Huo Tingkun, marketing director of HAI, says the aim in using the volunteers on the program three years ago was to find out how effective it was, and this entailed interviewing those taking part.

"We wanted to know how much weight they wanted to lose, to get them onto the program and then to hear their stories."

All volunteers managed to achieve their goals after three months, Huo says, and a photo shoot was arranged so that they could show off their trimmer bodies.

"Even I had not known how to eat correctly, and from then I started to learn about how to pair exercise with food."

HAI opened its first restaurant, Green Option, in Beijing in May and now takes orders for immediate delivery, whereas previously dishes needed to be ordered a day in advance.

Having a physical restaurant gives online customers a place to gather, Huo says, and Sweetie Salad is working with gyms offering free training courses for its customers.

Whilst restaurants specializing in healthy food are forming business partnerships with gyms or fitness studios, gyms are getting in the act by selling healthy food.

One such is A Plus Fitness, a chain in Beijing that opened a dining area in 2016 offering meals to members and walk-in customers.

One of the coaches at A Plus Fitness, Yu Xiang, says that in addition to his physical training duties he now asks members to take photos of each meal they eat and send them to him for comment.