Most people believe that exercising regularly will result in weight loss, but many people find that they end up putting on more weight in the form of fat, not muscle.
This can be attributed to some people eating more while engaging in physical activity, but for others who eat the same, there hasn’t been an answer.
Now, scientists have sought to discover the why exercise can lead to weight gain.
A new study, by Arizona State University's school of nutrition and health promotion, which was published last month in The Journal of Strength and Conditioning Research, looked at 81 sedentary but healthy women in their 30s who were overweight.
"Aerobic exercise training in women typically results in minimal fat loss, with considerable individual variability," the authors said.
"We hypothesised that women with higher baseline body fat would lose more body fat in response to exercise training, and that early fat loss would predict final fat loss."
All the participants had their BMI, body fat, weight and measurements recorded before being put on a three-month supervised exercise regimen which involved walking on a treadmill, at a fast pace, for 30 minutes, three times a week. They were also advised not to change their eating habits.
After 12 weeks, some women remained a stable weight, while others lost weight. But 55 had put on weight – some up to 4.8 kilograms.
Dr Glenn Gaesser, the study's lead author, told The New York Times that someone wanting to lose weight should weight themselves after four weeks of exercise because the study's participants who were losing weight after four weeks of exercise tended to continue to lose weight, while the others did not.
He also said if you didn’t see results after that period of time, you should "look closely at your diet and other activities".
The women in the study were found to be much fitter after four months of exercise.
"Fitness matters far more for health than how much you weigh," Dr. Gaesser said.