MEXICO CITY (AFP) - There may be no such thing as a free lunch, but the overweight in Mexico City will be glad to hear there is such a thing as a free subway ride.

Concerned health officials in the Mexican capital hope to get residents in shape by offering free journeys in return for them burning a few extra calories.

From Monday, dozens of stations in the city's metro system have been equipped with special machines that, in exchange for 10 squats, will tell passengers how many calories they burnt and give them a token for a free ride.

The so-called "health stations" are a novel way for health chiefs to draw attention to Mexico's dismal obesity levels.

According to official figures, 70 per cent of adults and nearly a third of children are overweight or obese, surpassing even the United States.

"Levels of excess weight and obesity concern us greatly. For me, it's the number one public health problem," the capital's health secretary Jose Armando Ahued Ortega said as he introduced the project, devised by leftist mayor Miguel Angel Mancera.

Thirty squat-counting machines have been installed across the city, which will also hand out pedometers to the first 80,000 users to help them track their energy output.

With more than five million daily users, the subway is a vital means of transport in the megalopolis.

Metro tickets in December 2013 went up from three to five pesos (S$0.30 to S$0.50), generating fierce criticism from commuters in a country where the minimum wage is 70 pesos a day.