When Paul Venables, an IT consultant, last visited McDonald's, he ordered a quarter pounder with cheese and large fries. Yesterday, he swapped the fries, at 460 calories, for a fruit bag at 42 calories, and held the cheese, knocking almost 500 calories from his meal.
Venables, 42, from Surrey, was inspired by McDonald's decision to put calorie counts on menus at its 1200 UK outlets from this week.
"Obviously, some people won't care, but I am concerned about the obesity problem in this country," he said.
In New York, where McDonald's has been publishing calorie counts since 2008, the average lunchtime purchase has dropped less significantly, from 829 to 785 calories.
Some 5000 UK food outlets will offer calorie-count menus by the end of the year.
Burger King, Starbucks, Pret A Manger, Pizza Hut and KFC have joined the voluntary scheme, as part of the Government's Public Health Responsibility Deal.
Pizza Express, Subway, Domino's Pizza, Nando's, Caffe Nero and Costa are yet to join.
Health Secretary Andrew Lansley said it would help fast-food fans "spot those hidden calories in their favourite foods and keep an eye on their waistlines".
He added: "One in six meals are now eaten outside the home and for those meals we often have no idea how many calories we are eating."
While Steve Bridges, 44, from Guildford, was not influenced in his choice of meal - a 490-calorie Big Mac - he believes it will have an effect.
"I eat healthily most of the time, and don't normally eat in McDonald's," he said.
"But it does make you more aware: it's interesting to see the companies which have opted out."
Edona Nurdini, 26, agreed that the decision to include calories would add to people's awareness.
"It also helps McDonald's to recover their reputation because they've never been associated with health," she said.
A survey for consumer publication Which? found two out of three of the 1000 adults questioned backed the policy of all fast-food chains, pizza stores and coffee shops publishing the number of calories on their menus.
Some people at McDonald's in Cannon Street in central London yesterday were surprised that their lunches were not more calorific.
Daniel Pryce, 25, who works in a law firm, chose a chicken sandwich (385 calories), small fries (230 calories) and Sprite zero (two calories).
"I don't come here that often, so I just eat whatever I want," he said.
"But eating a McDonald's isn't actually that bad."
A Big Mac contains around 25 per cent of the average woman's suggested daily calorific intake. It contains 10g of saturated fat, almost half the daily recommended amount, and 2.1g of salt, 40 per cent of the daily guideline.
Denniz Messi, 24, from Greece, said that even if the calorie counts were not sky high, it did not make McDonald's healthy.
"We all know this food is fattening, and not good for your health."