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In someone that is following a healthy diet and exercise regimen causing the loss of visible fat, but is not losing weight, it is usually because of increases in muscle mass because of the exercise. Within this context, muscle mass is not a health risk similar to fat mass, which is one of the limitations of using weight or BMI as the goals for weight loss. In the medical literature, there is much discussion that measures of fat mass are helpful to augment the use of weight or BMI. There are methods for determining the percentage of weight that is fat, called percent body fat, but this is not easily determined at home. There also discussion of using waist size or the waist to hip ratio as a measure of fat mass, and these can be more easily monitored at home. Since you stomach is getting flatter, then you would be having a decrease in waist size and in the waist to hip ration.
However, if you are losing fat, as seen by a flatter stomach or by objective measures of body fat, the increase in weight is not a health risk.
Over time, there will be a point at which muscle mass will plateau, and further loss of body fat will be reflected in a decrease in weight and BMI.
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