570 calories, guacamole 140 and tacos with all the trimmings up to 590.

Standing in the queue to pick up her food, Elissa was aware of the changes and was using the calorie guide to make her choice, mindful that she is trying to consume no more than 1,500 calories a day.

But with the calorie count for salads listed as between 155 and 823, she was a bit perplexed.

"If I order the 823 calories I'll be pissed," she said. "That would be half my daily intake."

Todd, by contrast, hadn't noticed the new calorie counts on the wall despite this being his regular weekly lunch spot, and was utterly unaware of the city's impending innovation. Does he watch his intake? "No, but I should do."

The umbrella group of the city's outlets, the New York State Restaurant Association, went to court last week to try to block the changes. It argued that the rule was an infringement of the first amendment, which prevents government messages being forced onto businesses.

However, the judge found that the new rules were part of a reasonable public interest in providing customers with accurate nutritional information.

Next door to Chipotle, Starbucks had also started to display its calories. That revealed that a venti Java chip frappucino packs in 600 calories. McDonald's had yet to post its counts. From today, all eyes will be on its outlets, together with the other fast-food chains. If they fail to meet the regulations by June 3 they will face fines.