Running on the treadmill in the health club might be good for the body, but refuelling yourself in the club restaurant after the workout can make you emerge fatter than before, say nutritionists.
The health experts point out that some gyms in Britain are serving up too many stodgy meals packed full of fat, salt and sugar.
In the worst cases, according to their analysis, the calorie count of some lunches would take the equivalent of a half-day working out in the gym to burn off.
"In most places if you choose well you can eat healthily. But some gyms offer things that are really not healthy such as fries and foods dripping with cheese, which sends out a mixed message. It could undo all the good work people are trying to do when they go to the gym," The Scotsman quoted Carina Norris, a nutritionist at Queen Margaret University in Edinburgh, as saying.
One expert said that some gyms could be serving up unhealthy dishes to maximise profits.
Lorraine McCreary, a dietician who runs Bothwell-based practice Diet Scotland, said: "Higher quality foods are more expensive. It is certainly cheaper to produce food that is high in fat because fat is a flavour carrier. But that is not to say that a half-pound burger is going to be a bad choice. If it is a good quality, lean burger it would be good for those who want something with higher protein. But if it is a lean burger it probably would be labelled that way."
Caroline Toshack, a personal trainer and director of Living Breathing Fitness in Edinburgh, said that people often over-indulged as a reward for a gym workout.
"People tend to over-estimate how many calories they have burned in a workout and under-estimate how many calories they're eating," she said.
Gym owners, however, believe that it is important to provide different options to customers.
A spokesman for David Lloyd Leisure said: "Our aim is to encourage a healthy lifestyle whilst providing members with choice, whether they're after a family meal, a snack on the go or an occasional treat."
Some, however, believe that more could be done to help customers make better decisions.
A spokesman for Green's Brassiere said: "We are committed to helping people make informed choices and we need to do more."