In a study published in the Journal of American Medical Association, it was shown that it does not matter what is eaten but rather how much is eaten, that determines fat levels. 25 healthy adults were given 1000 excess calories a day. However their diet varied in the amounts of fat and protein. After 8 weeks, all of them gained weight. The group fed a low-protein diet gained approx 7 pounds as compared to the 13.3 pounds gained by the normal-protein diet group and 14.4 pounds gained by the high-proteing diet group. The level of body fat however increased by the same amount in all groups.
It was previously believed that a high protein diet would lead to lower weight gain. However in the study, participants on a high protein diet had a higher weight gain which was essentially due to an increase in lean body mass. Study participants on a low protein diet gained less weight but acquired substantial body fat.
The study indicates that when the body has excess calories to deal with - it converts it into fat. It does not matter whether the calories are coming from fat, carbohydrates or protein. Therefore simply relying on BMI is not effective in assessing the health of individuals. It is the fat levels that matter not just the body weight.
If your diet is poor and low in protein, you will gain weight but it will be due to fat accumulation not lean body mass which someone with a healthy diet will have. Fat reduction therefore should be the focus of obesity treatment rather than simple weight loss.