Gym giant Anytime Fitness has just opened its 3000th site in 13 years, becoming the world's fastest-growing franchise and outpacing McDonald's and Subway.

Finally, it's fitness instead of fast food that we want to consume.

Getting our fix in any way that works is great. However, thankfully, a brisk walk in the fresh air beats slogging it out at the gym. At least if your aim is to be lean.

A new study has analysed the exercise patterns of people in Britain between 1999 and 2012.

Although the guidelines recommend at least 30 minutes of moderate intensity exercise each day, it is not known whether certain exercises are better than others.

Researchers from the London School of Economics looked at how regularly Britons engaged in 30 minutes or more of walking, moderate intensity exercise such as going to the gym, swimming, dancing, running and tennis, as well as heavy housework or heavy outdoor labour like chopping wood.

They then compared the waist circumference and body mass index (BMI) data of the people and found that those who regularly walked were leaner.

"The results thus provide an argument for a campaign to promote walking," the authors said.

"Focus on physical activity is less controversial as it would not be subject to political lobbying as is the case for 'fat' tax and other policies that aim to change consumption of junk foods in a person's diet."

The authors suggested that creating a policy "that 'every step counts' may be a step towards curbing the upward trend in obesity rates and beneficial for other health conditions".

Every step does count but it is a stretch to say walking is more beneficial than other exercise, says Associate Professor Emmanuel Stamatakis​ of the University of Sydney's Faculty of Health Sciences and Charles Perkins Centre.

"I would take the conclusions with a pinch of salt," Stamatakis says.

"It is hard to define the intensity and accurate duration of activities in questionnaires."

Swimming, for instance, may be splashing about or doing laps, he says, adding that people notoriously over report how long they spend exercising and the intensity.

That said, if perambulation is your thing, keep on walking.

"Walking has practical advantages," Stamatakis says. "You can build it into your everyday life. It's sustainable and there's no need to make extra time for it. You can just replace the bus trip or car trip with walking."

The 10,000 step per day challenge is an "excellent first target", but walking "is not necessarily the optimal type of exercise".

The "optimal" exercise involves three parts: more incidental exercise, more moderate to high intensity exercise and less sitting.

"All movement is important," Stamatakis stresses. "All physical activity is linked with lower BMI and waist circumference."