McDonald's salads are proving popular

Part of the fast-food giant's ongoing move to diversify away from just selling burgers and fries, the meal includes a salad, drink and Stepometer.

The latter is a small device that clips onto a person's belt and counts the number of steps they take.

Critics have charged that McDonald's new Grilled Chicken Caesar salad has more calories than its cheeseburger.

The company said it bought 700,000 Stepometers for August, but that this first batch will sell out in two weeks

"We have had to send for another 400,000 Stepometers from Poland to ensure we meet the projected demand by the end of August when the promotion ends," a McDonald's spokesman said.